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1.3-200 25-1500 0.7-150 13-3000 25-1000 Data downloaded from http://www.skcinc.com/prod/800-01071.asp drying paint which fit into this category but many other products used in the workplace as well as consumer products (e.g. toiletries and polishes) are a source of VOCs. Outdoors pollution from traffic contains a wide range of VOCs including benzene. A number of VOCs are irritants and several can have specific health effects e.g. benzene is a carcinogen. As with all chemicals the health effect, if any, depends upon the concentration of the pollut- ant and the period that people are exposed. There is evidence that people's sensitivity to pollutants varies considerably Commercially available sampling tubes are used to measure VOCs in the air. This is a stainless steel tube that contains a powdered adsor- bent. When the end caps are removed and air pulled through the tube, VOCs are removed from the air by the sorbent. When returned to the laboratory, the tubes are analyzed by thermal desorption (TD) followed by gas chromatography with a flame ionization detector to measure amounts of VOCs collected and/or a mass spectrometer (MS) to confirm the identity of the individual VOCs collected. The adsorbent used determines the range of VOCs collected. A porous polymer (Tenax TA) has been widely used and this is opti mum for compounds that have a boiling point between 75°C METERS AND MONITORS 29 and 280°C. Sampling and analysis using this sorbent are in accor- dance with the international standard ISOl6000-6. Other sorbents are more suitable for compounds of different volatility, for example, some detectors use Carboxen 569 (a carbon molecular sieve) for the analysis of carbon disulphide. Polyurethane foam samplers are used for semi-volatile com- pounds (SVOCs), such as biocides and chemicals associated with oil mists. Formaldehyde and other volatile carbonyl compounds - Formaldehyde is an irritating gas released by many products and through combustion of fuels. It is useful as a preservative and is a component of glues used to bind particles of wood in particleboard and similar products. In outdoor air it occurs as a product of com- bustion and photochemical reaction. Other carbonyl compounds in air can include acetaldehyde, acetone, acrolein and glutaraldehyde. Special cartridge units can be used to measure carbonyls in the air. This is a circular plastic sampler that contains silica gel coated with a chemical that absorbs carbonyls from the air pulled through it by a pump. The sampling and analysis is undertaken in accor- dance with the International Standard ISO16000-3. Other pollutants - There are a range of continuous monitoring equipment to determine gases including nitrogen dioxide, carbon monoxide, methane, carbon dioxide and ozone in air. In addition, measurement of particulates (PM10) and ultrafine particles can be undertaken. The standards do not recommend or endorse any specific sup- plier or manufacturer. HSOs can identify specific suppliers and evaluate the most appropriate instruments based on cost, avail- ability, dependability, reliability, and intended application. Active sampling normally requires a specialist to visit your site with a range of equipment to undertake the air sampling. Measurements of temperature and humidity are routinely made at the same time. Alternatively, the HSO can undertake the sampling alone; however close consultation with instrument suppliers is recommended. 2.1.3.4 Continuous Emissions Monitors Continuous emissions monitors have been built using most types of sensor technology. There are systems in use using electrochemical, infrared, ultra-violet, chemiluminescent and other measurements for the toxic gases, whilst oxygen measurements are mainly carried 30 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES out with electrochemical, zirconium or paramagnetic sensors. All these methods have their own advantages and drawbacks, be it technological or financial. Among the portable instruments most suitable for workplace environments are combustible gas indicators (CGI) which detect flammables, four gas meters which detect flammables, oxygen, carbon monoxide and hydrogen sulfide, and photoionization (PID) detectors and the colorimetric tubes discussed above which detect low level toxics. None of these technologies is capable of identify- ing unknown gases and vapors. 2.1.3.4.1 Photoionization Meters Description and Applications. Ionization is based upon making a gas conductive by the creation of electrically charged atoms, molecules, or electrons and the collection of these charged parti- cles under the influence of an applied electric field. The photoion- ization analyzer is a screening instrument used to measure a wide variety of organic and some inorganic compounds. It is also useful as a leak detector. The limit of detection for most contaminants is approximately 0.1 ppm. Calibration. The procedure for calibration involves applying the calibration gas (typically 100 ppm isobutylene) to the instrument and checking the reading. Special Considerations. The specificity of the instrument depends on the sensitivity of the detector to the substance being measured, the number of interfering compounds present, and the concentration of the substance being measured relative to any interference. Many models now have built-in correction or correla- tion factors. After calibrating the unit on isobutylene, select the gas to be measured. The instrument will automatically correct for the relative sensitivity of the gas selected. Some instruments are listed by an NRTL for hazardous locations. Check the operating manual for specific conditions. Maintenance. Keeping these instruments in top operating shape means charging the battery, cleaning the ultraviolet lamp window, light source and replacing the dust filter. The exterior of the instru- ment can be wiped clean with a damp cloth and mild detergent if necessary. Keep the cloth away from the sample inlet, however, and do not attempt to clean while the instrument is connected to line power. METERS AND MONITORS 31 2.1.3.4.2 Infrared Analyzers Description and Applications. The infrared analyzer is used as a screening tool for a number of gases and vapors and is presently the recommended screening method for substances with no fea- sible sampling and analytical method. These analyzers are often factory-programmed to measure many gases and are also user-programmable to measure other gases. A microprocessor automatically controls the spectrometer, aver- ages the measurement signal, and calculates absorbance values. Analysis results can be displayed either in parts per million (ppm) or absorbance units (AU). The variable path-length gas cell gives the analyzer the capability of measuring concentration levels from below 1 ppm up to percent levels. Some typical screening applications are: • Carbon monoxide and carbon dioxide, especially use- ful for indoor air assessments; • Anesthetic gases including, e.g., nitrous oxide, halo- thane, enflurane, penthrane, and isoflurane; • Ethylene oxide; and • Fumigants including e.g. ethylene dibromide, chloro- picrin, and methyl bromide. Calibration. The analyzer and any strip-chart recorder should be calibrated before and after each use in accordance with the manu- facturer's instructions. Special Considerations. The infrared analyzer may be only semi- specific for sampling some gases and vapors because of interfer- ence by other chemicals with similar absorption wavelengths. Maintenance. No field maintenance of this device should be attempted except items specifically detailed in the instruction book such as filter replacements and battery charging. 2.1.3.4.3 Toxic Gas Meters Description and Application. This analyzer uses an electrochemical voltametric sensor or polarographic cell to provide continuous anal- yses and electronic recording. In operation, sample gas is drawn through the sensor and absorbed on an electrocatalytic sensing elec- trode, after passing through a diffusion medium. An electrochemi- cal reaction generates an electric current directly proportional to the 32 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES gas concentration. The sample concentration is displayed directly in parts per million. Since the method of analysis is not absolute, prior calibration against a known standard is required. Exhaustive tests have shown the method to be linear; thus, calibration at a single concentration, along with checking the zero point, is sufficient. Types: Sulfur dioxide, hydrogen cyanide, hydrogen chloride, hydra- zine, carbon monoxide, hydrogen sulfide, nitrogen oxides, chlorine, and ethylene oxide. These can be combined with combustible gas and oxygen meters. Calibration. Calibrate the direct-reading gas monitor before and after each use in accordance with the manufacturers instructions and with the appropriate calibration gases. Special Considerations. • Interference from other gases can be a problem. See manufacturers literature. • When calibrating under external pressure, the pump must be disconnected from the sensor to avoid sen- sor damage. If the span gas is directly fed into the instrument from a regulated pressurized cylinder, the flow rate should be set to match the normal sam- pling rate. • Due to the high reaction rate of the gas in the sensor, substantially lower flow rates result in lower readings. This high reaction rate makes rapid fall time possible simply by shutting off the pump. Calibration from a sample bag connected to the instrument is the pre- ferred method. 2.1.3.4.4 Ozone Meter Description and Application. The detector uses a thin-film semicon- ductor sensor. A thin-film platinum heater is formed on one side of an alumina substrate. A thin-film platinum electrode is formed on the other side, and a thin-film semiconductor is formed over the plat- inum electrode by vapor deposition. The semiconductor film, when kept at a high temperature by the heater, will vary in resistance due to the absorption and decomposition of ozone. The change in resistance is converted to a change of voltage by the constant-current circuit. The measuring range of the instrument is 0.01 ppm to 9.5 ppm ozone in air. The readings are displayed on a liquid crystal display that reads ozone concentrations directly. The temperature range is 0°-40° C, and the relative humidity range is 10%-80% RH. METERS AND MONI TORS 33 Calibration. Calibrate instrument before and after each use. Be sure to use a well-ventilated area since ozone levels may exceed the PEL for short periods. Calibration requires a source of ozone. Controlled ozone concentrations are difficult to generate in the field, and this calibration is normally performed at SLTC. Gas that is either specially desiccated or humidified must not be used for preparing calibration standards, as readings will be inaccurate. Special Considerations. • The instrument is not intrinsically safe. • The instrument must not be exposed to water, rain, high humidity, high temperature, or extreme temper- ature fluctuation. • The instrument must not be used or stored in an atmo- sphere containing silicon compounds, or the sensor will be poisoned. • The instrument is not to be used for detecting gases other than ozone. Measurements must not be performed when the presence of organic solvents, reducing gases (such as nitrogen monoxide, etc.), or smoke is sus- pected; readings may be low. Maintenance. The intake-filter unit-Teflon sampling tube should be clean and connected firmly. These should be checked before each operation. Check the pump aspiration and sensitivity before each operation. 2.1.3.4.5 Mercury Analyzer-Gold Film Analyzer Description and Application. This instrument relies on a gold-film analyzer that draws a precise volume of air over a gold-film sensor. A microprocessor computes the concentration of mercury in milli- grams per cubic meter and displays the results on the digital meter. The meter is selective for mercury and eliminates interference from water vapor, sulfur dioxide, aromatic hydrocarbons, and particu- lates. However, hydrogen sulfide is an interférant. Calibration. Calibration should be performed by the manufac- turer or a laboratory with the special facilities to generate known concentrations of mercury vapor. I nstruments should be returned to the manufacturer or a calibration laboratory on a scheduled basis. Special Considerations. In high concentrations of mercury vapor the gold film saturates quickly. Check operating manual for more specific information. 34 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES Maintenance. Mercury vapor instruments generally contain rechargeable battery packs, filter medium, pumps, and valves which require periodic maintenance. Except for routine charging of the battery pack, most periodic maintenance will be performed during the scheduled annual calibrations. However, depending on usage, routine maintenance should also include burning mer- cury deposits off of the gold-film and changing the zero filter when necessary. See operating manual for specific instructions. 2.1.3.4.6 Direct-Reading Particle Monitors Description and Applications. Condensation-nuclei counters are based upon a miniature, continuous-flow condensation nucleus counter (CNC) that takes particles too small to be easily detected, enlarges them to a detectable size, and counts them. Submicrometer particles are grown to supermicrometer alcohol droplets by first saturating the particles with alcohol vapor as they pass through a heated saturator lined with alcohol soaked felt, and then condens- ing the alcohol on the particles in a cooled condenser. Optics focus laser light into a sensing volume. As the droplets pass through the sensing volume, the particles scatter the light. The light is directed onto a photodiode which generates an electrical pulse from each droplet. The concentra- tion of particles is counted by determining the number of pulses generated. Applications include the testing of respirators and dust monitors. The counter counts individual airborne particles from sources such as smoke, dust, and exhaust fumes. Models typically operate in one of three possible modes, each with a particular application. In the "count" mode, the counter measures the concentration of these airborne particles. In the "test" (or fit test) mode, measurements are taken inside and outside a respirator and a fit factor is calculated. In the "sequential" mode, the instrument measures the concentration on either side of a filter and calculates filter penetration. This instrument is sensitive to particles as small as 0.02 microm- eters. However, it is insensitive to variations in size, shape, compo- sition, and refractive index. Calibration. Check the counter before and after each use in accor- dance with the manufacturers instructions. This usually involves checking the zero of the instrument. Special Considerations. Reagent-grade isopropyl alcohol for use in these types of instruments is available from HSO. METERS AND MONITORS 35 • Dry the saturator felt by installing a freshly charged battery pack without adding alcohol. Allow the instru- ment to run until the LO message (low battery) or the E-E message (low particle count) appears. Some instruments allow you to remove the alcohol cartridge for storage purposes. • Remove the battery pack. • Install the tube plugs into the ends of the twin-tube assembly. Maintenance. Isopropyl alcohol must be added to the unit every 5-6 hours of operation, per the manufacturer's instructions. Take care not to overfill the unit. Under normal conditions, a fully charged battery pack will last for about 5 hours of operation. Low battery packs should be charged for at least 6 hours, and battery packs should not be stored in a discharged condition. 2.1.3.4.7 Combustible Gas Meters Description and Applications. These meters use elements which are made of various materials such as platinum or palladium as an oxidizing catalyst. The element is one leg of a Wheatstone bridge circuit. These meters measure gas concentration as a percentage of the lower explosive limit of the calibrated gas. The oxygen meter displays the concentration of oxygen in percent by volume measured with a galvanic cell. Other electro- chemical sensors are available to measure carbon monoxide, hydro- gen sulfide, and other toxic ga$es. Some units have an audible alarm that warns of low oxygen levels or malfunction. Calibration. Before using the monitor each day, calibrate the instrument to a known concentration of combustible gas (usually methane) equivalent to 25%-50% LEL full-scale concentration. The monitor must be calibrated to the altitude at which it will be used. Changes in total atmospheric pressure from changes in altitude will influence the instrument's measurement of the air's oxygen content. The unit's instruction manual provides additional details on calibration of sensors. Special Considerations. • Silicone compound vapors, leaded gasoline, and sulfur compounds will cause desensitization of the combus- tible sensor and produce erroneous (low) readings. 36 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES • High relative humidity (90%-100%) causes hydroxy- lation, which reduces sensitivity and causes erratic behavior including inability to calibrate. • Oxygen deficiency or enrichment such as in steam or inert atmospheres will cause erroneous readings for combustible gases. • In drying ovens or unusually hot locations, solvent vapors with high boiling points may condense in the sampling lines and produce erroneous (low) readings. • High concentrations of chlorinated hydrocarbons such as trichloroethylene or acid gases such as sulfur diox- ide will depress the meter reading in the presence of a high concentration of combustible gas. • High-molecular-weight alcohols can burn out the meters filaments. • If the flash point is greater than the ambient temper- ature, an erroneous (low) concentration will be indi- cated. If the closed vessel is then heated by welding or cutting, the vapors will increase and the atmosphere may become explosive. • For gases and vapors other than those for which a device was calibrated, users should consult the manu- facturer's instructions and correction curves. Maintenance. The instrument requires no short-term maintenance other than regular calibration and recharging of batteries. Use a soft cloth to wipe dirt, oil, moisture, or foreign material from the instrument. Check the bridge sensors periodically, at least every six months, for proper functioning. A thermal combustion-oxygen sensor uses electrochemical cells to measure combustible gases and oxygen. It is not widely used in the area offices. 2.1.3.4.8 Oxygen Meters These oxygen-measuring devices can include coulometric and fluorescence measurement, paramagnetic analysis, and polaro- graphic methods. The output of most electrochemical oxygen sensors is dependent on the partial pressure of oxygen in the atmo- sphere. They do not actually measure concentration directly. An instrument calibrated at sea level and used at higher elevations, such as mountains, will indicate a value lower than the actual concentration. METERS AND MONITORS 37 21.3.4.9 Bioaerosol Monitors Description and Applications: A bioaerosol meter, usually a two- stage sampler, is also a multiorifice cascade impactor. This unit is used when size distribution is not required and only respirable- nonrespirable segregation or total counts are needed. Ninety-five to 100 percent of viable particles above 0.8 microns in an aerosol can be collected on a variety of bacteriological agar. Trypticase soy agar is normally used to collect bacteria, and malt extract agar is normally used to collect fungi. They can be used in assessing sick- (or tight-) building syndrome and mass psycho- genie illness. These samplers are also capable of collecting virus particles. However, there is no convenient, practical method for cultivation and enumeration of these particles. Calibration: Bioaerosol meters must be calibrated before use. This can be done using an electronic calibration system with a high- flow cell, available through the HRT. Special Considerations: Prior to sampling, determine the type of collection media required and an analytical laboratory. The HRT can provide this information. This specialized equipment is avail- able from the HRT with accompanying instructions. Maintenance: The sampler should be decontaminated prior to use by sterilizer or chemical decontamination with isopropanol. 2.1.4 Batteries Many of the instruments described are powered by batteries. Battery care is important in assuring uninterrupted sampling. A pump battery pack, for example, should be discharged to the recommended level before charging, at least after every use. If the pump is allowed to run down until the battery reaches the low battery Fault condition, the pump should be turned OFF soon after the Fault condition stops the pump. Leaving some pumps ON for a long time after this Fault condition can damage the battery pack. Also, avoid overcharging the battery pack. Alkaline Batteries. Replace frequently before they become depleted, or carry fresh replacements. When replacing a battery, never mix types (alkaline, carbon zinc, etc.) or capacity and age. Doing so can have negative affects on all the batteries. Remove bat- teries if equipment will not be used for an extended period of time. 38 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES Rechargeable Ni-Cad Batteries. • Check the batteries under load (e.g., turn pump on and check voltage at charging jack, if one is available and this can be done safely) before use. See manu- facturer's instructions for locations to check voltage. Use 1.2 volts per Ni-Cad cell for an estimate of the fully charged voltage of a rechargeable battery pack. • It is undesirable to discharge a multicell Ni-Cad battery pack to voltage levels that are below 1.0 volts per cell; doing this will drive a reverse current through some of the cells and can permanently damage them. • Rechargeable Ni-Cad batteries should be charged only in accordance with manufacturer's instructions. Chargers are generally designed to charge batteries in approxi- mately 8 to 16 hours at a high charge rate. A battery can be overcharged and ruined when a high charge rate is applied for too long a time. However, Ni-Cad batteries may be left on a proper trickle charge indefinitely to maintain them at peak capacity. In this case, discharging for a period equal to the longest effective field service time may be necessary, because of short-term memory imprinting. However, do not let the battery run down overnight or longer. Turn the instrument OFF when the battery reaches the proper discharge level. Other Rechargeable Batteries: Other types of rechargeable batteries are being used in equipment such as lead-acid, nickel-metal hydride, etc. Make sure the manufacturer's instructions are followed concern- ing the handling and recharging of these types of batteries. 2.1.5 Adverse Conditions Adverse Temperature Effects High ambient temperature, above 37.8°C and/or radiant heat (e.g., from nearby molten metal) can cause flow faults in air sam- pling pumps. If these conditions are likely, use the pump with a higher operating temperature range, as opposed to a pump with a lower operating temperature range. Temperature can also affect the accuracy of instrument readings or operation. Check the operating manual for the proper operating temperature range. METERS AND MONITORS 39 Explosive Atmospheres • Instruments shall not be used in atmospheres where the potential for explosion exists unless the instrument is listed by a nationally recognized testing laboratory for use in the type of atmosphere present. Check the class and division ratings. • When batteries are being replaced, use only the type of battery specified on the safety approval label. • Do not assume that an instrument is intrinsically safe. Verify by contacting the instrument's maker if uncertain. Atmospheres Containing Carcinogens A plastic bag should be used to cover equipment when carcino- gens are present. Decontamination procedures for special environ- ments are available and should be followed after using equipment in carcinogenic environments. If at all possible, decontaminate the equipment after use on-site. 2.1.6 Annex-Instrument Chart The information shown in Table 2.2 below is for reference only. Not every compliance officer or field office will have every type of instrument. Many of the instruments can be found in and are available through a Corporate Loan program. Table 2.2 Gas and vapor meters. Type of Instrument Double-range meters 1 Triple range meters Quad range meters CO dosimeter Measured Substance Combustible gas, 0 2 Toxic, 0 2 , combustible gas 2 toxics, 0 2 , combustible gas CO Application Confined spaces Confined spaces Ggarages, indoor air quality 40 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES Table 2.2 (cont.) Gas and vapor meters. Type of Instrument Carbon dioxide meter Infrared analyzers Hydrogen cyanide monitors Hydrogen sulfide meters Mercury vapor meters NO and N0 2 meters Ozone analyzers Measured Substance co 2 CO, C0 2 , organic substances Hydrogen cyanide Hydrogen sulfide Mercury NO and N0 2 combustion o, Application I ndoor air quality (IAQ) Traces indoor air, leaks, spills Plants Farms, sewers Mercury plants, spills Water or air purification, IAQ 2.1.7 Bibliography A.M. Best Co. (AMB). 1990. Bests Safety Directory. AMB: Odwick, NJ. Hering, S.V., Ed. 1989. Air Sampling Instruments for Evaluation of Atmospheric Contaminants, American Conference of Governmental Industrial Hygienists: Cincinnati, Ohio. Cheremisinoff, N. P., 1999, Handbook of Industrial Toxicology and Hazardous Materials, Marcel Dekker Publishers, NY, NY. 2.2 Noise Testing and Monitoring 2.2.1 Introduction Hearing protection should be issued to employees: • where extra protection is needed above what can been achieved using noise control; • as a short-term measure while other methods of con- trolling noise are being developed. METERS AND MONITORS 41 The health and safety officer (HSO) should not use hearing pro- tection as an alternative to controlling noise by technical and organi- zational means. The national electricity health and safety standards require employers to: • provide employees with hearing protectors if they ask for it and their noise exposure is between the lower and upper exposure action values; • provide employees with hearing protectors and make sure they use them properly when their noise expo- sure exceeds the upper exposure action values; • identify hearing protection zones, i.e. areas where the use of hearing protection is compulsory, and mark them with signs if possible; • provide your employees with training and information on how to use and care for the hearing protectors; • ensure that the hearing protectors are properly used and maintained. Hearing protection can be used effectively by applying the follow- ing guidelines: • make sure the protectors give enough protection - aim at least to get below 85 dB at the ear; • target the use of protectors to the noisy tasks and jobs in a working day; • select protectors which are suitable for the working environment - consider how comfortable and hygienic they are; • think about how they will be worn with other protec- tive equipment (e.g. hard hats, dust masks and eye protection); • provide a range of protectors so that employees can choose ones which suit them. Hearing protection can be used effectively by avoiding the following: • provide protectors which cut out too much noise - this can cause isolation, or lead to an unwillingness to wear them; 42 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES • make the use of hearing protectors compulsory where the law doesn't require it; • have a 'blanket' approach to hearing protection - better to target its use and only encourage people to wear it when they need to. 2.2.2 Noise Monitors and Meters 2.2.2.2 Sound Level Meters Octave Band Analyzer. Some sound level meters may have an octave or one-third octave band filter attached or integrated into the instrument. The filters are used to analyze the frequency content of noise. They are also valuable for the calibration of audiometers and to determine the suitability of various types of noise control. Calibration. In normal operation, calibration of the instrument usually requires only checking. Prior to and immediately after taking measurements, it is a good practice to check, using a calib- rator, the ability of the sound level instrument to correctly measure sound levels. As long as the sound level readout is within 0.2 dB of the known source, it is suggested that no adjustments to the calibration pot be made. If large fluctuations in the level occur (more than 1 dB) then either the calibrator or the instrument may have a problem. Special Considerations • Always check the batteries prior to use. Use the micro- phone windscreen to protect the microphone when the wearer will be outdoors or in dusty or dirty areas. (The windscreen will not protect the microphone from rain or extreme humidity.) • Never use any other type of covering over the micro- phone (e.g., plastic bag or plastic wrap) to protect it from moisture. These materials will distort the noise pickup, and the readings will be invalid. • Never try to clean a microphone, particularly with com- pressed air, since damage is likely to result. Although dirt and exposure will damage microphones, regular use of an acoustical calibrator will detect such damage so that the microphones can be replaced. • Remove the batteries from any meter that will be stored for more than 5 days. Protect meters from extreme heat and humidity. METERS AND MONITORS 43 Maintenance. No field maintenance is required other than replace- ment of batteries. 2.2.2.2 Personal Dosimeters Calibration. Field calibrate at the measurement site according to the manufacturer's instructions both before and after each use. Use an acoustical calibrator that was designed to be used with the par- ticular model noise dosimeter being used. Special Considerations • Always check the batteries prior to use. Be very careful with the microphone cable. Never kink, stretch, pinch, or otherwise damage the cable. • Use the microphone windscreen to protect the micro- phone when the wearer will be outdoors or in dusty or dirty areas. (The windscreen will not protect the microphone from rain or extreme humidity.) • Never use any type of covering over the microphone (e.g., plastic bag or plastic wrap) to protect it from moisture. Such materials will distort the noise pickup, and the readings will be invalid. • Never try to clean a microphone, particularly with com- pressed air, since damage is likely to result. Although dirt and exposure to industrial environments will dam- age the microphones, regular use of an acoustical calib- rator will detect such damage so that microphones can be replaced. • Remove the batteries when the dosimeter will be stored for more than 5 days. Protect dosimeters from extreme heat and humidity. Maintenance. No field maintenance is required other than replace- ment of batteries. 2.2.3 Occupational Noise Exposure Standard 223.1 Allowable Levels of Exposure Protection against the effects of noise exposure shall be provided when the sound levels exceed those shown in Table 2.3 when mea- sured on the A scale of a standard sound level meter at slow response. 44 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES Table 2.3 Permissible noise exposures. 1 Duration Per Day, Hours 8 6 4 3 2 1.5 1 0.5 0.25 or less Sound Level, dB A Slow Response 90 92 95 97 100 102 105 110 115 1. When the daily noise exposure is composed of two or more periods of noise exposure of different levels, their combined effect should be considered, rather than the individual effect of each. If the sum of the following fractions: C(l)/T(l) +C(2)/T(2) C(n)/T(n) exceeds unity, then, the mixed exposure should be considered to exceed the limit value. Cn indicates the total time of exposure at a specified noise level, and Tn indicates the total time of exposure permitted at that level. Exposure to impulsive or impact noise should not exceed 140 dB peak sound pressure level. When noise levels are determined by octave band analysis, the equivalent A-weighted sound level may be determined as follows. When employees are subjected to sound exceeding those listed in Table 2.3, feasible administrative or engineering controls shall be utilized. If such controls fail to reduce sound levels within the levels of Table 2.3, personal protective equipment shall be provided and used to reduce sound levels within the levels of the table. If the variations in noise level involve maxima at intervals of 1 second or less, it is to be considered continuous. 2.2.3.2 Hearing Conservation Program The employer shall administer a continuing, effective hearing con- servation program, as described below, whenever employee noise exposures equal or exceed an 8-hour time-weighted average sound level (TWA) of 85 decibels measured on the A scale (slow response) METERS AND MONI TORS 45 or, equivalently, a dose of fifty percent. For purposes of the hearing conservation program, employee noise exposures shall be estab- lished without regard to any attenuation provided by the use of personal protective equipment. For purposes of establishing a program, an 8-hour time-weighted average of 85 decibels or a dose of fifty percent shall also be referred to as the action level. The following constitute the basic components of a hearing conservation program to be adopted and implemented: Monitoring: When information indicates that any employee's exposure may equal or exceed an 8-hour time-weighted average of 85 decibels, the employer shall develop and implement a monitor- ing program. Sampling: The sampling strategy shall be designed to identify employees for inclusion in the hearing conservation program and to enable the proper selection of hearing protectors. Where circumstances such as high worker mobility, significant vari- ations in sound level, or a significant component of impulse noise make area monitoring generally inappropriate, the employer shall use representative personal sampling to comply with the monitor- ing requirements of the program unless the employer can show that area sampling produces equivalent results. All continuous, intermittent and impulsive sound levels from 80 deci- bels to 130 decibels shall be integrated into the noise measurements. I nstruments used to measure employee noise exposure shall be calibrated to ensure measurement accuracy. Monitoring shall be repeated whenever a change in production, process, equipment or controls increases noise exposures to the extent that: Employee Notification: The employer shall notify each employee exposed at or above an 8-hour time-weighted average of 85 deci- bels of the results of the monitoring. Observation of Monitoring: The employer shall provide affected employees or their representatives with an opportunity to observe any noise measurements conducted pursuant to this section. Audiometric Testing Program: The employer shall establish and maintain an audiometric testing program as provided in this para- graph by making audiometric testing available to all employees 46 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES whose exposures equal or exceed an 8-hour time-weighted average of 85 decibels. The program shall be provided at no cost to employees. Audiometric tests should be performed by a licensed or certified audiologist, otolaryngologist, or other physician, or by a technician who has satisfactorily demonstrated competence in administering audiometric examinations, obtaining valid audiograms, and prop- erly using, maintaining and checking calibration and proper func- tioning of the audiometers being used. A technician who performs audiometric tests must be responsible to an audiologist, otolaryn- gologist or physician. Baseline Audiogram: Within 6 months of an employee's first exposure at or above the action level, the employer shall establish a valid baseline audiogram against which subsequent audiograms can be compared. Testing to establish a baseline audiogram should be preceded by at least 14 hours without exposure to workplace noise. Hearing protectors may be used as a substitute for the requirement that baseline audiograms be preceded by 14 hours without exposure to workplace noise. The employer shall notify employees of the need to avoid high levels of non-occupational noise exposure during the 14-hour period immediately preceding the audiometric examination. Annual Audiogram: At least annually after obtaining the base- line audiogram, the employer shall obtain a new audiogram for each employee exposed at or above an 8-hour time-weighted aver- age of 85 decibels. Evaluation of Audiogram: Each employee's annual audiogram shall be compared to that employee's baseline audiogram to deter- mine if the audiogram is valid and if a standard threshold shift has occurred. This comparison may be done by a technician. If the annual audiogram shows that an employee has suffered a standard threshold shift, the employer may obtain a retest within 30 days and consider the results of the retest as the annual audiogram. The audiologist, otolaryngologist, or physician shall review problem audiograms and shall determine whether there is a need for further evaluation. The employer shall provide to the person performing this evaluation the following information: • A copy of the requirements for hearing conservation as set forth in this section; METERS AND MONI TORS 47 • The baseline audiogram and most recent audiogram of the employee to be evaluated; • Measurements of background sound pressure levels in the audiometric test room; • Records of audiometer calibrations are required. Follow-up Procedures: If a comparison of the annual audiogram to the baseline audiogram indicates a standard threshold shift the employee shall be informed of this fact in writing, within 21 days of the determination. Unless a physician determines that the standard threshold shift is not work related or aggravated by occupational noise exposure, the employer shall ensure that the following steps are taken when a standard threshold shift occurs: • Employees not using hearing protectors shall be fitted with hearing protectors, trained in their use and care, and required to use them. • Employees already using hearing protectors shall be refitted and retrained in the use of hearing protectors and provided with hearing protectors offering greater attenuation if necessary. • The employee shall be referred for a clinical audio- logical evaluation or an otological examination, as appropriate, if additional testing is necessary or if the employer suspects that a medical pathology of the ear is caused or aggravated by the wearing of hearing protectors. • The employee is informed of the need for an oto- logical examination if a medical pathology of the ear that is unrelated to the use of hearing protectors is suspected. If subsequent audiometric testing of an employee whose exposure to noise is less than an 8-hour TWA of 90 decibels indicates that a standard threshold shift is not persistent, the employer: • Shall inform the employee of the new audiometric interpretation; and • May discontinue the required use of hearing protec- tors for that employee. 48 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES Standard Threshold Shift: A standard threshold shift is a change in hearing threshold relative to the baseline audiogram of an average of 10 dB or more at 2000, 3000, and 4000 Hz in either ear. In deter- mining whether a standard threshold shift has occurred, allowance may be made for the contribution of aging (presbycusis) to the change in hearing level by correcting the annual audiogram. Hearing Protectors: Employers shall make hearing protectors available to all employees exposed to an 8-hour time-weighted aver- age of 85 decibels or greater at no cost to the employees. Hearing protectors shall be replaced as necessary. Employers shall ensure that hearing protectors are worn: • By an employee who is required to wear personal pro- tective equipment; and • By any employee who is exposed to an 8-hour time- weighted average of 85 decibels or greater, and who: - Has not yet had a baseline audiogram; or - Has experienced a standard threshold shift. Employees shall be given the opportunity to select their hearing protectors from a variety of suitable hearing protectors provided by the employer. The employer shall provide training in the use and care of all hearing protectors provided to employees. The employer shall ensure proper initial fitting and supervise the correct use of all hearing protectors. Training Program: The employer shall institute a training pro- gram for all employees who are exposed to noise at or above an 8-hour time-weighted average of 85 decibels, and shall ensure employee participation in such program. The training program shall be repeated annually for each employee included in the hearing conservation program. Information pro- vided in the training program shall be updated to be consistent with changes in protective equipment and work processes. The employer shall ensure that each employee is informed of the following: • The effects of noise on hearing; • The purpose of hearing protectors, the advantages, disadvantages, and attenuation of various types, and instructions on selection, fitting, use, and care; and METERS AND MONI TORS 49 • The purpose of audiometric testing, and an explana- tion of the test procedures. The employer shall make available to affected employees or their representatives copies of this standard and shall also post a copy in the workplace. 2.2.3.3 Recordkeeping 2.2.3.3.1 Exposure Measurements The employer shall maintain an accurate record of all employee exposure measurements described in this section. 2.2.3.3.2 Audiometric Tests The employer shall retain all employee audiometric test records described in this section: This record shall include: • Name and job classification of the employee; • Date of the audiogram; • The examinees name; • Date of the last acoustic or exhaustive calibration of the audiometer; and • Employee's most recent noise exposure assessment. • The employer shall maintain accurate records of the measurements of the background sound pressure levels in audiometric test rooms. 2.2.3.3.3 Record Retention The employer shall retain records for at least the following periods: • Noise exposure measurement records shall be retained for two years. • Audiometric test records shall be retained for the duration of the affected employee's employment. 2.2.3.3.4 Access to Records All records required by this section shall be provided upon request to employees, former employees, and representatives designated by the individual employee. 50 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES 2.2.3.3.5 Transfer of Records If the employer ceases to do business, the employer shall transfer to the successor employer all records required to be maintained by this section, and the successor employer shall retain them for the remainder of the period prescribed in this section. 2.2.4 Bibliography The following references were consulted. Internet sources can be accessed by typing the identified reference on a well-known search engine such as "Google". OSHA Standards Recording and Reporting Occupational Injuries and Illnesses (29 CFR 1904) • 1904.10, Recording Criteria for Cases Involving Occupational Hearing Loss (for Recordkeeping) General I ndustry (29 CFR 1910) • 1910 Subpart G, Occupational health and environ- mental control - 1910.95, Occupational noise exposure Construction I ndustry (29 CFR 1926) • 1926 Subpart D, Occupational health and environ- mental controls - 1926.52, Occupational noise exposure • 1926 Subpart E, Personal protective and lifesaving equipment - 1926.101, Hearing protection Additional References (Internet Resources) • Acoustical Society of America • American I ndustrial Hygiene Association (AIHA) • American Speech-Language-Hearing Association • Guide for Selection and Use of Personal Protective Equipment & Special Clothing for Foundry Operators. METERS AND MONI TORS 51 OSHA and American Foundry Society (AFS) Safety and Health Committee (10Q), (2005), 914 KB PDF, 29 pages. This guide was developed by the American Foundry Society's (AFS) Safety and Health Committee (10Q) and is a product of the OSHA and AFS Alliance. It describes special considerations for the selection and use of per- sonal protective equipment and special clothing for work situations in metal melting and pouring opera- tions that present a risk of exposure to foundry hazards • E-A-R Hearing Conservation (and Hearing Loss Prevention) • Laborers 7 Health & Safety Fund of North America (Topics of Interest: Noise) • National Hearing Conservation Association (NHCA) (see also OSHA's NHCA Alliance page) • National Institute of Occupational Safety and Health (NIOSH) (Hearing Loss Prevention) Printed Resources • Alberti, P.W. Personal Hearing Protection in Industry. New York: Raven Press, 1982. • Beranek, L.L. Noise and Vibration Control. New York: McGraw-Hill, 1971. • Berger, E. H., et al. The Noise Manual. 5th ed. Fairfax, VA: American I ndustrial Hygiene Association, 2000. • Cheremisinoff, N. P., Noise Control in Industry, Noyes Publications, Park Ridge, N.J . (1996). • Code of Federal Regulations. Title 29, Parts 1900 to 1990. Washington, DC: US Government Printing Office, 1992. • Harris, CM. Handbook for Noise Control. 2nd ed. New York: McGraw-Hill, 1957. • Harris, CM. Shock and Vibration Handbook. 3rd ed. New York: McGraw-Hill, 1988. • Kryter, K.D. The Effects of Noise on Man. New York: Academic Press, 1970. • Kryter, K.D. The Effects of Noise on Man. 2nd ed. New York: Academic Press, 1985. • Olishifski, P.E. and E.R. Harford. I ndustrial Noise and Hearing Conservation. Chicago: National Safety Council, 1975. 52 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES • Field Operations Manual. OSHA Directive CPL 02-00- 045 [CPL 2.45B]. Washington, DC: US Government Printing Office, 1992. • Royster, J ulia and Larry. Guide 15: A Guide to Developing and Maintaining an Effective Hearing Conservation Program. Raleigh, NC: North Carolina Department of Labor, Division of Occupational Safety and Health, 1998. • Sataloff, J., and P. Michael. Hearing Conservation. Springfield, IL: Charles C Thomas, 1973. • Sataloff, R.T., and J.T. Sataloff. Occupational Hearing Loss. New York: Marcel Dekker, 1987. 2.3 Radiation Monitors and Meters 2.3.1 Introduction I nstruments used for radiation measurement fall into two broad categories: • rate measuring instruments and • personal dose measuring instruments. Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity). Survey meters, audible alarms and area monitors fall into this category. These instruments present a radiation intensity reading relative to time, such as R/hr or mR/hr. An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time. Dose measuring instruments are those that measure the total amount of exposure received during a measuring period. The dose measuring instruments, or dosimeters, that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the indi- vidual. An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units. The following provides general information on the selection and application of field instruments for monitoring radiation sources in the work place. METERS AND MONITORS 53 2.3.2 Light Description and Applications. The light meter is a portable unit designed to measure visible, UV, and near-UV light in the workplace. The light meter is capable of reading any optical unit of energy or power level if the appropriate detector has been calibrated with the meter. The spectral range of the instrument is limited only by the choice of detector. Steady-state measurements can be made from a steady-state source using the "normal operation" mode. Average measurements can be obtained from a flickering or modulated light source with the meter set in the "fast function" position. Flash measurements can be measured using the "integrate" function. Calibration. No field calibration is available. These instruments are generally very stable and require only periodic calibration. Special Considerations. Exposure of the photomultiplier to bright illumination when the power is applied can damage the sensitive cathode or conduct excessive current. Maintenance. Little maintenance is required unless the unit is subjected to extreme conditions of corrosion or temperature. Clean the optical unit with lens paper to avoid scratching. 2.3.3 Ionizing Radiation Description and Applications. The ionizing radiation survey meter is useful for measuring radon decay products from air samples col- lected on filters. Wipe samples collected on a filter can also be counted with this detector, and general area sampling can be done. Several types of ionizing radiation meters are commercially available. The survey meter with the scintillation detector can be used to mea- sure the presence of radon-decay products in a dust sample. The baro- metric pressure should be noted for ionizing radiation chambers. Calibration. No field calibration is available. Periodic calib- ration by a laboratory is essential and should be handled by the manufacturer. 2.3.4 Nonionizing Radiation Description and Applications. Various nonionizing radiation survey meters are available for measuring electromagnetic fields. 54 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES The frequency ranges covered by instruments are: 10 Hz to 300 kHz, 0.5 MHz to 6000 MHz, 6 GHz to 40 GHz, and the 2.45 GHz micro- wave oven frequency. These instruments are capable of measuring the electric field strength (E-field), magnetic field strength (H-field), or both depending on the instrument. Depending on the instrument, electromagnetic field strengths from power lines, transformers, video display terminals, RF induc- tion heaters, RF heat sealers, radio & television transmitters, micro- wave ovens and other sources can be measured. Calibration. No field calibration is available. Periodic calibration by the manufacturer is essential. Special Considerations. • Some of the instruments have an automatic instru- ment zeroing. Other instruments may require "zero- ing" the instrument in a "zero-field" condition. Check the operating manual for guidance. • Some units have a peak memory-hold circuit that retains the highest reading in memory. • Some units operate with either electric (E) or mag- netic (H) field probes based on diode-dipole antenna design. Total field strength is measured at the meter regardless of the field orientation or probe receiving angle. The diode-dipole antenna design of the probe is much more resistant to burnout from overload than the thermocouple design of probes used with other meters. Maintenance. No field maintenance is required other than replac- ing the alkaline batteries when needed. 23.5 Survey Meters for Radiation Detection There are many different models of survey meters available to mea- sure radiation in the field. They all basically consist of a detector and a readout display. Analog and digital displays are available. Most of the survey meters used for industrial radiography use a gas filled detector. Gas filled detectors consist of a gas filled cylinder with two elec- trodes. Sometimes, the cylinder itself acts as one electrode, and a needle or thin taut wire along the axis of the cylinder acts as the METERS AND MONITORS 55 other electrode (Figure 2.3). A voltage is applied to the device so that the central needle or wire become an anode (+charge) and the other electrode or cylinder wall becomes the cathode (- charge). The gas becomes ionized whenever the counter is brought near radioac- tive substances. The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode. This results in an electrical signal that is amplified, correlated to exposure and displayed as a value. Depending on the voltage applied between the anode and the cathode, the detector may be considered an ion chamber, a pro- portional counter, or a Geiger-Müller (GM) detector. Each of these types of detectors have their advantages and disadvantages. Abrief summary of each of these detectors is as follows. 2.3.5.2 Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode, which results in a collection of only the charges produced in the initial ionization event. This type of detector produces a weak output signal that corresponds to the number of ionization events. Higher energies and intensities of radiation will produce more ionization, which will result in a stronger output voltage. Figure 2.3 Gas filled detector. 56 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES Collection of only primary ions provides information on true radiation exposure (energy and intensity). However, the meters require sensitive electronics to amplify the signal, which makes them fairly expensive and delicate. The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation ener- gies. This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator. An ion chamber survey meter is sometimes used in the field when performing gamma radi- ography because it will provide accurate exposure measurements regardless of the radioactive isotope being used. 2.3.5.2 Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode. Due to the strong electrical field, the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas. The electrons produced in these secondary ion pairs, along with the primary electrons, continue to gain energy as they move towards the anode, and as they do, they produce more and more ionizations. The result is that each electron from a primary ion pair produces a cascade of ion pairs. This effect is known as gas multiplication or amplification. In this voltage regime, the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle. Hence, these gas ionization detectors are called proportional counters. Like ion chamber detectors, proportional detectors discriminate between types of radiation. However, they require very stable elec- tronics which are expensive and fragile. Proportional detectors are usually only used in a laboratory setting. 2.3.5.3 Geiger-Müller (GM) Counter Geiger-Müller counters operate under even higher voltages between the anode and the cathode, usually in the 800 to 1200 volt range. Like the proportional counter, the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas. However, this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions. This all happens in a frac- tion of a second and results in an electrical current pulse of constant voltage. The collection of the large number of secondary ions in the METERS AND MONI TORS 57 GM region is known as an avalanche and produces a large voltage pulse. In other words, the size of the current pulse is independent of the size of the ionization event that produced it. The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute. If the instrument has a speaker, the pulses can also produce an audible click. When the volume of gas in the cham- ber is completely ionized, ion collection stops until the electrical pulse discharges. Again, this only takes a fraction of a second, but this process slightly limits the rate at which individual events can be detected.Because they can display individual ionizing events, GM counters are generally more sensitive to low levels of radiation than ion chamber instruments. By means of calibration, the count rate can be displayed as the exposure rate over a specified energy range. When used for gamma radiography, GM meters are typi- cally calibrated for the energy of the gamma radiation being used. Most often, gamma radiation from Cs-137 at 0.662 MeV provides the calibration. Only small errors occur when the radiographer uses Ir-192 (average energy about 0.34 MeV) or Co-60 (average energy about 1.25 MeV). Since the Geiger-Müller counter produces many more electrons than an ion chamber counter or a proportional counter, it does not require the same level of electronic sophistication as other survey meters. This results in a meter that is relatively low cost and rug- ged. The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge). 2.3.5.4 Comparison of Gas Filled Detectors In the ion chamber region, the voltage between the anode and cathode is relatively low and only primary ions are collected. In the proportional region, the voltage is higher, and primary ions and a number of secondary ions (proportional to the primary ions origi- nally formed) are collected. In the GM region, a maximum number of secondary ions are collected when the gas around the anode is com- pletely ionized. Note that discrimination between kinds of radiation (El and E2) is possible in the ion chamber and proportional regions. Radiation at different energy levels forms different numbers of pri- mary ions in the detector. However in the GM region, the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated the event. The GM 58 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse. This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters. 2.3.6 Pocket Dosimeters Pocket dosimeters are used to provide the wearer with an immedi- ate reading of his or her exposure to x-rays and gamma rays. As the name implies, they are commonly worn in the pocket. The two types commonly used are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter. 23.6.1 Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen (Figure 2.4). The dosimeter contains a small ionization chamber with a volume of approximately two mil- liliters. Inside the ionization chamber is a central wire anode, and attached to this wire anode is a metal coated quartz fiber. When the anode is charged to a positive potential, the charge is distributed between the wire anode and quartz fiber. Electrostatic repulsion deflects the quartz fiber, and the greater the charge, the greater the deflection of the quartz fiber. Radiation incident on the chamber Figure 2.4 Direct read pocket dosimeter. METERS AND MONITORS 59 produces ionization inside the active volume of the chamber. The electrons produced by ionization are attracted to, and collected by, the positively charged central anode. This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position. The amount of movement is directly proportional to the amount of ionization which occurs. By pointing the instrument at a light source, the position of the fiber may be observed through a system of built-in lenses. The fiber is viewed on a translucent scale which is graduated in units of exposure. Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts. During the shift, the dosimeter reading should be checked frequently. The measured exposure should be recorded at the end of each shift. The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure. It also has the advantage of being reusable. The limited range, inability to provide a permanent record, and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter. The dosim- eters must be recharged and recorded at the start of each work- ing shift. Charge leakage, or drift, can also affect the reading of a dosimeter. Leakage should be no greater than 2 percent of full scale in a 24 hour period. 2.3.62 Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter. These dosimeters record dose information and dose rate. These dosimeters most often use Geiger-Müller counters. The output of the radiation detector is collected and, when a predetermined expo- sure has been reached, the collected charge is discharged to trigger an electronic counter. The counter then displays the accumulated exposure and dose rate in digital form. Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure. Some models can also be set to provide a con- tinuous audible signal when a preset exposure has been reached. This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instru- ment to achieve a maximum readout before resetting is necessary. 60 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES 2.3.7 Audible Alarm Rate Meters and Digital Electronic Dosimeters Audible alarms are devices that emit a short "beep" or "chirp" when a predetermined exposure has been received. It is required that these electronic devices be worn by an individual working with gamma emitters. These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount. Typical alarm rate meters will begin sounding in areas of 450-500 mR/h. It is impor- tant to note that audible alarms are not intended to be and should not be used as replacements for survey meters. Most audible alarms use a Geiger-Müller detector. The output of the detector is collected, and when a predetermined exposure has been reached, this collected charge is discharged through a speaker. Hence, an audible "chirp" is emitted. Consequently, the frequency or chirp rate of the alarm is proportional to the radiation intensity. The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen. 2.3.8 Film Badges Personnel dosimetry film badges (Figure 2.5) are commonly used to measure and record radiation exposure due to gamma rays, X-rays and beta particles. The detector is, as the name implies, a piece of radiation sensitive film. The film is packaged in a light proof, vapor proof envelope preventing light, moisture or chemical vapors from affecting the film. A special film is used which is coated with two different emulsions. One side is coated with a large grain, fast emulsion that is sensitive to low levels of exposure. The other side of the film is coated with a fine grain, slow emulsion that is less sensitive to exposure. If the radia- tion exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted, the fast emulsion is removed and the dose is computed using the slow emulsion. The film is contained inside a film holder or badge. The badge incorporates a series of filters to determine the quality of the radia- tion. Radiation of a given energy is attenuated to a different extent by various types of absorbers. Therefore, the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter. By comparing these results, the energy of the radia- tion can be determined and the dose can be calculated knowing the METERS AND MONI TORS 61 Figure 2.5 Personnel dosimerty film badge. film response for that energy. The badge holder also contains an open window to determine radiation exposure due to beta particles. Beta particles are effectively shielded by a thin amount of material. The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record, it is able to distin- guish between different energies of photons, and it can measure doses due to different types of radiation. It is quite accurate for exposures greater than 100 millirem. The major disadvantages are that it must be developed and read by a processor (which is time consuming), prolonged heat exposure can affect the film, and expo- sures of less than 20 millirem of gamma radiation cannot be accu- rately measured. Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives. Whole body badges are worn on the body between the neck and the waist, often on the belt or a shirt pocket. The clip-on badge is worn most often when performing X-ray or gamma radiography. The film badge may also be worn when working around a low curie source. Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation. A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body. 62 HANDBOOK OF INTERNATIONAL ELECTRICAL SAFETY PRACTICES 2.3.9 Thermoluminescent Dosimeters Thermoluminescent dosimeters (TLD) are often used instead of the film badge. Like a film badge, it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received, if any. Thermoluminescent dosimeters can measure doses as low as 1 millirem, but under routine conditions their low-dose capability is approximately the same as for film badges. TLDs have a precision of approximately 15% for low doses. This precision improves to approximately 3% for high doses. The advantages of a TLD over other personnel monitors are its linearity of response to dose, its relative energy independence, and its sensitivity to low doses. It is also reusable, which is an advantage over film badges. However, no permanent record or re-readability is provided and an immediate, on the job readout is not possible. A TLD is a phosphor, such as lithium fluoride (LiF) or calcium fluoride (CaF), in a solid crystal structure. When a TLD is exposed to ionizing radiation at ambient temperatures, the radiation inter- acts with the phosphor crystal and deposits all or part of the inci- dent energy in that material. Some of the atoms in the material that absorb that energy become ionized, producing free electrons and areas lacking one or more electrons, called holes. Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place. Heating the crystal causes the crystal lattice to vibrate, releasing the trapped electrons in the process. Released electrons return to the original ground state, releasing the captured energy from ionization as light, hence the name thermoluminescent. Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor. Instead of reading the optical density (blackness) of a film, as is done with film badges, the amount of light released versus the heat- ing of the individual pieces of thermoluminescent material is mea- sured. The "glow curve" produced by this process is then related to the radiation exposure. The process can be repeated many times. 2.3.10 Annex - Guide to Meter Selection and Applications Table 2.4 and Table 2.5 will help with the selection of appropriate meters through the comparison of their attributes and by their applications. T a b l e 2 . 4 R a d i a t i o n d e t e c t i o n d e v i c e s e l e c t i o n c h a r t . A t t r i b u t e R a d i a t i o n D e t e c t e d S e n s i t i v e M e d i u m R a n g e s O u t p u t S i g n a l ( V ) R e s o l v i n g T i m e ( s ) E n e r g y R e s o l u t i o n ( % ) G e i g e r - M u e l l e r ( G M ) T u b e a l p h a b e t a x r a y g a m m a G a s 0 . 0 4 m R / h r t o 5 0 0 m R / h r 1 I O 4 N / A I o n C h a m b e r a l p h a b e t a x r a y g a m m a G a s 3 m R / h r t o 1 0 , 0 0 0 R / h r I t ) * I O 4 N / A S c i n t i l l a t i o n a l p h a b e t a x r a y g a m m a n e u t r o n S o l i d L i q u i d 0 . 0 0 5 m R / h r t o 2 0 0 m R / h r o r t o 8 0 0 , 0 0 0 c / m 1 i o - 7 1 0 P r o p o r t i o n a l C o u n t e r a l p h a b e t a x r a y g a m m a n e u t r o n G a s t o 5 0 0 , 0 0 0 c / m i o - 2 1 0 * 1 5 S e m i c o n d u c t o r D e t e c t o r a l p h a b e t a x r a y g a m m a S o l i d t o 1 0 0 , 0 0 0 c / m I O " 3 i o - 9 1 » T a b l e 2 . 4 ( c o n t . ) R a d i a t i o n d e t e c t i o n d e v i c e s e l e c t i o n c h a r t .