I/EC
Safety
Portable Instruments Analyze Workroom Atmospheres Quantitative evaluation of impurities has provided the means for intelligent appraisal and effective control of occupational diseases by Joseph Zatek, Mine Safety Appliances Co.
Collection and identification of environmental contaminants associated with the use of organic and inorganic chemicals and metals are of the greatest importance today. There is a real need for measurement sampling for in-plant hazards because of the desirability of providing a safe working atmosphere which will not cause health impairment. An essential feature of this type of sampling program is to identify quantitatively air constituents in which a man works and to relate this together with other relevant factors to his medical history. Portable air-sampling devices have provided the tool for evaluating workroom atmospheres rapidly, should specific materials exceed hygienic standards. Another important and common application of these sampling instruments is determining the performance of various types of control equipment. Quantitative measurement provides knowledge of an efficient and economical control svstem.
In view of the growing awareness of industrial contaminants as a health hazard, the question of methods for accurately measuring lowconcentrations of potentially harmful gases and vapors arises. A number of commercially available instruments have been developed for specific analysis of industrial atmospheres. Instruments used for collecting or measuring these concentrations can be classified by their function. There are those which will collect a sample for subsequent microscopic, physical, or chemical analysis and those which will permit immediate indication of the amount of the specific contaminant for which they were designed. Subsequent A n a l y s i s
Air sampling devices employing impingement techniques arc commonly vised for detecting aerosol and dust concentrations. Impingement instruments with hand-operated or motor-driven pump draw
The Electrostatic Sampler is designed for use in the ceramic, electrical, steel, and chemical process industries, and those industries e n g a g e d in the production or use of radioactive materials
air at a relatively high velocity through a glass nozzle, and direct it so that it strikes the bottom of a glass flask partially filled with an absorbing liquid. This high velocity action causes the dust particles to be wetted and retained in the liquid. The sample is then examined and a count of the dust particles taken. These instruments equipped with a fritted tube and flask can be used to collect samples of a variety of gases and vapors, including acid gases or mists, sulfur dioxide, arsinc, methyl alcohol, and others. An Electrostatic Sampler can be used to sample dusts, fumes, and smokes. This instrument consists of a portable sampling head, high voltage power supply, telescoping support for the portable head, removable sampling kit containing four sample tubes with plastic tops, and two ionizing electrodes. The sampler operates on 117 volts, 60 cycle, alternating current. The power pack provides an adjustable high voltage potential difference of 12 to 15 kv. between the ionizing electrode and the collecting tube. Samples are collected at the rate of 3 cubic feet per minute on the tube at a collection efficicnev of nearly 100%. A number of automatic or multiple samplers arc available for collecting a scries of samples over an extended period. These units are equipped with small impingers or filter media, and permit quantitative sampling of conditions during the entire work period. Particulate sampling instruments have been developed based on impaction and thermal precipitation principles. The Casella Cascade Impactor collects particles on a scries of slides such that each slide collects material of progressively smaller median size. The instrument can be used to characterize size distribution of similar suspensions by measuring the VOL. 52, N O . 7
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SAFETY median size collected on each stage for a given suspension. The Thermal Precipitator is used to collect samples of dust or fume for determination of concentration present, or for particle-size determination. Its collection efficiency is essentially 100% for particles less than 5 microns in diameter. Operation involves passing air through a chamber which contains an electrically heated resistance wire and a cool surface. Air which contains dust to be measured is drawn through the chamber. Dust particles, unable to penetrate the dust-free space, are deposited on collecting slides which can be removed. Particles are then sized by standard microscopic methods. Modified vacuum cleaning devices have been developed for drawing air through a filter media where it is desirable to obtain a sufficiently large sample for the determination of free silica or other constituents of a dust or fume. Filter papers used for these high volume samplers include cellulose filters, glass-micron filters, and cellulose ester fibers. Filter paper collectors of the intermittent and continuous movement type can be adapted to the collection of many air contaminants. Direct Analysis
An important trend in recent years has been the development of accurate, direct reading instruments. The importance of this type of air sampling device cannot be overemphasized. They provide an immediate quantitative appraisal of a plant's atmosphere. Little or no training is required for their use by inexperienced personnel. The ideal detector is one that uses a small air volume, and indicates the amount of contaminant directly with the distinct coloration of the sample. The most desirable method is by means of detector tubes in which a bed of sensitive chemical, confined between two porous plugs, responds to a specific material by length of stain or color change proportional to the atmospheric concentration. A typical example of such a detector is the Colorimetric Tube developed by the National Bureau of Standards for carbon monoxide. This tube indicates concentration of 74 A
carbon monoxide present by a series of color changes. Concentrations from 10 to 1000 p.p.m. can be detected by this method. An important point to remember when using this type of instrument is that the absorbent gels are flow sensitive. Length of stain obtained for a given gas can be varied by changing sampling volume, sampling rate, diameter of the tube, and per cent impregnation. These factors must be held to standard values for a particular tube to obtain uniform test conditions. A few of the contaminants for which stain length detectors have been developed are hydrogen sulfide, sulfur dioxide, hydrocyanic acid, aromatic hydrocarbons, chlorine, mercury, and nitrogen dioxide. Certain particulates and gases can be spot tested by sampling a known volume of air through a high efficiency filter. A squeeze bulb or 500-ml. hand pump may be used as the aspirating device. A color stain is developed on the exposed filter area by the addition of freshly prepared reagent. Amount of contaminant present is then determined by comparing with a standard color chart. Indicating reagents for use in the field are now available in frangible glass ampoules with an indefinite storage life. Contaminants which can be measured by this method are lead and its compounds, arsine, boranes, chromic acid, hydrofluoric acid, and unsymmetrical dimethylhydrazine. Other Devices
There are a number of miscellaneous instruments which base their analytical approach on the characteristics of gas-air mixtures. They use adsorption, electrical conductivity, heat of combustion, spectrum adsorption, and thermal conductivity principles to measure gases that may be of hygienic importance. Heat of combustion is used in two types. The Carbon Monoxide Indicator provides continuous measurement of this gas in the atmosphere. Sample gases pass into a cell containing the catalyst Hopcalite, where CO is oxidized to CO2· Heat of this reaction is proportional to the CO present, and is measured on a meter calibrated in % CO.
INDUSTRIAL AND ENGINEERING CHEMISTRY
The second type depends on the change in the electrical resistance of a heated catalytic filament. Combustibles in the sample are burned on the filament which raises its temperature and increases its resistance in proportion to the concentration of combustibles in the samples. Improvements in the design and sensitivity of Combustible Gas Indicators have made it possible to incorporate in a single unit a dual scale capable of measuring toxicity as well as the explosive hazard. These indicators are normally equipped with a dual-scale meter reading from 0 to 10% of the lower explosive limit and 0 to 100% of the lower explosive limit. A third type of instrument contains a galvanic cell consisting of a zinc and hollow carbon electrode in a special electrolyte. A gas mixture flows through the interior of the carbon electrode and diffuses through the carbon to the interface with the electrolyte. Oxygen combines with hydrogen which has been brought to the electrode as hydrogen ions by an electric current generated by the cell itself causing polarization. This action is counteracted by the oxygen which acts to depolarize the cell and change the terminal voltage and meter reading according to the amount of oxygen in the gas being tested. The instrument has a range scale of 0 to 2 5 % and for oxygen deficiency testing, between 12 and 2 1 % , an accuracy of ± 0 . 5 % can be obtained. A number of instruments are available to determine the extent to which noise may be a hazard. These analyzers ascertain the exact nature of the distribution of noise in eight octave bands and provide a relatively complete specification of the noise. Special equipment is also available to measure peak pressure in abrupt sounds and to study vibration in moving parts.
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