Document yped4roNoRk0xd9qZDKwdb9KX

192 frank, a. patty 2. The Midget Impinger The midget impinger is similar to the standard instrument except that ifejf usually hand-operated and draws a sample volume of 2.8 liters per minute throuf an orifice 1 mm. in diameter. Each impinger nozzle must be calibrated, as devikl tions in nozzle size of over 50 per cent are found. This instrument has been shoWni to be as satisfactory as the standard apparatus for sampling most dusts in t}| range of 0.7 to 10 p, but it is not suited to the sampling of particles below 0.5|jt such as fumes. Its suitability for sampling asbestos fibers has been questioned^" can be operated by a hand crank and is therefore independent of a power sup"' and is suitable for use in flammable vapors or dusts. The entire impinger, comp! with 9 flasks, is contained in a relatively light and easily portable case. S. The Konimeter The konimeter is a dust-sampling instrument introduced by R. M. Kotze&L 1916, and later modified. In its present form (Zeiss Konimeter) it consr essentially of a small valveless spring-operated piston pump and a circular p| for impingement of the dust. When the piston is depressed it is held against, compression of the operating spring by a catch. Releasing the catch allows. piston to move rapidly to a stop and creates at the discretion of the operate). 2.5-ml. or a 5-ml. void, pulling atmospheric air through a round 0.5- to O.fijh orifice and impinging it against a circular glass plate held 0.5 to 0.6 mm. dipt* from and perpendicular to the impinging orifice. The circular plate, held4jj% metal rim, is covered with an adhesive film to retain the dust particles, and ha numbered, equally spaced sample positions. .> For use, the glass disk is cleaned, coated with an adhesive (glycerine jelly, recommended by the United States Bureau of Mines89), examined under a m| scope, and, if satisfactory, inserted in, the konimeter or a spare carrying vC; Usually several samples are taken in the same work area in order to estiiy average conditions. The instrument is not satisfactory for high dust concqfit] tions: even a 2.5-ml. sample of visibly dusty air is likely to be too much tq/Cjp For very low dust concentrations more than one 5-ml. sample may be taken-S1. same position of the disk. In the technique used by the United States Bureau Mines, the samplesnarejestima^e^byt-fiounting sectors.of,the. spots unfien,a,|r^_ scope.with 200^agnification^u4ng:!fldghtjfield;illumination, THe,iCOunts>(gga^ directly iCompareditbfmaBmge^cpjmts^but are .useful in estimating.-the effict|?' ness of dust-control methods,.andifor comparing dustiness in a situation whe$g, type of dust remains more or less constant but its concentration varies,);,TM numeroUff" conditions:'The-dust'" is"c6fihted' without prior treatment.'"E3sa, speed of sampling and counting are the chief advantages of the methodv);^ niques used in countries other than the United States vary from that de$&w[ but will not be elaborated upon here. ?i|ii " J. B. Littlefield, C. E. Brown, and H. H. Schrenk. U. S. Bur. Mines Circ. No. 6993,-y *> SAMPLING AND ANALYSIS OF CONTAMINANTS 193 4- Owen's Jet mpir ... `'`"njjs jet sampling device draws an air sample of 50 ml. or more through Mpistenin^chamber and then at high velocity through a narrow slot, usually 1 cm- wide. The lowered temperature resulting from greatly reduced passing through the slot causes condensation of moisture on any ^^^pM^rcj^j,..ahd the droplets are impinged against a cover glass where they ^S^^in'd*p|er dry, leaving the dust on the cover glass. The ribbon of dust thus Sm^^uniform.and is therefore difficult to count. It has all the disad- ^^^^j|ffeAonimeter and is, further, susceptible to different efficiencies de- rate of pull on the pump plunger. The instrument has a value for B** does the konimeter. The Bausch and Lomb dust counter is a .........- SpKhis device. JH ||L,B. ELECTROSTATIC PRECIPITATION JjM||jptrostatic precipitator has a high efficiency for collecting all types of This device utilizes the principle of the Cottrell precipitator drawn through a tube within which is a central conductor wire her electrode being formed by the tube or its conductor lining or toSggrt|i|he purpose of air sampling the tube diameter is usually between l1/* :?ufpififand the voltage used ranges from 8000 to 30,000. The relationship of ||l||n||gr, air flow, and voltage must be within certain limits in order to J^p^he'i'inaximum efficiency of 100 per cent^A_cgmmercially available port- ^^fflilar to one described by Barnes and Penney40 operates on a 110-volt jEffijiSfit producing under favorable circumstances a secondary voltage of ffljplt is important that this voltage be checked, and maintained at not .w-JllMf?0' precipitator is equipped with a fan and a flowmeter cali- approximately 3 cu. ft. of air per minute.. This volume, likewise, ^K^ked by a suitable meter. The dust content;;of the air is deposited ||||ftle'of the precipitator tube or, in some instfees, on the1 inner elec- ||||||ej}bsit should be carefully preserved and transported to a laboratory |hffi|pcbunting, or chemical analysis. This device is especially useful for ill!......... ,Mr' smokes, or other suspended matter with1 a particle''size'below in which range the impinger apparatus- is not-'sati's'factorilv for use in flammable atmospheres. `/i^'j'Tymparisoh of efficiency between the standard impinger and the electro^wSgciimator.for sampling fluoride fumes in a magnesium found^yvthe follow- ^^gbf^nterest. With two impingers in series, from 62 to 87 -per cent of OTnt"c611ected'^aF'found_in 'the'first''iftpiiipFahd~from~f3Tb'38 per fed. On the average there was 74 per cent in the first and 26, .per cent ^^feSamples collected simultaneously with a commercial -electric pre- ]fftOpO volts by spark-gap calibration) at the same location yielded IjPI^'per cent of the amount of fluorides found in the two impingers, the