Document MoRDmZ1Nbx2kk98V40p57X7O9

1 eg; 178 FRANK A. PATTY Ihexoneh Intermittent Exposure *"""X Constant Exposure O Intermittent Exposure to Commercial Moterlol IButonej aoi .02 03 04 06 .1 .2 .3 4 SB 3 4 6 8 10 VAPOR IN AIR, LOGARITHMIC SCALE -------------------------------------------------urer:centlby..volume------------ Figure 3. Relation* ^between concentration of vapors, of purified paraffin ..hydrocarb'offsAand odor intensity. B. PORTABLE RAPIDLY INDICATING DEVICES Nearly all physical sampling methods are open to the same criticism: tHcyqujfiSj not specific, and it is sometimes impossible to tell whether the physical chlfii'' ' measured* especially small ones, are due to the presence of a particular gas or'vftp or to some other cause; and in mixtures it is not always feasible to determine! percentage of each component. Nevertheless, the advantages of portability*^^ SAMPLING ANn ANALYSIS OF CONTAMINANTS 179 Availability of results make some physical instruments immensely if 1. Interferometer j. w^.-pcriraps the most dependable and most widely applicable physical instrument ^^d'cveloped for field use is the 50-cm. portable gas interferometer. This instru- measures minute changes in refractivity, so that it may success" iiily^h*t5Vise4lto determine low concentrations of contaminating gases or vapors aAnaSt^^bere, providing the refractivity of the contaminant is sufficiently ISflKjmjr^ffiii^that of air, which is 291.7 X 10"*' at 0 C. and 760 mm. Hg refractivity is a function of density, as well as of molecular strucKroSSil^jig^^toKe8 in pressure and temperature have an important bearing on w^SI&usli be carefully controlled. The light source must be of constant wave |J|j|ptpRisualIy an incandescent filament. The interferometer is a sensitive Ip^fetometer which, instead of measuring refraction directly, compares J$||ity of one gas to that of another. In air analysis, the usual standard of ^Ss* dry air, from which all carbon dioxide has been removed. This atmosphere must be at the same temperature and pressure as the atmosi^^Riamined. An interferometer may be calibrated against known mix- __ _ Wf in air, of the same order of concentration as atmospheric mixtures ^mwliicMt^ito be employed. Also, it many be calibrated for absolute refractivity $$$!!&$tlmt':is, refractivity factor, by increasing the pressure of dry air in one Ld- tb similar dry air at constant atmospheric pressure in the other.9 Once calibration has been accomplished, instrument scale readings may iftcdfto vapor percentages by employing any reliable data on the refrac- and gases. These data are perhaps most readily utilized when ^^'g'c^^Ksiunit refractivity change (U&R), that is, the change in the refractivity 25 C. and 760 mm. Hg pressure due to the admixture of 1 per cent 5 ;V fefhe data given in Table 3 were obtainedviby vaporizing weighed vents in a galvanized metal chamber having`;a volume of'604 cu. ft. jS^glilieJconcIntration of a known vapor may be determined as follows: '.. FjkV- i,ormair== ' 'Ife-...interferometer reading X refractivity factor (of instrument) fK ' unit refractivity change (of vapor) '^^pft;';in measuring benzene vapor, suppose the interferometer scale reading '^SS^divisions or 0.62 revolution of the indicator dial abo'ye its reading in ^^Hfidjeauh-'-revolution-is- equivalent-to a change in--refractmty*of~2-.91 -X ejner.eentage of benzene would equal: 4*. ""..I-, if. 0.62 X 2.91 X 10-9 = 0.125 (1250 p.p.m.) 14.4 X 10- .`-Edwards, V. S. Bur. Mines Tech. Paper No. 181, 1919; F. A. Patty, J. Ind. Hyg. (1939).