Document BRpykv674dQDrgo40aBVQZJ1L

2J6 casting room, revealed that there were 5.6 fibers 'to! present for fibers 5 pm or greater in length in the first segment of the filter while. 4.9 fibers 'ml were found in the sec ond and 5.8 fibers/mf were counted in the third segment. Sample D. taken in the mix ing room, shows an equally good distribution between the three segmen ts of the. filter. In the first filter segment 10.5 fibers/m! were counted for fibers 5 pm or longer in length, while 11.2 fibers m! were found in the second, and 9.8 fibers m/ were found in the third segment. Comparing the results of the analysis of. Sample C with Sample A, the superiority of the long plenum collection method is appar ent. The difference between the high and the low fiber count of Sample C is only 0.9 fiber, while Sample A shows a three times higher discrepancy or 2.7 fibers. The rela tionship between Sample D and B is even worse. Sample D shows only a 1.4 fiber count difference between the individual filter segments, while a 5.7 fiber count difference exists between the three filter segments of Sample B. The fiber count discrepancy of Sample B is slightly more than four times that of Sample D. The previously described modified sample collector with the 0.150-inch size air inlet' was found acceptable for all other flow rates as specified by the NIOSH standards. Since . the recommended flow rate of air is between 1 and 2.5 liters per minutes and the calcula tions for the lengthened plenum were made for 2 liters per minute of air flow, no differ ence is expected at either end of the flow rate range. It should be pointed out that the uniform distribution of heavier particles as seen here (i.e, > 5 pm) insures the uniform distribution of smaller particles (i.e. < 5 pm): Sample Preparation for Identification Since the index of refraction (ND) of most asbestos fibers is near to the refractive index of the chemically cleared membrane filter,: it becomes necessary to eliminate the pres June, 197J ence of the filter. This can best be accom plished using a low terhperature asher. This method uses a very slpw oxidizing process, that will neither harm; nor disturb even the finest particles of asbestos or other non- . oxidizable material. Tjhis instrument allows the oxidation reaction to occur at a low temperature by converting approximately 20% of the molecular. oxygen to atomic oxygen5 with the use of a radio frequency field. The atomic oxygen reacts with the oxidizable materials id the specimen chamr ber. A mechanical vacuum pump then re moves the voltatile oxidation products. This method also eliminate^ many other bxidiz- able airborn partidesj commonly found in industrial operations. ; Approximately four hours are required to lash" the filter. Sam- ., pies C and D were prepared by this ! tech nique. The pressure ojf the specinien cham-; < ber was maintained at ja pressure of 0.6 mm \ of mercury and 300 watts of power was used. ' The flow rate of thejoxygen was 150 pc/ : minute. Identification and Measurements of Asbestos Fibers An extremely convenient and easy method of measuring particles;is with an image an alyzing microscope (IAM). With the aid of an IAM it is not difficult to measure 50 to 100 fields in a relatively short time. The particles can be bracketed in various size groups, and the measuring of a previously mapped out area can be accomplished. Data collected from 50 fields requires approxi mately 15 minutes. In order to deterniine the number and size of asbestos particles present on a filter, using the IAM, a multi-process procedure is used. The specimen! is first ashed (as has been outlined) to. eliihinate the filter and any oxidizable particles such as lint. The area (or areas) of interest arc mapped out, measured, and all the! particles present are counted. This will give the total number of particles (asbestos and non-asbeStos). A sec ond measurement of tW exact same area is 8000 0335 HFM - 002039