INDUSTRIAL HYGIENE MICROSCOPY 



across the eyepiece scale to the right from 

 this point to another point where a line of 

 the eyepiece scale coincides with a line on 

 the stage micrometer scale. , 



If a large number of dust samples arc to be 

 examined, use of a micro-projector (Fig. 10) 

 has proved to be the preferred method. In 

 this case, the dust sample is projected onto 

 a screen. Particle size distribution is deter- 

 mined by comparing the size of the projected 

 images with a ruled area on the screen of 

 known size. Another procedure is to measure 

 the projected particles with a millimeter rule. 

 Knowing the magnification, the value in 

 microns for each division of the rule can be 

 determined. For example, if the choice of 

 optics and projected distance is such as to 

 give a magnification of 1000 X, a particle 

 having a diameter of 1 mm as measured with 

 the rule has an actual size of 1 micron. 



In preparation of a particle size distribu- 

 tion curve, the total number of grains meas- 

 ured may vary from 200 to 2000 dependent 

 on whether the dust sample contains a few 

 sizes or many sizes. The number of each 

 micron size counted is multiplied by the 

 cube of the diameter to obtain the relative 

 amount of that size present in the sample 

 and the percentage of the total amount is 

 calculated for each micron size. To obtain 

 an accumulative curve, the percentage of 

 the total sample larger than each size is 

 plotted against the diameter in microns. 



Considerable variation in the sizes ob- 

 tained can result dependent on the method 

 of collection. For example, disintegration of 

 particles by high-velocity impingement may 

 give an excess of fine particles. The use of 

 liquid collecting medium such as water may 

 result in solution of many particles. As pre- 

 viously indicated, due to proximity in index 

 of the collecting liquid and the dust, small 

 particle size material as examined by the 

 usual light-field microscope will be difficult 

 or impossible to see. As a result, these par- 

 ticles will not be included in the preparation 

 of a particle size distribution curve. 



Fig. 9. Filar micrometer 



^ 



Fig. 10. Micro-projector. 



Drinker and Hatch (6) suggest the use of 

 the Molecular Filter and Thermal Precipi- 

 tator for collection of dust for particle size 

 determination. Collecting efficiency is high 

 over the entire range of interest and the 

 particles are deposited without being subject 

 to physical stress. In the case of molecular 

 filter samples, as previously described, a 

 liquid of approximately 1.507 refractive in- 

 dex must 1)0 used for rendering the filter 

 transparent. Approximate information as to 

 size can be made by comparison with the 

 rulings of the hemacytometer and more 



409 



