Globe and circle graticules

The earliest of these graticules by Patterson and Cawood has 10 globes and circles ranging in diameter from 0.6 to 2.5 pm when used with a +2 mm lOOx objective-eyepiece combination and is suitable for thermal precipitator work [69]. 

Fairs [70] designed graticules covering a size range of 128 1 using reference circles with a root two progression in diameter except for the smaller sizes. He considered this system to be superior to the Patterson- 

Cawood where the series is much closer. He also described a graticule, having nine circles in a V2 progression of sizes, for use with the projection microscope [71]. This was incorporated in a projection screen instead of being in the eyepiece and was adopted by the British Standards Organization [11]. 

Watson [72] developed a graticule designed specifically to measure particles in the 0.5 to 5 pm (respirable dust) size range. 

May s graticule, [73] covers 0.25 to 32 pm in a root two progression of sizes (the lower limit is highly suspect). 

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The easiest diameter to measure is the Feret diameter but this is significantly larger than the other two diameters for most powders. It is probably best to reserve this diameter for comparison purposes and for rounded particles. Of the other two diameters, the projected area diameter is preferred since two dimensions are included in one measurement and the projected area is easier to estimate using globe and circle graticules than the length of the chord that bisects the image. 

Table 3.1 Illustrative example of the calculation of a size distribution by number using the British Standard globe and circle graticule...

Figure 14 Diagram of the modified Patterson-Cawood globe and circle graticule used for counting and sizing particles. (From Ref. 7, with permission of the Journal...

The globe-and-circle graticules are used to compare the projected particle area with a series of circles in the ocular graticule. The size of spherical particles can be expressed by the diameter, but for irregularly shaped particles various statistical... 

Replace the calibrated hairline by the globe and circle graticule provided and with the stage micrometer, calibrate either the globe or the circles. Using the calibrated graticule measure the projected area diameter of 10 particles in, as many fields of view as possible, but not less than 10. 

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