Multiple aperture method for powders having a wide size range

7 Multiple aperture method for powders having a wide size range (a) General 

If the size range of the powder is too wide to be covered by a single orifice, two or more aperture tubes can be used. As a general rule, if there is more than 2% of the distribution in the smallest size interval it is advisable to use a smaller orifice to determine the fine end of the distribution. 

Calibration is usually effected with the use of narrowly classified (monosize) spherical latex particles. Since the calibration particles are nearly monosize the pulses on the oscilloscope will be nearly uniform in 

If t is the average threshold setting as the pulse count changes by A then  

If the calibration constant as determined by equation (9.15) is significantly smaller Irom that using monosize particles it is likely that the whole range of powder has not been examined. In that case, equation (9.15) may be used to determine the fraction undersize by comparing the experimental value of with the expected value. 

---