Q Botch Mode 0 Continuous Mode 10-6 2 3 4 5 7 10-1 WEIGHED PARTICLE CONCENTRATION IN SAMPLE, gm/ml Figure 6. Check of particle mass calculated from population counts and directly weighedmates of minimum secondary coincidence error. Corrected counts were obtained simply by subtracting the intercepts from the corresponding observed number at any total particle concentration. The mass of particles calculated from corrected counts was plotted in Figure 5. The corrected points show a better agreement with the lower solid line (weighed) than the uncorrected points. It is clear from Figure 5 that most of the error was due to the secondary effect. In plotting the corrected mass, the 3% or so mass that remained below the threshold of channel 13 was ignored.Significant error in material balance can result from errors such as the existence of spurious counts, as evidenced in Figure 5. A material balance check on the population counts therefore is important. The mass obtained from counting (Equation 1) is plotted vs. the expected mass (from weighing) in Figure 6 for various narrow-size particles using both the batch and continuous modes of counting. Considering the smallness of the numbers, the agreement is satisfactory. No consistent bias was observed in the results of the mass balance experiments.
CONCLUSIONSIt has been demonstrated in the present work that the Model T Coulter Counters are accurate in the population mode. Spurious counts, however, existed for particles at the low-size end of the distribution below the threshold size of the measurement spectrum. Secondary coincidence error is in-dicated to be a problem in continuous crystal size distribution analysis. Such analyses are always based on a finite segment of size, but the distribution being measured is continuous and contains large numbers of particles below the lowest size scanned by the Coulter Counter. Some account of the excessive spurious counts that are expected in the lowest size channels has to be made to avoid erroneous conclusions. NOMENCLATURE K , = volumetric shape factor = mid-channel size, pm m, = calculated mass, g m, = weighedmass, g N = total true number of particles n = total observed number of particles Ani = average number in channel i t = time, min V = volume of stock suspension, ml Y = volume of suspension used for counting, ml a = sensing volume of an aperture, ml p = density of particles, g/cm3 r = average retention time, minutes LITERATURE CITED