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For powders of the same composition and of similar size distributions, Eq 1 is probably satisfactory. However, if particles can be packed differently and exhibit the same porosity, but with different pore structures, this relationship may not hold. For instance, two powders of the same chemical composition with different particle size distributions have been given the same Fisher subsieve numbers. Comparison of results from the Fisher subsieve sizer with those from other surface area and size distribution techniques may show no agreement and should not be undertaken.

When particle shape, size, and distribution are essentially the same and do not vary significantly from one production lot to another, Fisher subsieve analysis provides adequate comparisons for quality control purposes. The major limitations include constant sample packing and instrument calibration.

Operation. ASTM B 300 standard, "Average Particle Size of Powders of Refractory Metals and Compounds by the Fisher Subsieve Sizer," provides calibration and operating instructions for Fisher subsieve analysis. Standard ASTM C 721, "Average Particle Size of Alumina and Silica Powders by Air Permeability," also provides guidelines. These specifications are based on the Fisher subsieve sizer instruction manual (Ref 4).

The output of an air pump is regulated to a pressure head of 50 cm (19.7 in.) of water by the water level in the standpipe. At this constant pressure, the flow through the packed powder bed is measured by the water manometer. Calibration is accomplished by varying the leakage rate through the calibrating valves.

The powder sample is weighed to 0.1 g (0.003 oz) and placed in the sample tube. Using a rack and pinion, it is compacted with a 222 N (50 lb) force applied with a torque wrench. This technique minimizes sample-to-sample packing variation. Porosity is determined from the compacted sample height and the instrument calculator chart.

The sample tube is then placed in the air system, and after equilibrium is reached, the manometer water height is measured. The instrument calculator chart is used to obtain the average size. If the sample height is such (high or low) that the calculator chart cannot be used, the actual height (in centimeters) above the base line is measured for both sample height and manometer water height. These values are used in the following equation to provide calculated values. The calculator chart uses a modification of Eq 1:

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