Porosity [LA MPLA

and where L is sample height after compaction, cm; A is cross-sectional area of sample tube, cm3 (1.267); Mis weight of sample, g; Pis true, or absolute, density of powder material, g/cirf. H is height of water column above the base line, cm; P is overall air pressure, cm (50 cm, or 19.7 in., of water); and C is instrument constant (3.80).

If the sample weight is made equal to the true density, the equation is simplified:

Standardization. The Fisher subsieve sizer must be standardized daily (or more frequently, if necessary) according to the procedures given in the instrument manual (Ref 4). Monitoring of the sample post height, which sets the chart base line as the instrument zero, is frequently overlooked. This can be verified easily by placing two filter paper sheets between the two sample tube porous plugs located on top of the sample post. The rack is brought down in contact with the upper porous plug, and the pointer is checked to see that it is on the base line. If it is not, post height is adjusted until the measurements coincide. This is an infrequent but critical adjustment.

A method of standardization uses a Fisher ruby orifice mounted in a sample tube, which is calibrated by the manufacturer against the primary standard, the National Bureau of Standards Cement No. 114. Operating the Fisher subsieve sizer with the calibrating tube in place and the chart positioned to the proper porosity produces a water column of a predetermined height. This indicates a size on the chart that is equal to that engraved on the side of the tube. If it does not, the internal calibration valves are adjusted until the correct value is obtained. Other secondary standards may be used and include any uniform powder that has an accepted Fisher subsieve number.

Results. Because analysis results are obtained by interpolation of the pointer position between lines on the calculator chart, they should be reported with varying intervals and places after the decimal point, as indicated in ASTM C 721, Section 9.1. Reading and chart line intervals for given particle ranges are shown in the following:

Particle

Chart line

Reading

range,.-'m

interval,

interval,

Cm

Cm

0.2-4

0.1

±0.2(a)

4-8

0.2

±0.1

8-15

0.5

±0.1(b)

15-20

1.0

±0.2

20-25

5.0

±1.0

(a) For porosities less than 0.55, the reading interval for the 0.5-4 C-m range is+0.5 /Jm.

(b) For porosities greater than 0.6, the reading interval for the 11-15 C m range is ±0.2 /'m.

(a) For porosities less than 0.55, the reading interval for the 0.5-4 C-m range is+0.5 /Jm.

(b) For porosities greater than 0.6, the reading interval for the 11-15 C m range is ±0.2 /'m.

In one laboratory, a five-year weekly evaluation of the ASTM B 459 tungsten powder gave an average of 1.23 /Jm and a standard deviation of 0.012 /(m. Fisher subsieve numbers are generally agreed to be precise within ±5%. Because of the care taken in making the measurements, many laboratories report ±3% precision.

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