C Envelope Density
For example, in one series of tests on a typical granular product where the product volume
relative to the bed volume was varied from 6.9 to 41.7%, there was almost a 9.0% variability in
envelope density. At the highest percentage, the sample quantity may have been sufficient for
bridging of sample pieces to interfere with medium consolidation. At the lower percentage,
small errors in consolidation were magnified in the difference value. However, the envelope
volume within a ±1.3% error band was registered when the sample volume ranged from 30 to
35%. The current program for the GeoPyc automatically calculates the sample volume
percentage. This percentage is a useful guide to optimum performance and should always be
considered when assessing the validity of results.
Guideline 2. Select sample chamber dimensions, Dry Flo volume, and specimen quantity to
yield a sample volume percentage of at least 20%.
3. The error band was reduced to ±0.95% when another series of tests was run with the material
used in the above guideline and both Dry Flo and sample weights were held constant to the
third decimal place. The reported sample-to-bed volume varied only between 32.1 and 33.4%
in this case. Such control is not practical or even feasible in many instances, but this technique
should be considered when possible.
Guideline 3. Maintain constant all parameters susceptible to control for optimum
reproducibility.
4. Both the blank and test steps of an envelope density determination consist of an equal number
of preparation and analysis cycles.
Preparation cycles are unrecorded, repetitious, agitation and consolidation attempts intended
to orient the Dry Flo grains and the specimen into a uniformly mixed bed. Analysis cycles
follow the preparation cycles and yield statistical information on consolidated volumes. The
bed is expected to become more and more consolidated during the preparation cycles, but
little or no consistent increase or decrease in value should be evident in the analysis cycles.
Diminishing information can be gleaned once the cycles exceed a certain number. The results
presented above were primarily obtained with 10 preparation and 5 analysis cycles. Some
specimens require more, but fewer are adequate in other cases; 10 preparation and 5 analysis
cycles are good starting numbers.
Guideline 4. Choose the number of preparation and analysis cycles such that little or no
consistent increase or decrease in value is revealed by the recorded data.
ACCURACY
Follow the guidelines for reproducibility (see Reproducibility on the previous page). Those
guidelines must be followed, in conjunction with the guidelines listed below, to produce accurate
envelope density measurements.
1. Sample shape influences GeoPyc results, but the effect cannot be rigorously quantified
because shape itself is subject to infinite variation. The GeoPyc handles this problem by cal-
ibration. Two calibration values for each sample chamber, (conversion factors), are noted in
the operator’s manual included with the GeoPyc.
C - 2
GeoPyc 1365 Operator Manual
136-42800-01 (Rev -) — Dec 2016
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