Accurate and Reproducible Envelope Density Measurements

This article contains guidelines to achieve accurate and reproducible envelope density measurements with the GeoPyc®.


High reproducibility in any analytical measurement can often be achieved by performing tests in an identical manner using a single instrument, the same quantity of test material, and fixed instrument parameters. This is especially true with the GeoPyc technique, as it is extremely sensitive to procedural variations and deviations in test parameters.

When parameters are monitored to the fullest extent possible, reproducibility of results of about ±1.0% can be expected. Below is a description of these parameters and the criteria that must be observed to achieve this level.

Envelope density is calculated from specimen mass and envelope volume, that is, volume including both closed and open pores. A non-intruding, free-flowing, dry powder medium called DryFlo® is used to measure this volume. DryFlo® is confined in a cylindrical sample chamber with one of five diameters from 12.7 mm (0.5 in.) to 50.8 mm (2.0 in.).

The specimen volume is determined by subtracting the volume of consolidated DryFlo (blank run) in a sample chamber from the volume of the same consolidated DryFlo in the same chamber with the specimen included (test run). The medium bed is agitated through vibration and rotation, and the consolidation force is then slowly increased to the same set value in both phases of a test.

  1. The first criterion for a GeoPyc analysis is that the DryFlo consolidate indentically in the test and blank runs. When the medium was tested repeatedly, it was found that, almost without exception, it consolidates with a reproducibility of ± 0.34% or better in all size sample chambers for bed depths of one-half to twice the chamber diameter. When the bed depth is limited to the chamber diameter, a slightly better reproducibility of ± 0.25% is usually achieved. In any event, between one-third and one-quarter of the minimal overall error of ± 1.0% is due to the nonideal behavior of DryFlo.

Guideline 1. Start an analysis with a DryFlo bed depth a little less than the chamber diameter.

  1. Sample quantity plays a key role in reproducibility. The specimen extracted from a larger quantity of material must be sufficient to be representative of the whole. The minimum sample chamber size required for analysis is determined by the quantity of sample. A chamber should be selected where the sample constitutes at least 20% of the total sample-plus DryFlo volume when consolidated. A larger percentage of sample is preferable; however, remember that the sample must always be surrounded adequately by DryFlo.

Every envelope density result is derived from the difference in two volumes - the consolidated DryFlo and the consolidated DryFlo with sample. It is essential that this difference is as large as possible for mathematical significance.

For instance, 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 nearly a 9.0% variability in envelope density. At the highest percentage, the sample volume may have been enough to bridge sample pieces to interfere with medium consolidation.

At the lower percentage, the difference value magnified the small errors in consolidation. However, when the sample volume ranged from 30 to 35%, the envelope volume within a ± 1.3% error band was registered. The current program for the GeoPyc automatically calculates the sample volume percentage, which is a useful guide to optimum performance and should always be taken into account when analyzing the validity of results.

Guideline 2. Select sample chamber dimensions, DryFlo volume, and specimen quantity to yield a sample volume percentage of at least 20%.

  1. 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 DryFlo and sample weights were kept constant to the third decimal place. In this case, the reported sample-to-bed volume was found to differ only between 32.1 and 33.4%. This technique should be considered when possible, even though such control is not practical or even feasible in many instances.

Guideline 3. Maintain constant all parameters susceptible to control for optimum reproducibility.

  1. Both the blank and test steps of an envelope density determination contain an equal number of preparation and analysis cycles.

Preparation cycles are repetitious, unrecorded, agitation and consolidation attempts meant to orient the DryFlo grains and the specimen into a uniformly mixed bed. Analysis cycles follow the preparation cycles and yield statistical data on consolidated volumes.

The bed is expected to become more and more consolidated during the preparation cycles, but little or no consistent decrease or increase in value should be evident in the analysis cycles. Once the cycles exceed a specific number, diminishing information can be gleaned. The results presented above were primarily achieved with 10 preparation and 5 analysis cycles. A few specimens require more, but fewer are sufficient in other cases; 10 preparation and 5 analysis cycles are considered to be 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.


The guidelines must be followed for reproducibility described earlier in this article. Those guidelines must be followed, along with the guidelines listed below, in order to create accurate envelope density measurements.

  1. GeoPyc results are influenced by sample shape, but the effect cannot be thoroughly quantified because shape itself is subject to endless variation. This issue is handled by the GeoPyc through calibration. Two calibration values for each sample chamber, called conversion factors, are noted in the operator’s manual that comes with the GeoPyc.

The first conversion factor (calculated factor) is derived easily from geometry and mechanical couplings and it relates the plunger movement to chamber volume as if there were no sample shape influence. The second factor (adjusted factor) is altered to include an average shape influence experimentally determined from several different shapes.

Neither factor is likely to apply precisely to any specific specimen. True calibration for shape can only be achieved when the predetermined envelope density of a representative specimen of the material in question is used.

Preferably, the representative specimen is obtained from an evaluation procedure that was being followed before the introduction of GeoPyc. GeoPyc results can then be expected to track previous records. A fully nonporous specimen of the same shape as the material in question affords a degree of calibration; however, since it is nonporous, it cannot have the same surface texture and cannot be as satisfactory.

There is no real substitute for a truly representative specimen for calibration, because in the final analysis the GeoPyc works best as a comparison device. A GeoPyc user should set aside enough of the selected calibration material in order to recheck the calibration on a regular basis.

Guideline 1. Select for calibration a quantity of the material in question and determine its envelope density by the prior test procedure or some other method.

  1. Calibration itself will only be reproducible to the degree that the guidelines provided earlier for reproducibility is followed. As a result, the quantity of DryFlo, the weight of the representative sample, and the sample chamber size should be selected based on the amount of sample to be used later.

Additionally, all calibration tests should be performed with the same consolidation force and the same number of preparation and test cycles to be used in analysis.

Guideline 2. Conduct calibration tests using parameters identical to those to be used in analyses.

  1. Finally, it is essential to carry out a number of calibration tests and select the median as the conversion factor.

Guideline 3. Use the median value from a number of calibration tests as the conversion factor for the material to be analyzed.

This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.

For more information on this source, please visit Micromeritics Instrument Corporation.


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