The Volumetric Reference Capsule is a small cylindrical device (Volumetric Reference Capsule assembly) that is inserted into the penetrometer bulb (for solid samples) in the same manner as a sample.
Figure 1. Volumetric Reference Capsule assembly
The capsule assembly has its internal cavity separated from the exterior by a replaceable membrane which has numerous uniform, cylindrical pores. The porous membrane permits the reference capsule’s internal cavity to be evacuated of air at the same time as the penetrometer.
To begin the test, an accurate weight of the empty capsule assembly must be obtained. During an automatic test run, mercury initially fills the penetrometer to a few pounds per square inch but (at this pressure) the mercury does not yet enter the reference capsule’s internal cavity. A steady increase of pressure is then begun. At a specific higher pressure corresponding to the membrane pore size, mercury then enters the internal cavity of the capsule, and the volume of mercury is registered by the mercury porosimeter as if the capsule were a real monoporous sample.
Because of its design, mercury in the reference capsule’s internal cavity does not significantly come out of the capsule when the pressure is lowered back to atmospheric pressure. The mercury remains inside the capsule’s internal cavity where, upon removal from the penetrometer bulb, the mercury weight gain of the reference capsule can be determined on an NIST-certified balance.
True Volume of Mercury
The true volume of this mercury can then be determined by a computation using the certified handbook value for mercury density at the temperature of the capsule. Comparison of this true volume to the volume reported by the mercury porosimeter, as the total intrusion volume, will reveal the amount of any volumetric error in the entire measurement system of the mercury porosimeter, including the associated penetrometer.
Prior to regular use, the internal mercury volume capacity of each Volumetric Reference Capsule should be measured and recorded. This procedure is described in its accompanying manual. Although it is preferable to weigh upon each use, comparison of the capsule’s known (recorded) volume and the intrusion volume reported by the instrument has proven to be quite reproducible.
This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.
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