The bore of the cylinder can be seen to be enlarged at either end. 3.9 incorporates a threaded hole for extraction. The mixing that does occur is generally not a problem as long as the number of pressure cycles above about 0.5–0.6 GPa at room temperature is kept to a minimum. This is not a catastrophic failure since the seal is passive-the only pressure difference is due to seal friction which vanishes as contact is lost or due to the weight of the piston if the axis is vertical. Then at some pressure, it will lose contact and no longer completely seal the two liquids. The seal is generally an o-ring which suffers from glass transition under pressure that was mentioned previously. The isolating piston is very simple to implement and this is now used exclusively for volume makeup devices in the author’s laboratory. The bellows, however, adds a considerable dead volume since it cannot be fully collapsed and it is easily damaged when, for whatever reason, the volume displacement exceeds its design. The metal bellows is the most reliable method to assure that the liquids do not mix and that the difference in pressure is negligible. Perfluorinated hydrocarbons are useful here.
The meniscus then functions as an isolating piston. Then the two liquids will naturally divide into two regions one above the other separated by a meniscus. The pressure medium can be a material that is immiscible with the liquid under study and be of greater (or lesser) density than the liquid under study. One end of the cartridge is fitted with an isolating piston or metal bellows. The rheological experiment can be confined to a cartridge that fits within the pressure vessel. A volume makeup cylinder attached directly to the instrument and using an isolating piston.
In some special cases, perfluorinated hydrocarbons find use, to be discussed later.įigure 3.9. In the author’s laboratory, di(2-ethylhexyl)sebacate, di(2-ethylhexyl)adipate, and these diesters mixed with the light hydrocarbon are used as pressurizing media. The addition of 10% of a light hydrocarbon, commercially known as Coleman Fuel, to the diester reduced the response time to a few seconds.
For a pressure increase from 1.1 to 1.2 GPa the gauge required several minutes to fully respond to the increase. The pressure medium was a diester, di(2-ethylhexyl)sebacate, a standard for high-pressure work. For example, a 35 cm long tube of 1.5 mm inside diameter was used to connect an intensifier to a commercial manganin cell. The time for the effect of a pressure change to be communicated along a length of high-pressure tubing may be very long for a viscous medium. The damped response time may be quite long if the viscosity is that of the liquid being studied. This displacement will be damped by the viscosity of the liquid surrounding the force sensing component. In addition, if a force measurement is to be made by sensing of the elastic strain of a component, then a displacement occurs in association with that strain. If a thrust bearing of any type is operating inside of the instrument, it must be immersed in a low viscosity liquid. If the pressure is to be indicated by the input pressure to the pressure intensifier, then the same medium must be used consistently in the high pressure output side of the intensifier regardless of the liquid being studied. The manganin cell described above will not function reliably with a high viscosity medium. There are several reasons for this and some have already been discussed. In nearly all high-pressure viscometers and rheometers, there will be at least one liquid in addition to the liquid under study. Scott Bair, in High Pressure Rheology for Quantitative Elastohydrodynamics (Second Edition), 2019 3.6 Hydrostatic Media and Volume Compensation
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