Choose Proper Pump-Down Rates for Unfamiliar Sample Materials

There are a number of characteristics of an unfamiliar sample which can aid you in making a proper decision as to how aggressively the sample may be pumped down. Some of these characteristics are listed below. The assumption is always made that the sample material has been first dried in a shallow pan at 150 ºC or higher for one hour or in a vacuum oven.

Samples, whether in the form of fine powders, granules, or even larger pieces, require extreme caution if one or more of the following characteristics are noted or known to be the case:

  • a fine dust is raised upon stirring or shaking and the sample shows little sign of quickly settling
  • the sample is a finely powdered organic material
  • the sample is known to be microporous or mesoporous with pores of less than 100 Angstrom width
  • the sample is known or suspected to be a carbon
  • the sample is known or suspected to be a zeolite
  • the sample is known or suspected to be a fluid cracking catalyst
  • the sample has significant fine particle content below 10 mm
  • the sample leaves behind a visible deposit of fine particles when transferred from a weighing pan

Such sample materials should be evacuated at initial pump down rates of about 0.5 psia/minute or more slowly if of marked fineness until a pressure of 0.1 psia is reached. The pulsing of the second pump down path should be maintained until 100 mmHg is reached before the final shift to the third and most direct pump down path takes place. The penetrometer must be filled only about 1/3 full so that there is a margin of clearance between the stem bore and the sample bed.

Sample materials which are coarse, medium, or dense powders, or are composed of obviously non-shedding chunks, pellets, or extrudates, can be pumped down at rates of 1 to 2 psia per minute until a pressure of 0.25 psia is reached. The pulsing of the second pump down path should be maintained until 250 mmHg is reached before the third pump down path takes over.

Sample materials that obviously present little risk such as monolithic chunks or very coarse, dense granules usually can be pumped down at near maximum rates. For these, specify 5 psia per minute initial pump down followed by a change to the second path at 0.5 psia and finally a transition to the direct path at 500 mmHg.

In most cases, you should choose a conservative rate of evacuation to be on the safe side. When large numbers of samples of a material are to be run, the potential time savings may make it worthwhile to investigate as to whether faster pump down rates might be possible without risk. To do so, leave the capacitance transducer off the penetrometer base so that you may observe the sample during a trial pump down performed at slightly higher than the usual speed. Twenty percent increases in pump-down rates and pressures of path changeovers are reasonable trial increases.

Monitor closely during all portions of the pump-down sequence, especially as changes from one path to another occur. Should the sample bed or any particles begin to boil or move, immediately stop the pump down and return the sample to atmospheric pressure; reduce the pump down rate to a slower rate and try again. If the pump down was successful, then another twenty percent speed increase might be attempted. Continue until a suitable rate is found. Note that this procedure is not without risk and must be done very carefully.

The amount of time for which to continue the extended pump down after the lowest target on the third path has been met will depend upon the structure of the sample and what volatile materials may be present within it. Large chunks of porous materials such as sandstone or green ceramics or concrete may present a considerable diffusion barrier for gases and vapors traveling to their surface and may require extended exposure times to vacuum to sufficiently clear the internal pores of this obstructive matter which would prevent fully measuring the true porosity. Experience and prior knowledge of the sample can be used for guidance.

Additionally, a manual mode test can be done in which you pump the sample down to the target pressure, and then close it off from further vacuum and monitor the rate and ultimate value of the pressure rebound that results from the tardy release of gas or vapor. The amount of time before the rebound ceases or remains within tolerable limits will serve to guide you in choosing an extended evacuation time for future automatic runs of the sample material.

 
   Micromeritics Learning Center    Contact Us

Copyright © 2014 - 2017. Micromeritics Instrument Corporation. All rights reserved.