An Explanation Of Leak Rate

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Introduction
Personnel involved with freeze-drying technology must deal with evacuated vessels and the testing and criteria that assure that a vessel is free of significant leaks. Traditionally, industries that are involved with vessels subjected to evacuation have measured the relative "tightness" of these vessels by one of two criteria: "rate of rise" or "leak rate." At first glance, it may appear that these are simply two different terms for the same thing. However, a closer look reveals that this is not the case.

Basic Definitions
Rate of rise is the amount of pressure change in an evacuated vessel over a given period. (i.e. millitorr per minute, mbar per second, etc.) For example, if one evacuates a vessel to 100 mTorr, closes the vacuum valve, and then observes that after one minute, the pressure is 105 mTorr, then the rate of rise is quite simply 5 mTorr per minute. Mathematically the formula is:

Rate of Rise = Finish Pressure - Start Pressure Elapsed Time

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