Dewars outgassing

The Nova 1000 operates without the need for dead space determination, thus obviating Ae need for helium. It can generate a single BET analysis, a multiple BET analysis, and a 25 point adsorption and desorption isotherm together with total pore volume and sample density. The user places the sample in a calibrated sample cell and after outgassing transfers it to one of the two measurement ports. An optional five port degassing station is also available. The nitrogen adsorbate may be taken from a gas cylinder or from a Dewar flask. 

Achieving an acceptable vacuum in a dewar is not trivial. Cleanliness is essential because many materials will outgas over a period of time, destroying the vacuum. Do not handle the insides of dewars with bare hands. Use only approved materials (materials with low outgassing rates) in the vacuum space. 

The dewar pumpdown time is completely dominated by the outgassing of liquids, solids, and trapped gases on the surfaces inside the volume to be pumped. This is true even if one is very careful in handling the container. Because of outgassing, we cannot use observed pressures as simple measures of vacuum goodness. This is true in two ways ... 

Even the pressure inside the dewar is not a good predictor of the dewar vacuum life. We want to pump long enough to get the outgassing rate low enough so that - once the dewar is sealed up - the pressure will not build above some acceptable level until we are through with our test or other intended use. 

If outgassing continues at a rate of 1 x 10 torrL/s after we valve-off the dewar, the pressure will build up to 36 pm in 1 h. This is enough to conduct heat appreciably and degrade the hold time of the dewar. The resulting gases can also condense on the detector, contaminating or obscuring it. 

A thorough analysis of dewar vacuum requires that the leak and outgassing rates be broken down by species (water, nitrogen, methane, etc.). Tools for vacuum analysis include the following ... 

As we indicated already, the pumpdown time and resulting vacuum life depend on more than just the pumping speed. The outgassing of molecules often dominates pumpdown time and vacuum integrity, so it is desirable to minimize the contaminants in the dewar. 

Leaks are a throughput of gas, and have units of torr L/s or standard cm /s. A dewar with a volume of IL that must only keep the pressure below 0.1 torr for 4 h (14.4x10 s) can tolerate a leak (or outgassing) rate of 7 x 10 torr L/s ... 

Test personnel are faced daily with a decision about how long (or how well) a test dewar should be pumped. As we discussed earlier, adequate vacuum practice for long-life sealed dewars has been studied thoroughly, and many tools are available to assist in vacuum decisions and predictions residual gas analysis, outgassing rates, vacuum bake, time-temperature schedules, and so on. 

Test dewars are different in that we normally face daily (or faster) turnaround times, use fresh (un-outgassed) parts during every pumpdown, have frequent dewar modifications, and often do not impose production discipline. A typical scenario is as follows ... 

This scenario is subject to many criticisms The dewar walls absorb water and other gases while open. The unit being tested may be dirty, and certainly has some water on its surface. We do not know what the pressure was in the dewar when it was removed from the pump. (The gauge on the pump does not indicate the true vacuum in the dewar - see Section 13.4.5.) Minimal checks are made for possible leaks. No consideration is given to outgassing rates. There is no measure of how fast, and where, condensation occurs. 

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