This can mean either:
- Measuring the average outgassing rate of the chamber walls and fixtures
- Measuring the outgassing rate of some specific material
Both measurements are made in the same way. However, while the first is fairly easy, leading to reasonably reliable results, the second often involves finding the difference between two similar sized numbers and is, therefore, subject to large errors.
The chamber's average outgassing rate is measured using these steps (a specific example's values are quoted in parentheses):
- Estimate or measure the chamber's volume (53 liters)
- Estimate the chamber's inner surface area, including the area of any fixtures (15,200 cm2)
- Pump the chamber to its base pressure (6 x 10-7 torr)
- Isolate the chamber from the pumps and time the rise to some specific pressure (3 x 10-5 torr in 47sec)
- Calculate as in chart below
|Pressure change||3 x 10-5 minus 6 x 10-7 T||2.94 x 10-5 T|
|Pressure rise in 53 L in 47 sec||2.94 x 10-5 T|
|Amount of gas into 1 L in 47 sec||2.94 x 10-5 T multiplied by 53 L||1.56 x 10-3 T.L|
|Amount of gas into 1 L in 1 sec>||1.56 x 10-3 T.L divided by 47 sec||3.3 x 10-5 T.L/sec|
|Total gas load rate||3.3 x 10-5 T.L/sec|
|Average area outgassing rate||3.3 x 10-5 T.L/sec divided by 15,200cm2||2.2 x 10-9 T.L/sec/cm2|
|Outgassing rate in SI units||2.2 x 10-9 T.L/sec multiplied by 1333.2||2.9 x 10-6 W/m2|
The last line indicates the outgassing rate is similar to mechanically polished stainless steel after one hour under vacuum.
To measure the outgassing rate of some specific material, you do exactly the same test, only twice. First, with the chamber empty to find the background outgassing rate, then with a known area of the material in the chamber. The material's outgassing rate is the difference between these rates divided by the total area of the material. Note that in a fixed time period, the second pressure rise should always be greater than the first since the new material adds to the existing chamber rate. The method's problems are:
- The calculation involves the difference between two large numbers (always a source of error)
- It is particularly bad if the chamber outgassing rate is high and the material's rate is low.
- If the chamber has no load-lock and must be opened to introduce the sample, the chambers surfaces are re-loaded with gas that must be removed to achieve the previously determined background outgassing rate. Removing it obviously removes some of the adsorbed gas from the test material.
- The microscopic cleanliness and condition of a surface has a large effect on its outgassing rate. Measurement on nominally identical samples may give large differences in outgassing rate.
- The surface's vacuum history is very significant to its outgassing rate.
Which is why any specific material type can have such variable outgassing rates.