ASTM E595 outlines the process to test and requirements required to pass outgassing for space and other critical systems. It is generally considered the “standard” for all outgassing testing criteria, at least in my field.

What I am struggling to understand is why the TML (total mass loss) is adjusted down based on the WVR (water vapor recovered)?

The process states that the test sample will be exposed to a 50% RH environment for 24 hours then put into the test environment, which is some X hours at some T temperature (can’t remember exactly what these values are).

The mass lost in the test environment relative to the mass after the 24hr exposure to 50% RH is the TML.

Afterwards, there is an “optional” step where the sample is re-exposed to the same 50% RH environment for 24hrs and its mass is measured again. The mass regained can then be deducted from the previously calculated TML to get the adjusted value.

Why is this allowed? Does outgassing in space not care about water vapor? If so, why not?

Edit: to any one who is curious: I reached out to the ASTM standards people and got a response from the engineers responsible for outgassing standards and she said that they allow for the exclusion of WVR for the same reasons that SAE standard allows for the reduction of TMR (I may have gotten the acronyms wrong, sorry I’m drinking lol).

That is, water vapor is reduced in some but not all cases because generally speaking water vapor is not considered to be a major contaminant in space applications. I believe this is due to the fact that small amounts of water will sublimate in space and so it will not cause condensation issues and will simply dissipate into space.

However, it is ultimately up to the client/customer to determine whether or not they approve of the use of WVR to meet the 1% TML standard.