Outgassing vs offgassing — what's the difference?
In most vacuum work the two words are used interchangeably, and there is no single, universally agreed technical line between them. Both describe a material releasing gas under vacuum. So rather than argue about the label, the useful question is: which measurement do you actually need? Two very different tests hide behind these words, and getting the right one matters far more than the spelling.
Why the two terms get confused: Both "outgassing" and "offgassing" refer to gas coming out of a material once it's placed under vacuum or heat — gas that was adsorbed on surfaces, absorbed into the bulk, or trapped during manufacture. Different industries lean on different spellings: the space and vacuum-technology world tends to say "outgassing" (it's the term used across the ECSS and ASTM standards), while materials, packaging and building-science contexts often say "offgassing". Because they describe the same underlying phenomenon, they've become effectively synonymous — which is exactly why people conflate them.
The distinction that actually matters: When a customer comes to us about "outgassing" or "offgassing", they almost always need the answer to one of two quite different questions:
| Gas load | Contamination | |
|---|---|---|
| The question | How much gas does this release into my vacuum? | Which condensable species come off, and will they deposit on cold surfaces? |
| What’s measured | Total throughput → a specific gas-load rate | Mass loss and collected condensables (TML/CVCM); species identity |
| Typical units | mbar·L·s⁻¹·cm⁻² | % (TML/CVCM) |
| Sample type | Varying size, from material to assembly level | Material level; mass greater than 100 mg and less than 300 mg, to fit within a 1 cm³ crucible |
| Why you care | Pump-down time, achievable base pressure, gas load on a system | Degradation of optics, detectors, coatings, mechanisms |
| Common drivers | Vacuum-system and materials selection | ECSS-Q-ST-70-02C, ASTM E595, ECSS-Q-ST-70-05C |
| Our service | Offgassing testing | Outgassing testing (TML/CVCM) |
A material can be perfectly "clean" from a contamination standpoint and still impose a significant gas load on a system — and vice versa. That's the practical reason the two measurements exist separately.
How TS-Space uses the terms
To keep things unambiguous on our own reports, we tend to use "outgassing" for the general phenomenon and for contamination-oriented work, and "offgassing testing" specifically for the quantitative gas-load measurement — the accumulated-gas rate-of-rise test that produces a specific gas-load rate. But we're not precious about the words: tell us the question you need answered and we'll make sure you get the right measurement, whichever term you use for it.
Which do you need?
You're selecting materials for a vacuum system, worried about pump-down or base pressure, or need a gas-load figure to design against → you want gas-load quantification: Offgassing testing.
You're worried about deposits on optics, detectors or thermal surfaces, or working to a contamination standard → you want the contamination screen: Outgassing testing (TML/CVCM), with contamination analysis where species need identifying, usually alongside a Vacuum bakeout.
You need both → they're routinely run together; a bakeout with contamination monitoring followed by a gas-load measurement gives the complete picture.
Common questions
Is offgassing the same as outgassing? In practice, yes — the terms are widely used to mean the same thing. Any distinction is a matter of convention, not a fixed standard. What differs is the measurement you commission: gas load versus contamination.
What is offgassing testing? As we use the term, it's the quantitative measurement of a material's gas load under vacuum — its specific gas-load rate — by monitoring the pressure rise in an isolated chamber (the accumulated-gas, or rate-of-rise, method).
What units is an offgassing (gas-load) rate given in? A specific gas-load rate is typically reported in mbar·L·s⁻¹·cm⁻² — throughput per unit sample area.
Does a low gas-load rate mean a material is "clean"? Not necessarily. A low gas load doesn't guarantee low condensable contamination (TML/CVCM), which is measured separately. The two are related but distinct.