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I finally figured out that the nitrogen gas is used for the MELFI (minus eighty freezers). I can't figure out what the argon would be used for, though I can see how helium might be useful for refrigeration also. But what are these actually used for, if anything?

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    $\begingroup$ Nitrogen is used to dilute the oxygen in the atmosphere. It is not a 100% O2 environment on the ISS. They generate oxygen from water, but need additional gasses to get away from 100% Oxygen and the fire risks that entails. $\endgroup$
    – geoffc
    Jul 22, 2016 at 19:46
  • $\begingroup$ For the OP: What is the source of your information about these gases being onboard the ISS? $\endgroup$ Jul 22, 2016 at 20:25
  • $\begingroup$ Well, for example: books.google.com/… (but you can also find them in the SSP interface documents). $\endgroup$
    – Translunar
    Jul 23, 2016 at 21:16
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    $\begingroup$ That document says they are supplied for experiments. $\endgroup$ Jul 24, 2016 at 0:42
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    $\begingroup$ @OrganicMarble Do you know of any experiments that actually make use of them? And is there some reason they aren't just included in those payloads, but are instead part of the module's infrastructure? It seems a bit over-designed, but more importantly arbitrary — why these gases but not others? (Also, I thought argon was not allowed on ISS, since it's not scrubbable.) $\endgroup$
    – Translunar
    Jul 25, 2016 at 13:55

2 Answers 2

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Nitrogen Usage

The Minus Eighty Degree Laboratory Freezer for ISS (MELFI) uses high purity nitrogen for its Brayton cycle, not helium (source). The nitrogen ISS uses for other payloads and ECLSS isn't a high enough quality, so the the MELFI has its own source (if I'm remembering correctly from my Cold Stowage training at MSFC).

enter image description here (Image Source)

The compressor spins at 90,000 RPM on hydrodynamic gas foil bearings. Heat from the Brayton cycle is rejected to the LTL (Low Temperature Loop), which runs at 4°C. Jack "2Fish" Fischer made a great informational video on the MELFI while he was onboard the ISS. (He called down to Huntsville while I was on console and asked for a MELFI fact sheet. Much to my surprise, I saw this video a few days later with all the information I provided to him!)

Helium and Argon Usage

The Combustion Integrated Rack (CIR) uses argon and helium during their combustion tests with various fuels (source, source, source). Most of my in-depth knowledge is payloads related, so I'm unaware if any ISS systems use either of those gases as well. enter image description here (Image Source)

The various high pressure gas tanks flown for CIR had to be a limited volume that wouldn't induce a suffocating atmosphere if the entire contents leaked into the rack. Crew members handling them were required to have at least one hand on them at all times due to risk of propulsive venting, should the nozzle become damaged.

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Helium is used for cryo-storage, and enables freezers that are much colder than those making use of nitrogen.

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    $\begingroup$ Do you have any references for this answer? $\endgroup$ Apr 3, 2017 at 17:57
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    $\begingroup$ @JamesJenkins Personal communication with payload providers and CASIS. $\endgroup$
    – Translunar
    Apr 4, 2017 at 15:44
  • $\begingroup$ This is very cryptic. I don't understand what this one-sentence answer is saying. Are we talking about liquid helium and liquid nitrogen? And if so, why are liquid helium temperatures needed? $\endgroup$
    – user687
    Jun 3, 2017 at 2:54

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