Michael,

I would recommend a double-window ('double glazing') made from two thin transparent wafer substrates.

I've thought about this for a cryogenic (LN2) test chamber. Purge the space between the wafers with dry room temperature nitrogen, and I have a sheet of foam (with silicone) to attach the separate the two discs:

I bought a few 100mm discs of sapphire from:

https://www.universitywafer.com/

Thanks for the fast answer!

Are there a possibility to use a window without a purgation with dry room temperature nitrogen?

What is about the humidity in the cryogenic test chamber or could it be neglected? (Danger of steam up)

Michael,

You could, but the air between the windows will carry *some* humidity and you'll have frost on the inside window. The colder inside window will also cool the airspace between the windows, and so reduce the pressure - you might find air being drawn into that cavity till the pressure (and temperature stabilizes).

A pair of narrow plastic hoses, one to a dry N2 cylinder, one to ambient, would allow you to keep a flow of warm dry gas to keep moisture at bay.

Sealing the inside window against your chamber might be, ah, interesting. What is the chamber made from?

The problem is, i do not have the dry N2 and i have not the possibility to store it!

I am not sure what the chamber is made from, i think its aluminium!

:(

Maybe blow some air through a stack of silica gel?

Or a few puffs from a can of 'dust off' spray?

https://www.staples.ca/dust+off/directory_dust+off_20051_1_20001

If aluminium, silicone will stick quite well - so I'd start thinking about cutting a larger circular hole in the case, and gluing a sapphire disc to it. Then, make a hermetic seal with an impermeable blown polymer (even Armacel) and then the second disc glued to that. A sort of annular hamburger - with air space for the beef.

Thank you for the tip, i will try it!