A client called yesterday with a couple of good questions. Here is the first:
“Do you pull vacuums on liquid cylinders when you rehab them and if so to what level?”
Yes. Cryogenic liquid oxygen, argon and nitrogen have very low boiling points. Nitrogen for instance boils (converts to gas) at
-195.8 °C or -320.44 °F. Keeping argon, nitrogen and argon cold enough to maintain them in a liquid state requires specialized vacuum insulated containers. Cryogenic liquid cylinders are double walled vessels with a super-insulation wrapped inner container that holds the liquid and is inserted into a slightly larger outer vessel. The annular space (annulus) between the two vessels is used to insulate the liquid. After both vessels are sealed leak tight a vacuum is pulled on the annulus to remove virtually all atmospheric molecules and impede the transfer of energy.
To be an effective insulator this vacuum has to be pulled to very low levels. Leaks will result in a loss of vacuum and the need for rehab. West Cryogenics performs a helium mass-spec leak investigation at rehab to check for molecular level leaks. Then the vacuum is pulled to < 10 microns. The vacuum pull is done in two phases (hot and room temperature) to assure all moisture and other stray molecules are removed. A final inspection is performed after refinishing and other testing to verify that the vacuum has not risen due to latent moisture or a leak that needs to be found and repaired. If final inspection shows a weakening of the vacuum then the liquid cylinder goes back to mass-spec and vacuum restoration.
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