Hellium leaks through all normal seals. I've used Indium seals to get leak rates down as low as 0.0000000001 grams/sec, but that only works for normal ambient temperatures. (Customer requirements were no greater than ten to the minus six grams/sec. The Indium seals were orders of magnitued below that.) The stuff has a low melting point, which prohibits its use in my current application. Has anyone successfully sealed Hellium at higher temperatures? I'm talking about a number of different temperature ranges, some of them up to 1800 degrees F. It is possible that some future applications will be higher than that. I'm looking for any ideas for temperatures starting as low as 400 degrees F, since some of the appliacations are not so high. I understand that I will have to accept some leakage at rates higher than customers have found acceptable in previous applications. We just need to keep the leak rates as low as reasonable.