gas regulator question

[skunked_once]

I was trained not to use teflon tape on the threads of a gas regulator when attaching the regulator to the gas tank.
Another chromatographer at our location was trained to use teflon tape on the threads. Is one way correct or are both methods okay?
An explanation for your answer would be appreciated.

[Consumer Products Guy]

OK, maybe I'm not a plumbing expert, but I do it all here and around my home, and also was formerly an auto mechanic, but here goes:
PTFE (teflon) tape is used only for pipe threads, which are tapered so the pipe diameter actually gets larger; these are the only threads that get PTFE tape (or pipe sealant, but we don't use that in chromatography). Tube fittings (such as Swagelok) or flare fittings make a conical seal, and if this conical seal doesn't hold, it's extremely unlikely that any PTFE tape will help. Same goes for your connections of regulators to cylinders - the threads are parallel threads, just to keep stuff together: the seal is again a conical seal, and tightening the nut microscopically distorts the nipple end to make a seal. You could actually cause some problems if you got PTFE remnants into your regulator or gas lines. A leak at a cylinder could be due to a bad cylinder connection, or you may need to swap out the CGA fittting on your regulator for a new one (these are 1/4 inch pipe thread on modern regulators, use the PTFE tape on these).

[MikeD]

I concur with Consumer Products Guy. If the regulator bullnose surface is not sealing against the conical seat of the gas tank something is wrong with these surfaces - perhaps a scratch. Also loose fragments of PTFE tape can find their way inside the regulator. It's poor practice.