what would cause this.

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replaced low pres. guage on oxy reg. turn tanks on, the guage reads 100, the t-handle is loose. Thanks I also hear a hissing sound, but it does not appear to be the treads.
Reply:The diaphgram has a hole in it allowing the pressure to escape, that's the hissing sound. Also why the pressure reads 100 with no force on the screw, the pressure is not allowed to build up on the side of the diaghpram opposite the screw because it's escaping.....Mike
Reply:My guess is that the diaphram is stuck. Thats the part the T handle pushes against. It will act as if you had tightened the T handle.  The hiss could be from O2 leaking around the diaphram, O2 leaking from the relief valve, O2 leaking from the case or O2 leaking at a threaded connection. I would use some snoop and check the connections and case for leaks. A couple solid thumps with your hand when the unit is NOT pressurized, may unstick the diaphram. Did you by chance, grab the case with a big pump wrench when you installed the new gauge?