I'm looking at buying a vacuum gauge, but they nearly all have BSPP (male) threads - can anyone tell me how these are supposed to be sealed when fitted to the BSPP female? I've come across mention of copper crush washers and bonded washers, but i've no idea if this is the standard technique. Would the "crush washer" be sized to fit over the BSP thread, or inside the female?
thanks, graham.
Parallel threads normally seal on a flange and face - with a fibre or squashable copper washer in between. Does the male thread on the gauge have a flange?
If not, you may get away with some liquid PTFE thread sealant, but that's not ideal.
Doesn't look like much of a flange to me - this is typical of the ones I see:
A bonded washer is intended to go over the male thread and to seal between the faces of the two fittings. A crush washer is intended to fit inside the female fitting and be crushed by the extended male thread that pressure gauges have. That should allow you to get the gauge facing the right way when tightening it down and still get a seal. I have no idea whether vacuum gauges work the same way, as I have never needed vacuum.
Colin Bignell
That looks like it's designed to take a pipe cap nut, where the flare in the pipe does the sealing. Like brakes on a car.
On Saturday 06 July 2013 17:27 Roger Mills wrote in uk.d-i-y:
It seems depressingly common that random gauges come with BSPP threads. I had one supplied with a water pressure regulator (so it wa not a case of the wrong choice of base). There is no way it would reach down for a washer nor was there a decent flat shoulder to seal on.
I used several turns of Gas PTFE until it was "very tight" then wound it in to th eposition I wanted.
However, water is rather more forgiving than gas, so I think in the OP's case I would use a vacuum approved (gas grade) thread sealant.
I assume this is only for soft vacuum work?
ahh, thanks. Does one need a special female fitting suitable for a small crush washer or is t a standard shape of all female BSPP fittings?
graham
Tim Watts wrote on 06/07/13 20:04:
It's for a veneer press, so in the region of -750mBar (relative). I don't know if that counts as "soft", though apparently it is a "rough", "low" or "coarse" vacuum!
It does seem odd that nearly all the vacuum gauges in the uk are parallel threads, whereas it's all NPT in the states - I'd have thought a male + female tapered thread would be the ideal. otoh, if parallel is so common it must work.
g
I've never seen a female fitting where you can't use a crush washer, but I couldn't swear that they don't exist.
Colin Bignell
On 06/07/2013 20:22, graham wrote: ...
There would be no guarantee that you could get the gauge facing the right way with that arrangement.
Colin Bignell
I won't hold you to it then :)
graham.
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