Tim Watts wrote on 06/07/13 20:04:
On Saturday 06 July 2013 17:27 Roger Mills wrote in uk.d-i-y:

On 06/07/2013 17:07, graham wrote:
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.

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?


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