James,
I'm planning to install an aircraft altimeter for the same
purpose. An altimeter is simply a barometer with a wide range, so you
could use that instead. It has the sealed air chamber (aneroid) which
expands or contracts in response to changes in external pressure.
With the hatch closed and latched you can pump air out with an
external vacuum pump, turn the pump off, close the valve, take a reading
on the altimeter, wait a few minutes, then take another reading to see if any
air leaked back into the sub. This can even be done with the sub
unoccupied as long as the altimeter is positioned where you can read it
through a viewport.
In most cases you can use the adjustment knob to set the altimeter to 0
for the first reading. I plan to run the vacuum pump until the
gauge reads 5,000 ft. altitude, shut the valve, and wait 5 minutes. If
the altimeter reading drops to 3,000 feet for example, I know a seal is
leaking inwardly. One caution: If the sub is sitting in strong
sunlight, you could get some increase in internal air pressure just from heat
build-up.
This test is on my pre-dive checklist on every dive. During a
dive the pressure reading should change very little if your scrubber and O2
inlet are in balance.
Comments and criticisms are most welcome.
Jim
In a message dated 3/27/2012 7:36:00 A.M. Central Daylight Time,
jamesf@guernseysubmarine.com writes:
Hi All,
Mr Numpty here.
I kind of know the answer to this already, but i thought someone may be
able to clarify my mistake.
I was very pleased when i spotted just the sort of gauge i was after
mounted on the back of a sewage tanker. A pressure gauge but one
that reads a negative pressure as well as a positive. So it
starts at 0 and can go to -1 or +1 bar. Idea was that it would go
inside the cabin to monitor internal pressure. (I'd like it to be able
to measure a negative pressure as well as i intend to drop the internal
pressure to test for penetrator seal leaks).
I was just thinking about mounting it up and ive suddenly realised that
its not going to work is it? It must measure the port pressure
relative to the ambient pressure, which if i just mount it on a bulkhead
will always be the same no matter what the pressure actually is.
I suppose i'd have to make a pressure proof box sealed at
surface pressure and pipe the port to the inside of the
hull? So its kind of like a caisson gauge.
Just thinking aloud really.
James