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