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The barometer we used was actually called a " manometer " and was connected
to air ducts. We had a high capacity blower connected to the ducts, and would
pump up the ducts to 5 or 6 inches of water column, and measure the leakage rate
which had to be 1/2 % or less.
The tube was a clear vinyl at 3/8 inch dia. inside and was bent into a "U"
shape about 24 inches long.
The tube was attached to a rectangular plate with inch marks on both sides
of the "U" tube.
We used colored oil and it filled the bottom of the tube about 4
inches deep.
Both ends of the tube were open, one being connected to the ducts that we
were pumping up.
As the pressure in the ducts rose, the oil would climb the open side of the
"U" giving us a reading of pressure in the ducts relative to the ambient room
pressure.
Now this was a pretty accurate reading considering the simplicity of the
device.
For our purposes, we would set the tube in the sub, and plug one end with
the hatch open, measuring atmospheric pressure as " zero" on the scale. The
level of oil will be equal on both sides.
With the hatch closed, and the sub submerged, If the pressure in the
sub rises, the air in the closed portion of the tube will compress slightly and
the oil will rise due to the increased pressure on the still open end of the
tube. If the internal pressure drops below 1 atmosphere, the oil will rise on
the open side of the tube.
Now, because the one end is closed, we won't get accurate readings of water
column, but we will get an indication that the pressure differential across the
open and closed ends has changed. Venting or adding air to the cabin will be
required to equalize the indicating oil level.
Adding some air will be fairly easy, just crack the valve on a scuba
tank a little. Lowering the air pressure will be a little more difficult.
Probably a small 12 volt air compressor connected to a snorkel pipe or a small
container like an empty propane tank would be enough, or if the pressure/volume
wasn't that much, maybe just vent through the snorkel as you near the
surface.
Now I know this isn't very hi tech, but with a little testing, it should be
easy to grade the scale to reflect the changes in internal pressure by attaching
a gage to the sub hull, closing the hatch, and pumping the sub up while inside
and marking on the scale how much pressure relative to 1 atmosphere is
reflected. That way we could establish where the safe depth to open the
snorkel is for any given over pressure reading.
Because the tube is closed on one end, and the air bubble in that end must
compress for the oil to rise, the first inch or so of rise will be easily
detected, but as the pressure increases in the sub, the rise will be less
evident, so the markings on the scale will become closer as the pressure
differential increases. Again, using a gage viewed through the window,
we should be able to mark the scale at each portion of atmospheric change
up to what ever over pressure level we are anticipating might occur.
Not very hi tech, but it appeals to my Scottish nature. Not necessarily
cheap, but definitely thrifty! Frank D.
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