The "Submersible" in SPG means.... ??

[Javik]

I've been trying to find a specific explanation for what is special about a "Submersible" mechanical pressure gauge vs a normal land-based pressure gauge, and I haven't found anything sufficiently detailed yet.

I believe for a standard surface pressure gauge, the casing is basically "rain/dust-tight" but is not also water-tight. So if it were put underwater, water would seep inside around the Bourdon tube, potentially leading to corrosion and eventual jamming of the mechanism.

Some surface gauges are also filled with oil, and I don't know if these are different from a submersible gauge. It seems like the main surface application for these, are for systems where the gauge may be colder than the surrounding air, so the meter can be covered with moisture condensed out of the air and will rust out and fail rapidly if not protected.


There are several possible answers for "Submersible":
- Option 1: The case of the metering chamber around the Bourdon tube is sealed and filled completely with an oil or other liquid to keep out water, salt, contaminants. (Don't know if this would work, since the Bourdon tube apparently needs a compressible space to expand into.)

- Option 2: The case of the metering chamber around the Bourdon tube is sealed and filled partially with an oil or other liquid to keep out water, salt, contaminants, plus a small roaming air bubble to provide the space needed for the Bourdon tube to compress and expand into.

- Option 3: The case of the metering chamber around the Bourdon tube is sealed and only filled with air to keep out water, salt, contaminants



There is a second set of questions, regarding whether the submersible gauge casing is designed to be unaffected by exterior pressures, or if the case flexes and compresses slightly and is affected by exterior pressures.

- Option 4: The sealed case is strong enough to maintain 1 ATM around the Bourdon tube, even when submersed to its maximum depth. This type of gauge shows an absolute unvarying reading of the tank pressure relative to 1 ATM regardless of diving depth. When the tank reads 0 at depth, this is an absolute value and will not change as the diver surfaces.

- Option 5: At depth, the exterior water pressure outside the casing also compresses the case, filling liquid, and the Bourdon tube, causing a differential pressure reading error of the gauge that increases as depth also increases. However, the differential error is small since even at 5 ATM / 132 ft, the submersed differential is only 73.5 PSI for a 4000 PSI gauge reading, causing it to display 3926.5 PSI.


A meter that only can show a differential reading may potentially be used as a hidden emergency safety factor since a tank that reads 0 differential PSI at depth will regain usable pressure for maybe 0.25 - 2 last breaths, for an emergency decompression stop that might only be able to last a few seconds but is better than nothing, as the diver surfaces back to 1 ATM.

So... ?

 

[Akimbo]

The entire Bourdon tube and face/needle assembly is housed in a one-atmosphere pressure-resistant housing. Using an industrial glycerin-filled would read the HP-pressure minus bottom pressure, aside from not lasting long in sea water or fitting the HP hose connection.

 

[Tbar]

Some comments in green

There are several possible answers for "Submersible":
- Option 1: The case of the metering chamber around the Bourdon tube is sealed and filled completely with an oil or other liquid to keep out water, salt, contaminants. (Don't know if this would work, since the Bourdon tube apparently needs a compressible space to expand into.)
The tube is curved and will straighten when pressure is read, so it will not significantly increase its volume, does not expand.

- Option 2: The case of the metering chamber around the Bourdon tube is sealed and filled partially with an oil or other liquid to keep out water, salt, contaminants, plus a small roaming air bubble to provide the space needed for the Bourdon tube to compress and expand into.
See 1

- Option 3: The case of the metering chamber around the Bourdon tube is sealed and only filled with air to keep out water, salt, contaminants
See 1, keeps water out so there is no corrosion

- Option 4: The sealed case is strong enough to maintain 1 ATM around the Bourdon tube, even when submersed to its maximum depth. This type of gauge shows an absolute unvarying reading of the tank pressure relative to 1 ATM regardless of diving depth. When the tank reads 0 at depth, this is an absolute value and will not change as the diver surfaces.
Generally the case will sustain pressure otherwise it would mechanically fail i.e. at 10m the pressure will double and hence the volume halve. The may be some flex in the case. The relative change in the internal pressure compared to the tank pressure is low. 1ATM ~= 1bar, the tank range for most divers will be 230bar to 50bar.

- Option 5: At depth, the exterior water pressure outside the casing also compresses the case, filling liquid, and the Bourdon tube, causing a differential pressure reading error of the gauge that increases as depth also increases. However, the differential error is small since even at 5 ATM / 132 ft, the submersed differential is only 73.5 PSI for a 4000 PSI gauge reading, causing it to display 3926.5 PSI.
Yes see 4

So the case needs to be strong enough to resist mechanical failure (crushing) under depth pressure and must be sealed to stop water entering and hence corrosion. Gauges used above water, such as an IP gauge are not built to the same requirements.