How does moisture enter tanks?

[lowwall]

Mechanical SPGs measure gauge pressure, which by definition is the difference between absolute pressure and ambient pressure. However it gets a little tricky as the SPG's only has one outlet, so "ambient pressure" or the 0 reading is determined by whatever it was when the system was sealed, i.e., when the regulator is hooked up to a cylinder with a closed valve.

So two SPGs could show different readings on tanks with identical amounts of gas depending on the ambient pressure when they were rigged up.

Thus you will have theoretical small changes in the SPG reading as you move to areas of higher or lower pressure, but the reality is that the EN250 standard for SPG accuracy is only ±10% below 100bar/1450psi. They just aren't accurate or precise enough to notice 10 or 20 psi differences.

The only way you'd see change in the real world would be from something like rigging a bottle at the bottom in a diving bell (or purging a closed pony in a deep chamber). Leave it closed and you might see the needle rise off zero as you ascend and the gas in the hoses tries to expand and thus exerts pressure on the gauge. Or, if you have your own airplane, you could try taking a rigged pony with closed valve to altitude.

Edit: on further reflection, some of this is wrong and I've used strike through text to indicate it. Assuming the internal volume of the hose does not change, then the pressure shown on the gauge will not change. See my next post (2 below) for more details.

 

[Gone for diving]

You could assemble a spg in a vacuum to read absolute, the housing is good for vacuum, but will really only change your gauge by 14.7 psi and that nothing in the range of you tank,

But gauge question was, at 800 ft the water pressure is 400 psi absolute pressure, and the tank gauge is at 400 psi does the gauge on the tank read 800psi at the surface?
(At least that's how I understood the question)

I say no it will read exactly the same, 400psi. At that depth 400psi is locked in your tank and you can't use it until you ascend,

Which by the way is why you are to keep the reg in you mouth during a CESA
You unlock abit more air in your tank,

 

[lowwall]

But gauge question was, at 800 ft the water pressure is 400 psi absolute pressure, and the tank gauge is at 400 psi does the gauge on the tank read 800psi at the surface?
(At least that's how I understood the question)

I say no it will read exactly the same, 400psi.

Essentially correct as it's a closed system.

Maybe it will be easier to visualize if we take the hose out of the equation and think of a button reg that's screwed directly into an HP port on the first stage with the valve open.

Remember PV = nRT? Thus P = nRT/V. In our case, you have a fixed volume (V) with a fixed number of gas molecules in it (n). R is a constant. So, as long as the temperature stays the same, your system is always going to have the same pressure (P). It doesn't matter what happens outside the system as long as the internal volume doesn't change.

If course, steel and aluminum aren't perfectly rigid and the volume will increase slightly as you ascend from 800 feet. Thus a high precision SPG would show a small decrease in pressure.

 

[lowwall]

Putting it all together. Take two identical cylinders, fill them with enough gas to measure 400 psi at the surface. Attach a reg with SPG to one and open the valve. It will read 400psi. Leave the other valve closed and its reg detached.

Now load everything into a diving bell and start descending. As you descend, the bell tenders will need to keep adding gas to keep the bell dry. Stop when the gas in the bell is at 400psi, which will be around 900' (275m).

During the entire descent, the SPG that's been attached to a tank will read 400psi. Now attach the other reg to the other tank and open the valve and the needle will.... not move off zero.

Ascend to 450'. The first SPG will still read 400psi, the second will still read 0psi. Close both tank valves and purge a second stage on both regs. Reopen the valves and both SPGs will now read 200psi.

 

[Gone for diving]

I don't see how hooking up a reg set at 900ft will change anything,,,
If the 2 gauge are identical and normal spg gauges, and assuming they will hold 900ft pressure and both tanks are same pressure, they will read the same on the gauges, there will be no difference,

 

[lowwall]

I don't see how hooking up a reg set at 900ft will change anything,,,
If the 2 gauge are identical and normal spg gauges, and assuming they will hold 900ft pressure and both tanks are same pressure, they will read the same on the gauges, there will be no difference,

SPGs technically shows psig (pounds per square inch gauge) not psia (pounds per square inch absolute): 0 psig is any ambient pressure at the gauge's inlet. 0 psia is a vacuum, while sea level pressure is about 14.7 psia

No matter what the ambient pressure is, an unconnected SPG will read 0. You can take it to the top of Everest or 1000' deep and it will still read 0.

Hook it up to a pressure source and it will read the difference between that ambient pressure and the absolute pressure of the source. If the ambient pressure and the absolute pressure of the source are the same, it will read 0 psi (or bar). Go to outer space, vent your tank and then hook up the SPG and it will read 0. BTW, this is also the only time that psig will equal psia.

Remove the SPG, close the valve and take that tank and SPG in a bell to 900'. The unconnected SPG will still read 0. Open the tank valve (slowly) and gas will rush in until it equalizes with the pressure of the gas in the bell. Once it has settled, hook up your SPG and it will read ..... 0. Why? Because there's no difference in the pressure between what's in the tank and ambient. The number of gas molecules in the tank is irrelevant to the gauge. All it knows is that there's nothing pressing on its inlet. Hook a reg up and you'll find the gauge isn't trying to trick you, you can't breathe any of that gas at that depth.

Leave everything hooked up and head for the surface. The SPG will continue to read 0 the whole way. Once you surface, close the valve and carefully remove the SPG or purge a reg to get rid of the excess gas between the valve and SPG inlet. Now replace the SPG, open the valve and it will read ... 400psi!

 

[BlueTrin]

Found this interesting to read

Pressure measurement - Wikipedia

en.m.wikipedia.org

Depth gauge - Wikipedia

en.m.wikipedia.org

 

[Gone for diving]

I am confused why we keep talking about vacuum when a standard spg can't read vacuum, I understand it there and does effect the gauge but spg can't read that fine nor can it read lower than zero...

The new question is.... a standard SPG
With the hose removed and open,
At 900ft, will it read 400psi?
I say yes it will. It will act like a mechanical depth guage,

Thanks for that link @BlueTrin,

That what I have been trying to say,

I only have ever opened old plastic spg, because the can be disassembled, They are oring sealed at the base and at the lens, no water can get in. So the guage has pressure relative to atmospheric pressure as the zero point,

So any tank pressure reading is relative to the surface not the depth,

Anyone want to do a quick dive to 900ft test this ?
Take a hoseless spg along and see what it reads.

 

[lowwall]

Good question. My illustrations are all based on gauge pressure as showing 0 at ambient pressure, not at 1 atmosphere. If it's a completely sealed system that is calibrated to show 0 at 14.7psi, then the indicated pressure will change as the ambient pressure changes.

I can find examples of transducer-based industrial SPGs that work either way, but I don't see a definitive example for analog scuba SPGs.

 

[broncobowsher]

Likely to reach the crush depth of the plastic housing. At which point the pressure one both sides of the bourbon tube will be equal and it will read 0