How does moisture enter tanks?

[BoltSnap]

I guess that is not true.
The High pressure gauge is directly connectet to the tank (of course via the first stage) and not manipulatet by the first stage. On the gauge you don't have a correction of the ambient pressure.

The pressure gauge is reading the difference between the internal pressure and the ambient/external pressure, the PSIG or gauge pressure. When the pressure inside the tank is equal to the pressure outside the tank, the gauge will read "0PSIG. There is no manipulation or anything here, it is purely how the pressure gauge works.

 

[Gone for diving]

If the gauge is showing 350 - 400 psi while at depth, then it is a none issue since the gauge is showing 350 - 400 psi above ambient pressure or PSIG. If you start from the Surface with 350 - 400 PSIG then go down to 800ft, then that maybe an issue.

Don't think so,
But you sure had me thinking I was wrong for a minute,
( but I was referring to surface pressure earlier)

Every plastic pressure gauge I have taken apart was sealed no water can get into the gauge,
So if it was assembled at the surface
It should pretty much be surface reference,

If the guage was sealed and liquid filled, it should read ambient pressure plus tank pressure,

Pressure gauge reads direct from the tank, even if the hose comes out of the regulator,

I think you are thinking intermediate pressure, for the bcd or second stage,
A gauge hooked up to bcd hose with a flooded or sealed liquid gauge,
It would definitely stay a steady 150ish PSI no matter the depth,

 

[BoltSnap]

Don't think so,
But you sure had me thinking I was wrong for a minute,
( but I was referring to surface pressure )

Every plastic pressure gauge I have taken apart was sealed no water can get into the gauge,
So if it was assembled at the surface
It should pretty much be surface reference,

If the guage was sealed and liquid filled, it should read ambient pressure plus tank pressure,

Pressure gauge reads direct from the tank, even if the hose comes out of the regulator,

I think you are thinking intermediate pressure, for the bcd or second stage,
A gauge hooked up to bcd hose with a flooded or sealed liquid gauge,
It would definitely stay a steady 150ish PSI no matter the depth,

So when the pressure gauge reads 0PSIG on the surface, this means that there is no pressure at all inside the tank, vacuum?? And when you take a pressure gauge that is reading 0PSIG at depth of 30m to the surface, what is it going to read? 0PSIG still or something else?

 

[Gone for diving]

So when the pressure gauge reads 0PSIG on the surface, this means that there is no pressure at all inside the tank, vacuum??

Yes as long as the guage is accurate,
We are talking within a few psi here, a digital gauge should be much more accurate)
(don't want to muddy the water but a digital gauge like the ones I hydraulic test with, zero at whatever pressure is on the hose when I push zero and will read negative below that,)

And when you take a pressure gauge that is reading 0PSIG at depth of 30m to the surface, what is it going to read? 0PSIG still or something else?

It will read negative,
but most mechanical gauges have pin so the needle can't go lower than zero, ( has to do with the internals)

The one way to make it read zero at 30meters is to be at 30m in a chamber and let that pressure into the gauge housing, and seal it,
If its air good chance it will blow the gauge apart at the surface,

Another way (depending on housing rigidity) is to fill guage with a liquid, like clear oil with no air bubble, then the defection of the plastic case should act as a sealed first stage and give you close to ambient pressure inside,

Or just drill a hole in the housing,

The housing of the pressure gauge is a pressure vessel,

That why you can't really use a hydraulic pressure gauge well, they are filled with glycerin, and they have a vent that can be opened and close, and they have lots of deflection in the plastic lens, sealed they will read useable air at that depth,

Pound per square inch - Wikipedia

en.m.wikipedia.org

 

[broncobowsher]

You really think you can read 15PSI on an analog SPG? That isn't even the width of the ink on the tick mark. Gauges have more sticktion in the movement, not to mention backlash in the gears that can be seen with 15 PSI.

How many hundreds of PSI swing do you get between a warm and cold tank?

Get into the design of the gauge of what the reference pressure is. You want fun stuff? You measure tire pressure in reference to ambient pressure, but the TPMS that turns the light on the dash to tell you that a tire is low is based on absolute tire pressure. Go to the top of a tall mountain, tire pressure will go up with a gauge. You are comparing to a lower reference point. But the absolute pressure will be unchanged.

My digital pressure gauge I use with my booster will read a vacuum. I have a vacuum pump and will suck down the O2 bottle before filling, mostly because I can. It read 0 until there is enough vacuum and it will read -7PSI. Nothing between.

 

[Gone for diving]

Oh I totally agree,

That isn't even the width of the ink on the tick mar

My AC gauge set does
Just need a finer gauge, that why digital is nice you can read low pressure relatively accurate and high pressure,

I thought the question was about if a conventional diving spg, reads remaining air in the tank at depth, or surface air pressure,
(is it compensating or reading aprox sea level pressure)

 

[BlueTrin]

Shouldn’t it just read the pressure without compensating?

I.e. if it displays 1 bar, I’d expect it to be at the same pressure than at the surface?

Is there a convention?

In fact I just realised I don’t know how SPG work internally

Is it just working against some kind of spring? Or just against a gas at a certain pressure?

 

[broncobowsher]

Your A/C gauge won't like it if you put 3,000+ PSI into it.

Question for you. How much does the needle move around in the zero range? Does it change with air pressure (weather fronts, altitude, etc.)?

My tune up gauge (automotive) the zero is zero regardless of ambient pressure. The needle doesn't always rest in the same spot, but that can change by just bumping it.

 

[broncobowsher]

Shouldn’t it just read the pressure without compensating?

I.e. if it displays 1 bar, I’d expect it to be at the same pressure than at the surface?

Is there a convention?

In fact I just realised I don’t know how SPG work internally

Is it just working against some kind of spring?

It is a piece of tubing squashed somewhat flat and coiled up slightly. As pressure is applied to the tube, it tries to straighten out. Think of your HP hose when you pressurize it, the hose straightens out and stiffens up. A very minor version of that inside the gauge. Take that slight movement, run it through some watch gears to magnify the change, use that to spin the dial.

The hose/tube tries to straighten out because there is more pressure inside trying to get out. But if the pressure is the same on the outside of the tube (ambient pressure) as the inside, there is no difference and no force. You can alter the pressure delta between the inside and the outside of the tube while keeping the interior pressure the same The gauge reads the difference in pressures.

A digital gauge typically has a vacuum on one side, a strain gauge measuring the pressure trying to get into the vacuum. So it has no ambient reference point. Zero is established electronically.

 

[rjack321]

Explaining all this was a real challenge for me as an OW scuba instructor, because the language in the course seemed to indicate that just getting low pressure in the cylinder would allow water to get into the tank. As a consequence, I believe a lot of people, including dive shop employees and instructors, believe that is true.

It's kind of crazy that people can think in ATAs for their SAC rate calculations. But they can't translate them into PSI of water and compare with PSI in the tank.

A virtually empty tank with just 100 psi in it has to be taken 200feet deep before water pressure exceeds the tank pressure. Getting all worried about breaking the "500psi on the boat" training rule because by some alchemy at 400psi they will get water in them is just wrong. You have to take that tank with 400psi past 800ft depth before water pressure exceeds that tank pressure.