Does an spg measure vs atmospheric or ambient pressure?

[lowwall]

thanks so an SPG is measuring a relative pressure vs atmospheric pressure (not vs ambient pressure) so the reading isn’t impacted by depth.

A second stage regulator will stop delivering air once absolute tank pressure is less than ambient pressure; so in theory at 40m deep the regulator will stop delivering air when spg indicates 4 bar left (40m deep = 5 bar absolute pressure, 4 bar on spg = 5 bar absolute tank pressure).

When doing calculations of bottom gas needs, what I’ve seen done is calculating the total tank capacity in L at 1 atm (tank pressure shown on spg * tank capacity) and the expected gas consumption at depth (SAC in L/min * time at depth * absolute ambient pressure) and making sure that total gas consumption is less than 2/3 of total tank capacity.

But if the plan is to dive at 40m, in theory wouldn’t it be better to keep 1/3 of *actual* usable gas?
Which would mean not considering total capacity as spg tank pressure * tank capacity but [spg tank pressure + 1 - ambient pressure at planned depth]*tank capacity?

In practice I agree it wouldn’t make a material difference, I’m just interested in the perfect theoritical case to confirm my understanding.

In your perfect theoretical case 2/3rds is quite close since you'll be breathing those final liters of gas at an average pressure of 1.2 to 1.3 atmospheres absolute. So go ahead and turn your theoretical dive when your SPG reads 2/3rds + 1.2 bar.

Of course, this is all nonsense. No SPG is that accurate, no depth gauge is that accurate, no regulator can provide gas all the way to ambient pressure, the "rule of thirds" isn't an actual rule, and SAC rates constantly vary unless you are @scubadada.

 

[Rogerdd]

In your perfect theoretical case 2/3rds is quite close since you'll be breathing those final liters of gas at an average pressure of around 1.3 atmospheres absolute.

If we’re doing some deco, I’d be doing this rule of third for the back gas and for the deco gas independently.
So the total backgas needed would be breathed at a deeper depth

 

[lowwall]

If we’re doing some deco, I’d be doing this rule of third for the back gas and for the deco gas independently.
So the total backgas needed would be breathed at a deeper depth

I was adding to my post when you posted this. Please see my second paragraph above.

Edit: Also, are you familiar with the concepts of significant digits and false precision?

Significant figures - Wikipedia

en.m.wikipedia.org

False precision - Wikipedia

en.m.wikipedia.org

When it comes to planning gas consumption or reading an SPG, the lowest significant number is the tens for bar (hundreds for psi).

 

[Edward3c]

If we’re doing some deco, I’d be doing this rule of third for the back gas and for the deco gas independently.
So the total backgas needed would be breathed at a deeper depth

Having planned your dive, how close to the gas calculations did the real dive end up?

 

[inquis]

@Rogerdd, you're neglecting (or are not aware of) non-ideal gas effects. The pressure drops faster when the tank is full vs half full when removing the same amount of gas. The tank will have more than 2/3 of the starting amount when you reach 2/3 of the starting pressure.

in theory wouldn’t it be better to keep 1/3 of *actual* usable gas?

Some people try to do that. They will compute thirds after taking 200 psi (or 15 bar) off the top so the regulator will supposedly still function in the face of 9.5 bar IP above ambient. However, it's simply not necessary because Thirds of a non-ideal gas works in your favor. If you happen to use 3x the gas consumed from that first third (e.g., 240 bar to 160 bar), you will NOT be at 0 bar.

In other words, Nature has already done what you're suggesting. Keep the math simple when you're in the water.

 

[bradlw]

so an SPG is measuring a relative pressure vs atmospheric pressure (not vs ambient pressure) so the reading isn’t impacted by depth.

not quite.
The SPG is measuring pressure relative to whatever the pressure is around (outside) the gauge

Think about it. How would the gauge see what the atmospheric pressure is, when it's sitting 30ft under water? The only thing it can "see" is what is around it....

Yes, when you're out of the water that is tealtive to atmospheric pressure.
but when you are submerged that same gauge is measuring the diffence between the tank pressure and the surrounding water column pressure,
that's what ambient means... it's whatever the surrounding environement is.....

 

[Rogerdd]

@Rogerdd, you're neglecting (or are not aware of) non-ideal gas effects. The pressure drops faster when the tank is full vs half full when removing the same amount of gas. The tank will have more than 2/3 of the starting amount when you reach 2/3 of the starting pressure.

Do you mean that when tank is full (gas under high pressure), the air inside the tank is not behaving like an ideal gas, meaning it’s less compressible (for a fixed amount of molecules under a fixed volume, the pressure will be higher than an ideal gas).

Formally PV = z(gas, P, T)*nRT with z typically > 1 and and z decreases as P drops

So under fixed volume and constant temperature, as gas is consumed (n decreases) -> P drops; but the lower the pressure -> the slower the drop

Some people try to do that. They will compute thirds after taking 200 psi (or 15 bar) off the top so the regulator will supposedly still function in the face of 9.5 bar IP above ambient.

Yes earlier in my simple case I considered the regulator could deliver air until:
absolute tank pressure (spg reading + 1 atm) = ambient pressure
but I guess it would be more like until:
absolute tank pressure = ambient pressure + IP gauge pressure (say constant 10 bar versus ambient pressure).
So using 15 bar in your example is dimensioned for a dive up to 40m, if we didn’t take into account the non-ideal behaviour of the gas that is in our favour in terms of conservatism.

not quite.
The SPG is measuring pressure relative to whatever the pressure is around (outside) the gauge

Think about it. How would the gauge see what the atmospheric pressure is, when it's sitting 30ft under water? The only thing it can "see" is what is around it....

Yes, when you're out of the water that is tealtive to atmospheric pressure.
but when you are submerged that same gauge is measuring the diffence between the tank pressure and the surrounding water column pressure,
that's what ambient means... it's whatever the surrounding environement is....

Have you seen landphil post (#8)? He’s saying it’s sealed, you’re saying it isn’t. Either conclusion is valid if the premise is correct. Question is specifically about that premise. I think landphil is correct.

 

[bradlw]

Have you seen landphil post (#8)? He’s saying it’s sealed, you’re saying it isn’t. Either conclusion is valid if the premise is correct. Question is specifically about that premise. I think landphil is correct.

ok, that's an interesting twist that I hadn't thought of, and it seems that @landphil knows something that I don't know, and had not considered as a possibility!

It makes sense that it's sealed, of course!

and I'll assume that @landphil is correct that it's sealed to maintain a set/calibrated internal pressure...to keep ambient pressure changes OUT.... and not instead environmentally sealed so that the ambient pressure can be sensed but seawater can't get in (in a similar way to how many first stage regulators are sealed)
which is what I would have assumed, if I had considered that. It's been a very long time since I really looked at an SPG closely
From an operational standpoint though, I think environmentally sealed might just maybe make more sense. Seems to me that what's of most interest, in a way, is the pressure inside the tank. That pressure is limited relative to ambient, right? It's not limited relative to 1atm.
and then from that point what the diver wants to know is that the tank is full (relative to ambient ideally so that it's both full and safe) at the start of the dive and to monitor it so the dive can be ended with some safety margin

Regardless...Nit picking I suppose since the difference between ambient and 1 standard atmosphere is a relatively small fraction of the pressure in a tank even the low pressure at the end of the dive

 

[rjack321]

If spg shows 200 bar at the surface (gauge pressure), that means the absolute pressure in the tank is 201 bar.

If we go down to 30m and don’t use any gas, the absolute pressure in the tank is still 201 bar (because the cylinder doesn’t deform).
But would the spg still show 200 bar, or 197 bar?

Basically 1) is an spg a gauge pressure vs atmospheric pressure (closed chamber in the spg), 2) or vs ambient pressure (atmospheric + hydrostatic at whichever depth the reading is made)?

Bourdon tube gauges measure gauge pressure. They read zero at the surface, whether that surface is the bottom of the Marianna trench or outer space. If the gauge is not attached to a tank and taken to a different pressure it still reads zero. The little (usually copper) tube in there has no way of sensing what the outside pressure of the world at the moment might be.

So yes the tank at 200 gauge bar is at 201 bar absolute before your dive. A full tank at 30, 60m, 100m still shows 200 bar on the gauge. Other kinds of non-bourdon tube pressure sensors can be calibrated to add the ambient pressure (whatever it might be) and read in absolute. We don't use those kinds of sensors in scuba diving.

 

[bradlw]

The little (usually copper) tube in there has no way of sensing what the outside pressure of the world at the moment might be.

just a little trivial nit, if that's the case then that's not really gauge pressure in the context of SCUBA diving.

That's more like a differential pressure measurement with 1ATM as the reference (or whatever the reference pressure is on the outside of the tube/inside of the sealed case)

Gauge pressure is a differential pressure measurement referenced to ambient.