Using a regulator in cold water.

[Angelo Farina]

Consider this: there is an inverse relationship between the volume and the temperature of a gas. The air passing through your first stage valve expands dramatically. All other things equal (and they aren't, at least not in your regulator, where humidity, materials and frictive heating confuse the issue) the temperature drop can exceed 10F for every 1/4 ata. If the supply cylinder is at 3000 psi and the IP is set to 140psi, you're looking at a whole lot of adiabatic cooling.

This is all entirely wrong, I am sorry. Expansion cooling occurs only if entropy is kept constant, that means a fully reversible expansion process. A gas turbine goes quite close to it.
But in a lamination valve (as the first stage of a regulator) no work is obtained by the gas expansion, hence the entropy grows significantly. What remains almost perfectly constant is the enthalpy.
Assuming air is a perfect gas (which is almost perfectly, but not entirely), the temperature is directly proportional to entalpy: H = cp*T . Hence, as enthalpy is constant, also the temperature will not change.
The only way the expanding gas has to transform part of its enthalpy into another form energy is by converting to kinetic energy, that, is creating a very fast air jet.
That's the reason for, while breathing normally (hence with low speed of gas), the reg does not freeze. Instead, when you press the purge button creating a very fast flow, then some energy is transformed from enthalpy to kinetic, hence the expansion becomes more adiabatic, and the temperature drops.

 

[lowviz]

OoooooKaaaay. I had thermo in the late 60's/early 70's. I'm good up to G=H-TS.

After that, I'll believe anything...

 

[Angelo Farina]

OoooooKaaaay. I had thermo in the late 60's/early 70's. I'm good up to G=H-TS.

After that, I'll believe anything...

I will try to make it simple. It is just the good old principle of energy conservation.
When you pack the air in a cylinder, you are storing a lot of energy as potential (elastic) energy.
When the air expands, it releases this energy.
If you manage to convert it to something useful (as making a turbine spinning, and doing some useful work, for example driving an alternator and producing electricity) then the gas releases all this potential energy. And expanding without getting heat from the surrounding, it becomes very cold (down to well below 0 C, that's the freezing point of water).
If instead the expansion is not transferring this energy to everything else, this energy remains in the gas, keeping it at the same temperature as when it was at high pressure, inside the cylinder.
The more energy you extract from the expanding gas, the colder it will be at the reduced pressure.

 

[CaveSloth]

in a lamination valve (as the first stage of a regulator) no work is obtained by the gas expansion.

I think this is the key to your argument and I would like proof of this assertion. I don't really know how the valve works, but I can't imagine how the gas can't or doesn't transfer any of its potential energy to SOMETHING.

 

[Storker]

This is all entirely wrong, I am sorry. Expansion cooling occurs only if entropy is kept constant, that means a fully reversible expansion process.

So, when my 2nd freeflows and freezes up - which has happened a few times - that's "a fully reversible expansion process"? I don't think so, and I also think that you've got your thermo dead wrong. Cooling doesn't happen only on isentropic expansion, it also happens on e.g. adiabatic expansion.

Assuming air is a perfect gas (which is almost perfectly, but not entirely), the temperature is directly proportional to entalpy: H = cp*T . Hence, as enthalpy is constant, also the temperature will not change.

Well, now we're into "not even wrong" territory.

 

[lowviz]

@casual observers to this thread: This could ONLY happen on SB...

 

[Marie13]

@casual observers to this thread: This could ONLY happen on SB...

Eh? I must have missed something...

 

[Angelo Farina]

So, when my 2nd freeflows and freezes up - which has happened a few times - that's "a fully reversible expansion process"? I don't think so, and I also think that you've got your thermo dead wrong. Cooling doesn't happen only on isentropic expansion, it also happens on e.g. adiabatic expansion.

Well, now we're into "not even wrong" territory.

In both cases the expansion is almost perfectly adiabiatic. Adiabatic means without heat exchange, and this is true in both cases, as the expansion is fast, and there is not enough time for a significant heat exchange between air and the environment.
The difference is between an expansion with no energy conversion (hence all the originally enthalpy is available to the gas after expansion, keeping its temperature almost the same as before expansion) and an expansion where a significant amount of energy is transferred to another form of energy (kinetic, in case a fast flow is emitted).
Of course a purely iso-enthalpic expansion and a purely iso-enthropic expansion are just the two extreme cases, and the reality always lies in between. If the gas flow is slow, still a small amount of energy is transformed to kinetic energy, so the expansion is never perfectly iso-enthalpic, and there is always some temperature decrease.
And even in a full open-valve expansion a fraction of the initial potential energy is not fully converted to kinetic energy, hence the temperature drop is not as large as the theoretical reversible adiabatic formula would predict.
But the faster the air flow, the larger the temperature drop. This is easy to verify...

 

[Ayisha]

...And then there is this single slide I created to try and explain it in brief:
View attachment 557579

From your graphic above: "Freezing in 1st stage usually contributes to freeflow, but can sometimes result in restricted air flow."

Now you've complicated matters with downstream regs, which fail open (ex freeflow), and upstream regs (ex the seating mechanism sits upstream of the orifice), which fail closed (ex give no air).

On a side note, I always have complete confidence in my Sherwood Blizzard regs: balanced, environmentally sealed, downstream regs.

 

[John C. Ratliff]

In both cases the expansion is almost perfectly adiabiatic. Adiabatic means without heat exchange, and this is true in both cases, as the expansion is fast, and there is not enough time for a significant heat exchange between air and the environment.
The difference is between an expansion with no energy conversion (hence all the originally enthalpy is available to the gas after expansion, keeping its temperature almost the same as before expansion) and an expansion where a significant amount of energy is transferred to another form of energy (kinetic, in case a fast flow is emitted).
Of course a purely iso-enthalpic expansion and a purely iso-enthropic expansion are just the two extreme cases, and the reality always lies in between. If the gas flow is slow, still a small amount of energy is transformed to kinetic energy, so the expansion is never perfectly iso-enthalpic, and there is always some temperature decrease.
And even in a full open-valve expansion a fraction of the initial potential energy is not fully converted to kinetic energy, hence the temperature drop is not as large as the theoretical reversible adiabatic formula would predict.
But the faster the air flow, the larger the temperature drop. This is easy to verify...

Okay, please explain the Combined Gas Law:

(P1 x V1) / T1 = (P2 x V2) / T2
Ideal gas law - Wikipedia

Also, please explain why, upon filling a tank, the temperature of that tank is noticeably warmer?

And, why my MR-12 II regulator second stage froze up in 39 degree fresh water?

And why, my Sherwood Magnum Blizzard regiulator, with a fully protected first stage (piston spring is in air) and heating vanes of copper in the second stage, doesn't freeze up?

Now, for all you really worried about freeze-up, there is a paper (which I'll find) about regulator recommendations for the Antarctic diving. Also, know that most double hose regulators are virtually immune to freeze-up, as both the first and second stage units are housed in air, and not subjected to ambient water.
[abstract] SCUBA REGULATOR PERFORMANCE FOR UNDER-ICE SCIENTIFIC DIVING OPERATIONS

SeaRat