ice particles from MK25?

[WarmWaterDiver]

Moisture from the scuba tank isn't impossible, looking over various breathing gas specifications. But, this would be more likely to freeze up in the innards of the first stage, as the adiabatic cooling effect, if completely adiabatic in practice (pretty much requires a perfect vacuum), is only about 3 degrees F. In actual practice, we experience some approach to adiabatic effect. That being said, one's mouth is a moist environment with essentially 100% relative humidity at mouth temperature, which is greater than second stage regulator innards temperature. So, after the first exhaled breath, the second stage is a subcooled moist environment. This is where the 3 degrees comes into play as systems with higher thermal insulatingproperties (plastics and such polymers) will then allow a greater approach to 100% adiabatic effect than materials who are good thermal conductors such as metals, like DA has mentioned. These materials allow your surroundings to effectively be used as a 'heat sink', even if those surroundings are 'cold' water. Note the second stages that incorporate heat exchangers with fins for extended surface area for cold water diving - they're designed for more use of the environment's capacity as a 'heat sink' than others.

Now, in the first stage, the potential temperature drop achievable if a 100% approach to adiabatic operation is much more significant, and of course varies with tank pressure. The higher the tank pressure on any given inhalation cycle, the greater the 100% adiabatic approach temperature drop theoretically achievable. The as-filled moisture content of the tank is a big part of the equation here, and can vary significantly. Again, your environment acts to some degree as a heat sink. There are posts on this board regarding cold-climate area rescue workers experiencing first stage freeze-ups as air is less dense and is a less effective heat sink than water, and can be a fluid at lower temperatures than water. So, since the ice passed into your mouth rather than lodged in either your first or second stage at some flow passage area, it's likely moisture that left your body with your exhaled breath. But, moisture in the tank fill gas isn't something to dismiss.

 

[donclaypool]

The same thing used to happen to me when the water temp got into the 30's with my trusty ol' MK5 with 109 2nd (pictured in avatar). Unless it's too annoying, I wouldn't worry, mine never freeflowed once.
Don

Diving cold water with my new SPMK25 and ice landed on my tounge.
Assuming it was getting frosty I quickly stuck in my Xstream (on an H valve).
Now I understand why people love em. But my question is, where does frost
come from? Is it the second stage seat?

 

[FFMDiver]

would it be better to use an old metal cyclone 2nd as a heat sink?
Thanks for all the informative commentary.

The same thing used to happen to me when the water temp got into the 30's with my trusty ol' MK5 with 109 2nd (pictured in avatar). Unless it's too annoying, I wouldn't worry, mine never freeflowed once.
Don

 

[donclaypool]

would it be better to use an old metal cyclone 2nd as a heat sink?
Thanks for all the informative commentary.

I'll have to be honest and opt out in deference to those cold water gurus up north. I have seen plastic bodied regs freeze up and freeflow. I am not intimately familiar with the newer regs as my rig is the only one I've dived extensively. But metal, like you're implying, is the old low tech solution.

When I bought my regs, used, I asked about their cold water worthiness as I'd be diving the Bering Sea. The seller produced an old magazine with the picture of my reg hanging out the mouth of a research diver in the arctic, his head protruding from a hole in the ice. SOLD!

Don

 

[donacheson]

Isn't it moisture from breathing, not water (from the tank or otherwise) that condenses inside the second stage?

It would seem to be the most likely source, but the expansion can't be close to adiabatic. For example, assuming that after expanding in the first stage and passing through the hose to the second the air will be close to 25C, adiabatic expansion from 130 psia to 30 psia would produce about a 100C drop in temperature, but nothing like this occurs in the real world. (I've assumed the reg intermediate pressure is 100 psi more than ambient). One simply doesn't breathe air from a second stage nearly that cold.
But how cold? I don't know. Cold enough to condense and freeze the moisture in one's breath on a part of the second stage? Could be, especially in cold water when the diver's breathing hard.

 

[WarmWaterDiver]

donacheson,

Here's a link to a thread where I ran the calculations for air temperature drops if 100% approach to adiabatic conditions was achieved, on page 4 post #32. I don't get anything close to what you show for the drop between the first stage IP and mouthpiece of the second stage. I used Redlich-Kwong general thermodynamic correlations if I remember right - what correlations are you using that result in 100 degrees C drop for 100 PSI drop?

http://www.scubaboard.com/showthread.php?t=47081&page=4&pp=10

 

[donacheson]

donacheson,

Here's a link to a thread where I ran the calculations for air temperature drops if 100% approach to adiabatic conditions was achieved, on page 4 post #32. I don't get anything close to what you show for the drop between the first stage IP and mouthpiece of the second stage. I used Redlich-Kwong general thermodynamic correlations if I remember right - what correlations are you using that result in 100 degrees C drop for 100 PSI drop?

http://www.scubaboard.com/showthread.php?t=47081&page=4&pp=10

I used the thermodynamic equation Tl/Th = (Pl/Ph)^0.286, where Tl and Pl are the temperature and pressure on the low pressure side and Th and Ph on the high pressure side; ^0.286 denotes raising to the power of 0.286. For Pl=30 psia, Ph=130 psia, and Th=300K, Tl=197.2K.

As a check, the dry adiabatic lapse rate in the atmosphere is known to be about 10C per kilometer. For the standard atmosphere rom the surface to 1 km, Ph=1013 mb, Th=290K, Pl=900mb (all values approximate), Tl=280.4K.

 

[WarmWaterDiver]

I used the thermodynamic equation Tl/Th = (Pl/Ph)^0.286, where Tl and Pl are the temperature and pressure on the low pressure side and Th and Ph on the high pressure side; ^0.286 denotes raising to the power of 0.286. For Pl=30 psia, Ph=130 psia, and Th=300K, Tl=197.2K.



As a check, the dry adiabatic lapse rate in the atmosphere is known to be about 10C per kilometer. For the standard atmosphere rom the surface to 1 km, Ph=1013 mb, Th=290K, Pl=900mb (all values approximate), Tl=280.4K.

donacheson,



Still struggling with your approach - based on your post, the last 100 PSI of differential pressure has more drop in temperature (100 C = 212 F) than the first 2800 and some change differential from the tank to the first stage intermediate pressure (25 C = 77 F) (if using a 3000 PSI tank pressure as one example, but in any case, a tank pressure of > or = 130 psia or else there is no DP for gas to flow across the first stage to the second stage - or first stage intermediate pressure area, depending on perspective).



That what I saw as a first level check of your post, and that data doesn't 'add up'.



I used Hysys.process, a well developed process simulation software tool that I use on the job. Looked over my notes, I used ideal gas thermodynamic properties package. I'll summarize the results again. For the second stage, the basis I used was 125 PSI differential between ambient and intermediate pressure (often listed as suggested setting for cold water diving on this board). For the first stage drops, I used the125 PSIG I.P. and tank pressure as the tank pressure decreses during the dive, and highest pressure drop (which yields highest temperature drop) would be the first inhalation - remember, we're talking about 100% adiabatic (theoretical maximum temperature drop) operation here.

3 degrees F drop for 110 PSI drop (i.e. I.P. drop across second stage valve)
36.5 degrees F drop for 1675 PSI drop (1800 PSIG tank pressure minus 125 PSIG I.P.)
46.5 degrees F drop for 2275 PSI drop (2400 PSIG tank pressure minus 125 PSIG I.P.)
58 degrees F drop for 2875 PSI drop (3000 PSIG tank pressure minus 125 PSIG I.P.)
60.3 degrees f drop for 3375 PSI drop (3500 PSIG tank pressure minus 125 PSIG I.P.)
67.3 degrees F drop for 4225 PSI drop (4350 tank pressure minus 125 PSIG I.P.)


http://www.hyprotech.com/products/family.asp?ID=1



Maybe you've got a typo somewhere???

 

[donacheson]

donacheson,



Still struggling with your approach - based on your post, the last 100 PSI of differential pressure has more drop in temperature (100 C = 212 F) than the first 2800 and some change differential from the tank to the first stage intermediate pressure (25 C = 77 F) (if using a 3000 PSI tank pressure as one example, but in any case, a tank pressure of > or = 130 psia or else there is no DP for gas to flow across the first stage to the second stage - or first stage intermediate pressure area, depending on perspective).



That what I saw as a first level check of your post, and that data doesn't 'add up'.



I used Hysys.process, a well developed process simulation software tool that I use on the job. Looked over my notes, I used ideal gas thermodynamic properties package. I'll summarize the results again. For the second stage, the basis I used was 125 PSI differential between ambient and intermediate pressure (often listed as suggested setting for cold water diving on this board). For the first stage drops, I used the125 PSIG I.P. and tank pressure as the tank pressure decreses during the dive, and highest pressure drop (which yields highest temperature drop) would be the first inhalation - remember, we're talking about 100% adiabatic (theoretical maximum temperature drop) operation here.

3 degrees F drop for 110 PSI drop (i.e. I.P. drop across second stage valve)
36.5 degrees F drop for 1675 PSI drop (1800 PSIG tank pressure minus 125 PSIG I.P.)
46.5 degrees F drop for 2275 PSI drop (2400 PSIG tank pressure minus 125 PSIG I.P.)
58 degrees F drop for 2875 PSI drop (3000 PSIG tank pressure minus 125 PSIG I.P.)
60.3 degrees f drop for 3375 PSI drop (3500 PSIG tank pressure minus 125 PSIG I.P.)
67.3 degrees F drop for 4225 PSI drop (4350 tank pressure minus 125 PSIG I.P.)


http://www.hyprotech.com/products/family.asp?ID=1



Maybe you've got a typo somewhere???

No typo. I can derive it easily from the First Law (but I'd have to examine the derivation closely to determine if any Second Law has subtly crept in.) The only case I was considering was the 100 psi drop across the second stage, reasoning that by the time the air had flowed from the first to the second stage it would have warmed back to ambient.

The equation I used treats air as an ideal gas. That's a reasonable assumption. In any case, the expansion in either stage is not even close to adiabatic.