temperature drop in first stage...

[will_tekkie]

As I recall, most second stages operate with an input pressure about 100 psi more than ambient, so a calculation can be made of a typical adiabatic temperature drop in a second stage. Since T1/T2=(P1/P2)^0.286 (both teperatures and pressures are absolute), the adiabatic temperature drop in 33 feet of 68F sea water would be about 180F and in 100 feet of 68F sea water, about 130F. Obviously, we don't breath air chilled that much, so the transfer of heat within the second stage is very important - and difficult to estimate theoretically, thus requiring measurement.

i´m agree again...obviously a adiabatic expansion is not according to the actual process (as we said at the beginning of the threats) so if you read the article above you will see some lab tests made in regulators that can help a lot...real temps are shown...therefore as we made above the heat transfer can be estimated from the presures and temps measured in the cases presented...of course all values are rough and the real ones should depends of each case, there are too many factors involved...a dive in the sea is not the same than a reg tested in a lab.

 

[sndt1319]

Well we could assume that the water moving around the diver is flowing laminerly. Also we could assume that the ocean is a heat reservor and therefore has a constant temperature for use in the convection and radiation calculation. To calculate the Reynold number we could further assume a constant velocity of the water. For the flow of the air from the first stage to the second stage I think we could assume turbulent flow as the transition from the first stage to the hose would trip the flow into turbulent once the air moved. How to model the point at which the air starts to accelerate though I don't know about. Would the hose act as a reheater for the air leaving the first stage? It seems like you would want to increase the rate of heat transfer from the water to the air if the air is really as cold as that article implied. So you would want thin metal regulators right? A better question would be at what temperature drop would you want to have the 2nd stage insulated to trap the heat you give off as opposed to have the thin metal 2nd stage to ues the heat in the water to heat the air going into your body? If the air heats up in the hose, it might be one more reason to go the long hose route.

 

[pufferfish]

If the air heats up in the hose, it might be one more reason to go the long hose route.

Now there is a reason for a long hose I have never seen before on any list

 

[WarmWaterDiver]

I think the flexible hydrocarbon based materials used for the hoses act as pretty good thermal insulators instead of thermal conductors relative to say metal in the first stage - and to use the water one is diving in as a heat sink for the air in between the first stage and second stage, metal with extended surface area like the heat exchanger built into the Aqua Lung Glacia second stage inlet as an example seems it would be quite a bit more efficient than a longer hose of a flexible material with higher R value for thermal insulation. The question would then come down to how much extended surface area is enough? This would again depend on the temperature of the gas leaving the first stage, which is a function among other things of differential pressure (what tank pressure is available for that breath) and what approach to isothermal instead of adiabatic heat transfer a particular first stage achieves. Also, a stationary diver in an environment with no discernable current (sound familiar to photographers at times?) would have to rely on whatever flow occurs by natural convection across the first stage and second stage hose for any water flow in these areas to transfer heat. All in all, WAY too many variables to model, a BUNCH of empirical data for a BUNCH of conditions would have to be taken and attempted to correlate (like building a neural network program for expert systems).

 

[Norm]

As you and others have implied, lots of factors to consider.
I know that a temperature drop of 50 degees F is not at all uncommon below ambient!
Norm

 

[sndt1319]

I think what we need to decide is why do we want to know the temperature drop. What do we plan on doing with it? Also, as has been stated there are a lot of variables to this problem. Instead of looking at the whole system we should break down the whole system into four parts: the tank, the first stage, the hose, and the second stage. Then we can start to put values with our variables. Where we are really interested in what is going on in the system are at the minimum and maximum values. So we would want to look at the values for natural convection in the ocean as well as the maximum safe operating current for say a commercial diver when we talk about the external cross flow velocity and the max and min operating preasures for the tank. Then we would want to look at a combination of values at both the max, min, and mean values. If we found a base equation using only variables like x or y we could put that into a program like Maple or Matlab and then write a program to put our max, min, and mean values into the variable. But again we would need to know exactly what usable data we are after. Personally I think that the temperature drop isn't what we should be directly after but rather how the diver is effected by that temperature drop. How much body heat is lost to heating the air and how that effects core temperature at the end of the dive. (By the way, if any of you are students at Oregon State University, I think this could make a cool senior project for next year and would be interested in working on it.)

Brian

 

[WarmWaterDiver]

Well, attempting to use science to ascertain the maximum safe moisture content of one's compressed breathing gas supply to minimize potential for a first stage internal freezeup is the main application I see. See Pufferfish's use of both the empirical and theoretical data in this regard on page 5 of the thread I posted on page 2 of this thread (link below to page 5). Note also that rescue workers in a gas environment must have a far closer approach to adiabatic operation of the first stage of their breathing apparatus than scuba divers in a liquid environment, and what such workers in Pufferfish's neck of the woods have experienced. I think you might have a pretty decent senior project on your hands for addressing these real-world issues.

http://scubaboard.com/showthread.php?t=47081&page=5&pp=10

 

[pufferfish]

I think that would be an excellent project to look at taking different types of regulators under fixed conditions and vary the moisture content of the gas and see when they fail.

Last winter we did about two dozen dives at 2 atm under the ice with a bunch of divers and I kept track (when I wasn't freezing) of the freeflows. The gas was checked that week and had a dewpoint of < -80 C which is somewhere well below -100 F. Most divers were using 3000 psi tanks but a couple had 3400 psi. On the first dive of the day when the second stages were dry and the tanks still warm it became pretty obvious that the piston regs were a liability except for the Sherwood regs. The MK25AF/S600 would start out ok but after about ten minutes into the dive would slowly start to free flow and if you were unlucky it would go into a full free flow. I used it for a dive and it was fine but one never knew what it might do. You can't dive like this under the ice. It was on the second dives when the tanks had been sitting in the snow and the second stages were wet that divers had problems with their second stages. Outdoor air temp was about -5 C and water temp about 1 C or 34 F.

What really stood out over two different weekends was the fact that the Apeks ATX200 demo reg we had never freeflowed once even on a 3400 psi tank, being used by divers with poor cold water technique, and tanks sitting in the snow for four hours. The reg was used by six different divers of all levels and not one first stage freeflow. I only saw the Apeks second stage freeflow once and this was on a second dive right at the beginning but the second stage had been sitting in the snow and was wet. The diver put the second stage in the lake to thaw and no problems after a few minutes.

That was enough empirical data to convince me of the benefits of a diaphragm reg over a piston reg in those conditions. Apeks sold three sets of regs that weekend I believe.

What I think would be very interesting to do now is to take the Apeks regs and see how sensitive they are to internal freezing by varying the moisture content of the tank air. Some researchers (both Canadian and US )believe that it is really the materials within the first stage that determine freeflow characteristics and not so much the moisture content of the air.

I'd say a cool project would be to take the best known cold water regs (Apeks, Posiedon, Sherwood) and expose them to air with different moisture contents to find out which ones are the most freeze resistant as moisture rises. Of course tank pressures, sac rates, water temps, etc. would have to be fixed.

Dr. Neil Pollock a researcher at Duke University (see article in this month's Alert Diver by him) did some empirical research a decade ago on the Sherwood Maximus regulator at McMurdo Station in the Antarctica. Have a look at this attachment from a paper he wrote in 1995 called "Sherwood Maximus Regulator Temperature and Performance During Antarctic Diving" Mastro, J and Pollock, N.

The water temperature was -1.8 C and the dive was a photographic survey. The diver's sac rate was about .51 scfm. A thermocouple was inserted in the body of the first stage. I'm not an engineer but looks much more like an isothermal situation under the ice vs the pure adiabatic one the firemen must deal with up here during the winter at 1 atm.

 

[sndt1319]

I would love to test all those regs. It would make a great senior project and I am sure I could find a professor to sponser it. So now if someone would just send me an Apex ATX200, an Abyss(don't want to leave out you Mares guys), a Blizzard, an Extreme, and a Mk25/S600 I can get started on it right away. Of course since I can't currently afford my own reg yet the winner might not be returned. Also the others might be slightly altered so I can attatch thermal couples and such. So any volunteers?

Brian

 

[pufferfish]

Here's another figure showing the mean first and second stage temps for six different divers. Pretty consistent temps it seems and the heat retention plate was only significant at the deeper depths where the delta was about 1.5 C warmer with the plate when below 90 fsw. The mean temp difference with that plate across all depths was only .6 C.

No you can't drill any holes in the body of my ATX 200 for thermocouples :11:

Maybe Apeks and Sherwood might help you out. Don't know about SP though as they didn't fair so well at McMurdo either.