Cold Water 2nd Stages

[TN-Steve]

Just wanted to see if what I think I know passes the "SB" test.

Things that would make a good cold water second would be things like a metal body, metal air tube inside, and often a finned heat exchanger where the hose mounts to the body. All of these are to help bring heat from the water into the reg, and even if the water is 40 Deg, it helps to keep it from freezing.

I'm wondering if a 90 deg swivel would also help, since it's a fair sized piece of metal with a lot of surface area, or is it too far away from the actual orifice / valve seat to do much good?

Since a reg like the Conshelf XIV is metal, would it by defination (or in practice) be considered a "Cold Water Reg".

I know that in a lot of cold water setups, the IP is set a bit lower, in the 125 - 130 range rather than 140-145. Is that to also take some of the heat loss from expansion off of the 2nd stage?

Not asking for any particular reason, just trying to get a few things to click.

Thanks,

Steve

 

[tbone1004]

part 1, basically yes.

part 2, not really, too far away. It is too far before the final reduction in pressure and the rubber hoses actually get the IP air to close to ambient temperature by the time it gets to the second stages, it's the major drop in pressure that kills it.

http://divelab.com/download/procedu...baRegulatorFreezing-ChillingFacts-4-9-14.pdf/
read that if you want to get all the info

 

[bl6394]

Cosign what tbone1004 said.

In my Scubapro equipment tech clinic, it was suggested to use a swivel on the second stage in cold water (e.g. < 40 F. fresh water) for just that reason. More metal in contact with water = larger heat sync = helps prevent the second stage from freezing up.

 

[tbone1004]

Bjorn, I think the bigger one is to not use hose covers on either end of the hoses, that seems to work quite a bit better than a little extra metal. It isn't going to hurt it obviously, and most of my second stages have swivels for comfort, but I've done more cold water diving with out them than I have with them with no issues. Biggest one is to not turn into a Hoover, make sure the exhalations are long and slow so it gives the body time to warm back up

 

[bl6394]

tbone - again we agree. I don't use hose covers - but that was also mentioned during the clinic.

With good technique - even a Mk 25 can be used in cold fresh water. But with bad technique - it is a nightmare. I've had many inexperienced students OW, AOW and Deep use a high performance piston regs in cool fresh water - (42-45) and and free flow. Generally - if I'm not sure about them - I prefer to see them on a environmentally sealed diaphragm regulator like the Mark 17 and a second stage with lots of metal (G250V). That's a pretty bullet proof combination - even for my biggest Hoovers.

For that matter - the Hollis regs (DC1 / 212) are really solid in cold water too.

 

[tbone1004]

Bjorn, my Poseidons don't do so bad in the ice ;-)

 

[Subcooled]

it was suggested to use a swivel on the second stage in cold water (e.g. < 40 F. fresh water) for just that reason. More metal in contact with water = larger heat sync = helps prevent the second stage from freezing up.

Given that the water is not one degree below freezing

How much does that little piece of metal affect, actually? The gas is not staying in that part for many seconds, is it?

I hope you understand that the more you exhale the more moisture there will be in the 2nd stage. Avoid physical excertion and stay fit. Don't brethe too keenly.

 

[halocline]

I know that in a lot of cold water setups, the IP is set a bit lower, in the 125 - 130 range rather than 140-145. Is that to also take some of the heat loss from expansion off of the 2nd stage?

The thought behind lowering the IP is to reduce flow from the 1st stage to the 2nd. Theoretically, lower IP does mean a smaller pressure gradient at the 2nd stage orifice, and hence less adiabatic cooling. But I seriously doubt that has any real-world affect. The lower flow does, though.

 

[TN-Steve]

The thought behind lowering the IP is to reduce flow from the 1st stage to the 2nd. Theoretically, lower IP does mean a smaller pressure gradient at the 2nd stage orifice, and hence less adiabatic cooling. But I seriously doubt that has any real-world affect. The lower flow does, though.

I'm not quite sure I'm tracking on the lower flow part of your answer, help me out. Regardless of the IP I'm going to be breathing the same volume of gas in a given time frame. For the sake of argument, lets say 20 lpm. At a higher IP, the downstream valve wouldn't be open quite as far, at the lower IP one would have to breathe a bit harder to open the downstream valve enough to get the same volume. (I'm ignoring the effect that venturi would have on breathing effort since I don't think that it factors into this)

The paper that Tbone posted really helped me start to get a grip on the mechanics going on. It also looks like that it can be highly correlated to breathing effort, (flow), and that once a freeflow occurs (intentional or otherwise) it becomes a positive feedback cycle and only gets worse from that point forward.

Thanks for making my brain work on a Sunday evening

Steve

 

[tbone1004]

Steve, it's still early and I've had a long couple of weeks with work so I didn't follow you completely, but hopefully this helps

The 1st and 2nd stages are both capable of delivering much more air than any 2 people could need. Lowering the IP does have some merit for lowering the temperature drop, but the lower flowrates and slightly higher cracking pressure help to prevent freeflows which from what I've heard is the big push for that change. The cracking pressure can then be adjusted back with adjustable second stages. Poseidon solved this on the Jetstream with the Dive/Pre-Dive switch and when it is in predive it is all but impossible to freeflow.

Regarding the flowrates, the flowrate is determined by your breathing rate essentially because the demand valve is never open all the way. Take big huge fast deep breaths and it will open farther and flow more, this is a double edged sword though because you need it to be fast enough to the point that there is enough time for it to stay closed and warm back up to ambient.


FWIW this is pretty much only an issue in true ice diving scenarios, people blow "cold water" way out of proportion and unless you are diving in sub 40F water or diving in sub50F water with below freezing air temperatures, this is mostly a nonissue.