Piston vs. Diaphragm

[scubabunny]

I thought all Scubapro 1st stages came with T.I.S. (thermal insulating system), which makes it safe for cold water..except for extreme ice-diving. Note..from what I understand, that is NOT an enviormental seal..which makes it safe to dive in contaminated water. Am I completely off-course on this?

In Guam, cold water diving is not a big selling point, but as we sell to alot of military that get transfered to all points...I need to know the difference.

Thanks for any info.

 

[turnerjd]

Scubabunny,

Firstly an old thread

http://www.scubaboard.com/showthread.php?threadid=1083

The key point is that in freezing it is a mixture of things that happen. However, from basic physics, the greatest pressure change leads to the greatest cooling effect.

Tank - Regulator intermediate pressure (ie pressure change within the first stage) is 232bar - 10 Bar = 222 Bar on a full tank (assuming a 10 Bar IP - it is normally about 9.5 or so.

Regulator IP - ambient (inhaled) pressure = 10 Bar - due to the design of first stages, IP is always ambient + about 10 Bar.

So, in the second stage we have only a small pressure change, and in the first stage we have a large pressure change - hence the greatest temperature change, and the probable cause of the problems will be in the first stage.

With a change in pressure in the first stage it is quite often that the first stage is well below freezing. - Go out on a cold winter morning, and look at divers ready to get in the water. If they have been breathing off their reg before they enter the water (which is a bad idea) then there is often ice on the first stage which is condensed humidity from the air.

So, to overcome this, there are two solutions:- ScubaPro's TIS, and the 'Keep it Dry' of everybody else.

Fundamentally, The scubapro idea is that they allow as much water as possible into the reg workings so that the negative heat energy (ie coldness) is carried away by the water. This is fundamentally the wrong direction to go. If you want to avoid freezing - why have water flowing through/into the first stage????????

Everybody else takes the attitude that if there is no water in there it can't freeze. There are several ways of doing this, Apeks have their dry seal membrane at the bottom, which acts on a diaphragm to provide the ambient pressure part of the first stage workings. Sherwood have their dry-bleed system - which is essentially a system that allows a small ammount of air to escape, and hopefully taking any moisture away with it. Other systems include filling the inside of the first stage with oil.

As for second stages - they need a heat exchanger and a reasonable waterflow round it so that the air that arrives from the second stage isn't too cold, but otherwise they aren't too big a problem.

To test this all out, I put my old Apeks T20, which is their bottom of the line second stage, without any fancy controls or heat exchanger on my DST first stage, and dived in Stoney cove in the middle of winter (yes I did have my pony with a different reg setup incase it didn't work!!) and I had no problems with freezing whatsoever.

By far and away the best way of avoiding problems cold water diving the keys (that I have learned/observed) are

1) don't breath off the reg untill you are in the water
2) go slowly and steadily - don't rush round all the time
3) Get the correct exposure protection - uncomfortable and cold divers will overbreath a reg causing problems
4) Relax:- it's the same as diving in the tropics - It's not hard!

HTH

Jon T

 

[scubabunny]

Hmmm....a cold winter morning..forgot what they are! hehe

Seriously, I have never dove anywhere that didn't have palm trees so I don't know ANYTHING about all that. But, like I said, alot of people we sell to are transfering to Washington State..or Alaska, and they ask these questions.

Thanks for your info, and I will definitely check out the thread.

Which reminds me..is there an "archives" section so people can do serches to get the relating threads? If not..this may be a good idea. There is a TON of info on this board, and going through every past thread is sooo time consuming.

Oh..if this already exists..can someone please point me in the right direction????

 

[jmsdiver]

A previous thread sounded like a plug for Apeks. So, the question is why the affection for a diaphragm reg? What bennies (save being sealed) does a diaphragm offer over a piston? I have been diving strictly piston regs and find them very easy to breath from. Just curious....thanks

 

[scorpiofish]

None. Differences in performance are basically nil. Piston fans say it is a simpler design and easier to service. While true, the difference is negligible.

If you look at the actual testing of high performance regs, you will notice that both piston and diaphragms are well represented.

Scubapro has made a tidy profit with pistons now haven't they? The MK20 piston has always been the first choice over the MK14 diaphragm design.

If I needed cold water defense, I would prefer to just have a diaphragm instead of having to pack silicone in a piston.

Bottom line is that it doesn't really matter any more. Performance is what counts.

 

[Ontario Diver]

jmsdiver;

I think it was the thread that I started that you are referring to.

It turned into a bit of an APEKS love fest, but if you notice it was really aimed at a specific scope of diving. There was support for Scubapro as well.

The split between piston and diaphragm seems to be trivial. Both can be either very good or very bad. Piston is technically much simpler but under a failure condition has a greater chance of catastrophic failure, the intermediate pressure is managed by the ratio of the piston ends and is difficult to tune. Diaphragm has more components but less prone to failure and has creeping failures. Intermediate pressure is managed by the strength of the return spring and tunes a bit easier. Both need good servicing and annual may not always be enought for super keen divers.

A number of the reg comparison articles I researched didn't even tell whether the first stage was diaphragm or piston.

My final thought is that for the majority of divers (60-80 ft, warm or cool water, no cave or wreck penetration, minimal environmental conditions) will do very well with either style. For extreme cold, the way the diaphragm works seems to be better.

(by the way, this may not all perfect and true, tis only what I've learned and understood in the last month) Let the flames begin!

 

[Lost Yooper]

First of all, whether the reg has a sealed first stage or a piston first stage they are all capable of freeze up. I've had a Zeagle T50D freeze up a couple times, and I've heard of a lot Scuba Pro's freezing up. The only reg that seems to be a reliably good cold water reg is the Posiedons (piston firsts). The key to cold water (less than 40F) performance is lowering the IP.

The Apeks line has an unbelievable track record for reliablity and extremely low maintenance among extreme technical cave divers. Some of these guys haven't so much as touched the guts of their Apeks regs for years, and they're doing dives that none of us would even contemplate. However, the consensus seems to be that a finely tuned Scuba Pro is a slightly better breather than a finely tuned Apeks.

Quite often you will see tech divers using Scuba Pro's for their deco regs due to their "easier" breathing. The other reason they use Scuba Pro's for their deco regs is that a piston reg can be literally taken off a bottle underwater and placed onto another bottle and used. Granted, this would only be done in a pinch, but it can be done with a piston first stage.

As for the majority of divers, I doubt most can tell the difference in breathing between an Apeks reg and a SP reg. It basically comes down to if you want a clean and dry first stage, parts availabilty/service, and of course, price.

Later.

Mike

 

[neil]

Mike,
Are you sure Poseidens are piston regs? I don't think so, but I'm not that familiar with them.
Also, why wouldn't you be able to take a diaphragm reg off one bottle and put in on another U/W?
That makes no sense to me. Once you de-pressurize it, you should be able to switch any reg U/W, although you're probably going to get some water in either one. Remember, (or learn ) once the pressure is off, both type of valves are open. If you can find out why piston regs are considered better for this, I'd like to know.

Ontario Diver,
Actually, intermediate pressure is regulated by the main spring in both types. (It's not really a "return" spring, the spring is an opening force on the reg).The diaphragm reg's spring is externally adjustable while the piston's is usually not. (there have in the past been piston regs with external adjusments). In the newer SP designs, the IP is adjustable with different thickness seats, rather than spring shims. Intermediate pressure is the closing force on first stages; the spring is an opening force, so the IP is set by the strength of the spring.

neil

 

[Lost Yooper]

Hey Neil,

The older 300's were pistons, but maybe the newer firsts are diaphrams. I guess I haven't paid much attention. I see some guys putting rubber caps full of alcohol on the firsts to keep them from freezing. Bob3 would know for sure. Regardless, the Posiedons are famous for their cold water performance. I don't like 'em though.

I heard that piston firsts will allow the water to move through the first stage and into the hoses, and a diaphram won't without damaging the guts of the first stage. You might be able to get away with it in a diaphram first, but a rebuild would be required. This hole thing can be avoided by simply using DIN for everything because if it doesn't break, then you don't have to worry about fixing it underwater.

Take care.

Mike

 

[neil]

Originally posted by Lost Yooper
Hey Neil,

I heard that piston firsts will allow the water to move through the first stage and into the hoses, and a diaphram won't without damaging the guts of the first stage. You might be able to get away with it in a diaphram first, but a rebuild would be required.

Take care.

Mike

Mike,
I see what they mean about the piston reg; the water would only collect on the HP side of the piston while the reg was de-pressurized (where there isn't much except the face of the HP seat and the small end of the piston now sealed against it), while the water in a diaphragm reg could possibly get onto the balance chamber and HP seat right away. Good point, but I'd wanna rebuild either one! Of course I'm assuming that it's seawater.
In a piston reg, some water could still get up through the piston into the IP side, and that's where the piston slides. I've seen regs ruined by corrosion in this area. Sea water is nasty stuff.

Neil