scubapro question

[sylvester]

I have and use a mk25af and s600, my experainces in the past have shaken my faith in scubapro in cold water, Since my 1st big freeflow, and having the mk25 upgraded to AF, I have not had it in cold (below 40) water, but my LDS told me It wouldnt hurt to detune it a bit on the IP pressure prior to going cold and deep to prevent problems. He mentioned a bit as 5psi.

So my question is, those of you who dive the same rig in cold deep water do you do this? What do you lower your intermediate pressure to?

I know some of you will tell me not to use scubapro in the cold, or to get a Posiedon, thanks anyway, but I have the Scubapro and will continue to use it.

 

[Greg Barlow]

Lowering the IP a small amount will not hurt the regs performance to a point that you will notice.

I use Mares regs and service those and some other brands. I typically reduce the IP on all models that will be used in water below 50 degrees. Mares calls for 142-152psi, while I limit the setting to 135-140. Other manufacturers often follow the same protocol with pressure reduction.

The AF conversion should also help with the issue. The problem with free flows is that it can be difficult to identify the cause of the initial problem.

Greg Barlow

 

[Genesis]

Ok, for those who want to know WHAT detuning actually does, here 'ya are...

1. Detuning reduces the maximum flow rate through the regulator. If you have a maximum pressure differential of 150psi, you can flow more gas than if the differential is 125psi. That is where it "helps", to a degree, as the adiabatic cooling is determined by pressure change x volume. Less volume, less heat drawn out.

2. HOWEVER, note that other term - pressure CHANGE. Detuning INCREASES the pressure change, which does the OPPOSITE of what you want. The result is not quite intuitive, in that you'd think you'd always gain from turning down the pressure. This is not quite so.....

3. There is a third factor, and that is SUPPLY pressure. Low pressure tanks are less-likely to freeflow than high pressure ones, because the delta-P is lower, and thus there is less heat bled off across the first stage orifice.

Now all this assumes that the first stage is what has frozen - either externally OR the vapor in the tank's gas. Note that the latter can happen EVEN WITH A FULLY SEALED FIRST STAGE. This is a significant risk if a tank is filled in a warm place with warm ambient air and then taken into a very cold place for diving. In other words, if you're going ice diving, filling your tanks at 90F is probably a bad idea.

If the SECOND stage is what has frozen, then maximum airflow is the only limiting factor. Detuning will help here, as turning down the pressure reduces the maximum flow rate.

Just realize that what you're doing here is (intentionally) reducing the ultimate flow perforamance of your reg. That this "works" without making the reg impossibly-hard (or dangerous) to breathe is proof positive that these things today are marketed as "superflow" without regard to the REAL requirements of the job - that is, beyond a certain point (that which you can actually consume, worst-case) extra flow actually does you no good and CAUSES the problem you're now trying to prevent!

 

[DA Aquamaster]

Genesis is talking about adibatic cooling - the "cooling" that occurs when the pressure drops from the tank pressure to the intermediate pressure.

The air molecules have to take up a lot more speace indivudually and need to increase their kinetic energy to acoomplish this so they absorb heat from the reg and ultimately from the water.

The greater the pressure drop the greater the adibatic cooling (well almost, it's not quite a linear function but close enough). So a 2250 psi steel 72 will minimize the adibatic cooling issues compare to a 3000 psi AL 80 or a 3500 psi HP steel tank. Lower pressure is definitely better. I dove my run of the mill MK 25 D400 all season on steel 72's in water as cold as 39 degrees with no problems. My MK 25 AF 3000 psi tank equipped buddy had several freezeups on the same dives and our SAC rates are for all practical purposes identical. So in my opinion goign to a low pressure tank will do a lot more for you than detuning the reg.

Flow rate is also important and one of the MK 25's problems is that it can move a whole lot of air and place an excessive demand on it's thermal insulating system. Genesis is right in regard to lowering the IP aggravating the situation because it increases the pressure differential and the consequent adibatic cooling. But the amount of this increase is very very small compared to the benefits of reducing the flow rate and the volume of air producing the cooling effects in the first place.

So in the end you gain more by lowering the flow rate by lowering the IP than you give up by increasing the pressure differential by 15-20 psi. You reduce the tank pressure by 15-20 psi in the first few breaths anyway. If you are real concerned underfill your tank by 20 psi and call it good.

What I have heard recommended by the SP rep is to reduce the IP to 125 psi and then crank down the adjusting knob on the second stage to prevent any free flows. This basically turns your high performance reg into a lower performing reg that breathes like a rock.

Personally I don't think any of this is needed if you use a low pressure tank and are careful not to suck a lot of air and/or simultaneously breathe air and use a power inflator or dry suit inflator. You also want to avoid any longs blasts on the inflator and want to avoid testing or breathing off the reg until the first stage is fully under water.

 

[Genesis]

... is that these "high performance" regs that freeze all deliver more air than any two divers could EVER need at one time, even at stupid depths, and at those depths you'll be on Helium-based gas anyway (which flows easier)

So what is the point of that, other than to cause freeflows?

 

[DA Aquamaster]

I agree completely. I saw no benefit to the MK 25 and went back to my tried and true Mk 10's. The Mk 10 delivers more than enough air at depth and yet not the excessive amount delivered by the Mk 20/25. It also uses the very reliable but slightly more maintainence intensive Silicone Protected Environmental Chamber.

I got the impression that SP moved away from the SPEC system as the regs are easier to service if you do not have to deal with last year's silicone. And that the service is cheaper if you don't have to refill the ambient chamber. But even with Christo-lube at $27.00 for two ounces you can still do five or six regs per tube so it would only add $5.00 or so to the annual service cost. I'd pay that for reliable freeze protection.

 

[Genesis]

There are 3 seats in a rebuild kit, and most of the time two are usable (one without shims, one with)

This gives you twice the rebuilds for the same price if you're doing your own and have a box full of O-rings

 

[DeepScuba]

Genesis and DA:

BRAVO!!!

I want you two to come down and explain this very same thing to my shop........

is that these "high performance" regs that freeze all deliver more air than any two divers could EVER need at one time, even at stupid depths,

AND

Just realize that what you're doing here is (intentionally) reducing the ultimate flow perforamance of your reg. That this "works" without making the reg impossibly-hard (or dangerous) to breathe is proof positive that these things today are marketed as "superflow" without regard to the REAL requirements of the job - that is, beyond a certain point (that which you can actually consume, worst-case) extra flow actually does you no good and CAUSES the problem you're now trying to prevent!

I just last week, had argued the exact same point FOR ANY HIGH PERF REG.

It's stupid, that a reg (1st stage) has to deliver THAT much, because in fact it doesn't, and only diminishes the safety aspect beyond what 2 divers could effectively use while breathing hard at 250ft.

Damn you guys are smart!

I will say one thing, and I've said it before. "We" MUST have some VERY DRY air up here, as our divers use plenty of Mk20/25 with top IP's and have NO freeflows while in 30-39 degree water all the time. Our end on Lake Ontario never sees temps at depth of probably more than 42.

It truly amazes me that you guys have the problems you say you do, as it's just NOT a problem up here. It's gotta be the moisture in the air, or something along those lines.

 

[Norm]

"Way back when", Sherwood used to recommend that you run a slightly love I.P. on the Blizzard than on any other models. I think those days are gone. With things such as sealed first stages, balanced second stages, better compressors, metal second stages, metal parts within a second stage, teflon coatings and the like, when the reg isn't exposed to operator error, freeze ups can usually be prevented. Every single freeze up I have ever seen was, (in my opinion), due to operator error:
Breathing off a reg before being entering the water, a predive freeflow, rapid Valve-On mistakes by an anxious diver, (which is hard on a reg for more than one reason), moist air in your tank, using a warm tank in COLD water, using a tank that has been exposed to extreme cold before a dive, (although I have never personally had a problem with this), or diving DEEP in COLD water and placing a HIGH DEMAND on your regulator to deliver, are but a few of the common mistakes that divers make when diving in a cold environment.
The one I cannot explain is a friend that had a freeze up on his new reg on every dive precisely at 80'.
Norm