My Scubapro MK 20 & G500 Froze Up For The First Time

[cleung]

[QUOTE
I think your Mk20 should be fine for those temps, with a little adjustment.
First, make sure it's been upgraded to the composite piston that Scubapro marketed for the Mk25. Second, lower IP to 125 and consider going even lower. Third, try to accept a slightly higher cracking effort from your second stage, which may help to minimize excessive gas flow from easy triggering. Of course, don't make it harder to breathe than you are comfortable with. Don't test breathe out of the water at the surface - just a tiny check of valve function, preferably with your reg just underwater before submerging.

I'll bet your set will continue to perform just fine at 8°C or below![/QUOTE]

If the IP was adjusted to lower, would this affect my reg when I bring it to tropical warm waters? This is the same reg that I've been using for both warm and cold water diving.

 

[Angelo Farina]

A question. I read here that for cold water it is recommended to lower the IP.
I suppose that doing this improves the situation for the second stage, but make it worse for first stage.
Adiabatic cooling is proportional to the pressure ratio, so lowering the IP will increase the pressure ratio on the first stage...
So I suppose that this recommendation makes sense only for a sealed diaphragm first stage, which is very difficult to freeze.
I would not recommend it on an unsealed piston, such as the MK20.
What do you think?

 

[rsingler]

If the IP was adjusted to lower, would this affect my reg when I bring it to tropical warm waters? This is the same reg that I've been using for both warm and cold water diving.

Nope. When you tune a second stage to the firsts IP, it's all the same. You can dive the lower IP full time. You may get a raised eyebrow from the shop if your ask for an IP of 115, but there's no risk to it. Your G500 can handle that just fine.
At some point in reducing IP, the second stage can't provide a light enough spring pressure to not breathe stiffly, but for a G500, that's probably beliw 100 psi. Another reason I love center balanced designs like the new D420. I have normal breathing right down to 60 psi IP. Why might that matter? Because if I get stupid and ruin my tank right down to 100 psi before I notice I'm almost OOA, my second stage will still breathe easily right to the end.
Now there are older guys than me that say you should use a downstream second stage that breathes harder as IP drops, so you get some warning that your tank is getting low. That's too old school for me. When I'm OOA, I don't want to be fighting for that last breath.

Yeah, go ahead and tune for cold water year round. Just be prepared to be firm with your LDS, and ask for a cracking effort of 1.2" from your G500 at that IP.

 

[rsingler]

A question. I read here that for cold water it is recommended to lower the IP.
I suppose that doing this improves the situation for the second stage, but make

3000 to 135 vs
3000 to 115?
I can't imagine the adiabatic cooling will be that much worse. But I'm not a physicist.

 

[Angelo Farina]

3000 to 135 vs
3000 to 115?
I can't imagine the adiabatic cooling will be that much worse. But I'm not a physicist.

The pressure ratio changes from 22.22 to 26.00, an increase of more than 15%.
It looks significant to me...
And, if I remember correctly, the ratio should even be raised to an exponent larger than 1...

 

[rhwestfall]

There was definitely a big flow of air running into my mouth while I also felt/heard bubbles from behind my head which is where my first stage is near. Also one of the other divers in my group saw air coming out from behind me as well. And yes, it was a yoke system but I most definitely felt the constant rush of air rushing into my mouth. All of this seemed to subside a bit after a bit of time on the surface as I made my way to shore.

I really didn't do anything different on my next two dives 2 days later except it was in 18 degrees C water temp, another location and two other rental tanks. So the water temp was a 10 degrees difference.

Had that happen to me at the bottom of Gilboa when entering the trailer.... then worked fine up in the shallows. Happened again at depth on the Crystal on Erie. Rebuild time....

 

[rsingler]

The pressure ratio changes from 22.22 to 26.00, an increase of more than 15%.
It looks significant to me...
And, if I remember correctly, the ratio should even be raised to an exponent larger than 1...

You always make me think, @Angelo Farina!
Okay, so I looked up isoentropic adiabatic cooling.
The formula when pressure changes are known is:
T2 = T1 times (P2/P1) raised to the 0.375 power, if we assume that gamma for air is 1.6.

[The nitty gritty is that P2/P1 is raised to the power of:
(gamma-1)/(gamma), or
(1.6-1)/(1.6), or
(0.6/1.6), or
the 0.375 power.]

Using a temp in Kelvin of 280°K (~45°F) for T1 to start, the formula shows a difference in temperature drop of only 9°F between the two pressures. Probably because we're raising to a fractional power. (Results are 87°K vs 82°K for 135 and 115 psi IP).

The wrinkle is that the formula gives us an isolated temp drop to around -300°F for that instantaneous adiabatic cooling. And that's not what happens in the water. Lots of things are slowing down the process. But at least the formula shows us that the theoretical difference in drop is not statistically significant.

If I'm using the right formula, lol!
Thanks for making me work for that one, Angelo!

 

[Angelo Farina]

@rsinhler, that formula holds for a perfect gas doing an isoenthropic (reversible) expansion, like in a turbine.
Cooling in a reg is driven by entirely different phoenomena: irreversible expansion of a real gas. Furthermore there is an additional effect, which is called the Joule-Thompson effect, also causing cooling.
I do not have my thermodinamics book by hand, but I remember that the formulas were different and more complex...

Only thing I remember is the value of gamma, which is 1.41 (biatomic molecules such as N2 and O2). But gamma refers to the equation for isoenthropic reversible expansion of a perfect gas, which says p*v^gamma = const.
But, as said, under the conditions inside the first stage, air cannot be considered a perfect gas, and the expansion is entirely irreversible, as no work is produced...

 

[cleung]

I really don't understand all this technical detail as my top of mind question is still --- do I need to get a new regulator?

 

[rhwestfall]

Clint:


I'd suggest it be serviced... it may have not been done right the last time, or something is wrong with a component (service item: seat, o-ring, etc.). Stuff happens....

That being said, I'd also ask the tech to not set the IP at the upper range of the manufacturer's specifications. Within, but at the lower end..