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

Discussion in 'Regulators' started by cleung, Oct 14, 2020.

  1. cleung

    cleung Contributor

    # of Dives: 100 - 199
    Location: Mississauga, Canada
    773
    80
    [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.
     
  2. Angelo Farina

    Angelo Farina Marine Scientist

    # of Dives: 1,000 - 2,499
    Location: Parma, ITALY
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    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?
     
    couv likes this.
  3. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

    # of Dives: 500 - 999
    Location: Napa, California
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    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.
     
  4. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

    # of Dives: 500 - 999
    Location: Napa, California
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    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.
     
  5. Angelo Farina

    Angelo Farina Marine Scientist

    # of Dives: 1,000 - 2,499
    Location: Parma, ITALY
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    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...
     
  6. rhwestfall

    rhwestfall Woof! ScubaBoard Sponsor

    # of Dives: 200 - 499
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    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....
     
  7. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

    # of Dives: 500 - 999
    Location: Napa, California
    5,880
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    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!
     
    couv and stepfen like this.
  8. Angelo Farina

    Angelo Farina Marine Scientist

    # of Dives: 1,000 - 2,499
    Location: Parma, ITALY
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    2,849
    @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...
     
    couv and rsingler like this.
  9. cleung

    cleung Contributor

    # of Dives: 100 - 199
    Location: Mississauga, Canada
    773
    80
    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?
     
  10. rhwestfall

    rhwestfall Woof! ScubaBoard Sponsor

    # of Dives: 200 - 499
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    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..
     

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