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What's the actual effect on diving with a reg that has the wrong IP?

Discussion in 'Regulators' started by Bigd2722, Jun 8, 2015.

  1. Bigd2722

    Bigd2722 Solo Diver

    # of Dives: 500 - 999
    Location: Winter Park, fl
    For example; if my reg has an IP of 110 (should be 135) does it breathe harder? Does it respond differently at depth? Does it respond differently at different tank pressures? And vice versa, if the IP is too high?

    I'm not asking about the effect on the equipment just how the diving/breathing is affected.

    By the way, this is not an actual scenario, just a curiosity.

  2. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    for "normal" regulators, i.e. scubapro/apeks style, which is basically everything except Poseidon, technically nothing, technically everything.

    If the second stage is tuned for the IP of the first stage, whether it is low or high, within reason there is no measurable effect. If the second stage is tuned for 135, and the IP is low then the cracking pressure is slightly harder because you now have to compensate for the spring pressure that is not being offset by the IP, if the IP is high, then it will start freeflowing because the IP overcomes the spring pressure.

    With Poseidon Xstream and Jetstreams, the opposite is true, not enough IP will cause the regulator to freeflow, and too much IP can cause it to lock up *though it will start freeflowing at a certain pressure but they require OPV's to keep the IP down so they breathe normally. The pro to this is that in a HP seat failure, the second stage will still breathe normally with a leak in the first stage, where on a traditional second stage you will be breathing from a freeflow. The con is some people wouldn't notice the freeflow behind their head if they're oblivious....
    Most second stages can be tuned between about 110 and 150 and stay within cracking effort and WoB numbers.
  3. halocline

    halocline Solo Diver

    If you have a balanced 2nd stage, the IP has little effect on the cracking effort, because it's IP that is re-routed and provides the opposing force keeping the 2nd stage closed. So when the IP goes down, pressure on both sides of the 2nd stage valve go down, but it's not completely equal. Balanced 2nd stages have some 'downstream bias' meaning the amount of balancing air pressure is slightly lower and is supplemented with a light mechanical spring. This is to allow the regulator to free-flow in the event of an IP spike.

    If you have an unbalanced 2nd stage, changes in IP will have a dramatic effect on cracking effort, because the opposing force is a mechanical spring. Unbalanced 2nds need to be tuned to a specific IP to work even reasonably well. Balanced 2nds also get tuned to a specific IP to work their best, but are much more tolerant of a range of IP.

    There's another issue, that being flow rate of the first stage. The higher the IP, the greater this is, and it means that there is higher pressure air in the hose, which also means a slight performance increase even with tuned 2nd stages. As an example, I can tune my best performing balanced 2nd stage to a balanced 1st (MK10) at 120, then do the same thing at 135, and I'll notice a difference in testing. It's not a cracking effort issue, its a flow issue, and the greater the venturi assist in the 2nd stage, the more it seems to change.

    But that doesn't mean that higher IP is necessarily better. I keep mine pretty low, around 125, and accept the slightly lower flow rate because it's still PLENTY of air. If you have an atomic set (wasn't that your thread about the unstable IP?) I'm sure it's the same. Technically it will flow more air at 135-140, but since the flow rate is so high, low IP (120-125) will still perform great. And there might be less of a free-flow sensitivity. 110 is pretty low, though, I wouldn't leave it there.

    There's no effect on how the regulator compensates for depth, it works exactly the same way regardless of IP. Same for tank pressure, except theoretically as the tank pressure approaches IP. By that time you're either on the surface or looking for an alternate source of air.
  4. axxel57

    axxel57 Solo Diver

    The downstream force of the IP is an important factor for the performance of a 2nd stage.

    In conjunction with other factors like the diameter of the orifice, the spring tension, the lever advantage, the seating force and the Diaphragm force it produces at a certain IP range and adjustment an optimal cracking and inhalation effort.

    There is quite some difference the 2nds are reacting to a lower IP between an unbalanced and a balanced 2nds, but it is not that big as some might expect.

    Usually translates in my tests a 3-6 psi IP drop into a 0,1inch/O2 increase in cracking effort in unbalanced 2nds and a 6-9psi drop into a 0,1inch/O2 increase in balanced 2nds when the 1[SUP]st[/SUP] stage is adjusted to an IP of 125 - 145psi (tank test pressure 3000 and 600psi).

    So if the 1[SUP]st[/SUP] stage was originally adjusted to an IP of for example 140 and the 2nds adjusted according to that IP to 1inch/O2, and for whatever reason the IP drops to 110psi at full tank I would expect an increased cracking effort of about 0,5 – 1inch for unbalanced and an increase of approximately 0,3 – 0,5 inch/O2 for balanced 2nds.

    That does not put in account the decreasing IP over the course of the dive.

    So if the IP is ‘wrong’ in this example I think one would feel the difference with an unbalanced 2[SUP]nd[/SUP] very clearly, but also with a balanced 2[SUP]nd[/SUP].

    It would be a different thing if the 1[SUP]st[/SUP] stage was adjusted on purpose to that low IP of 110psi and the 2nds were adjusted using that low IP.

    I see also another problem in adjusting the IP quite lower than the specs of the manufacturers.

    When breathing from a regulator the dynamic IP will decrease from the static IP. How much depends on the type of regulator. With an IP adjusted below the specs the dynamic IP will decrease more than normal.

    This will increase the inhaling effort in the beginning of the inhaling process even more until the Venturi assist kicks in which will reduce, neutralize or override this increasing inhaling effort.

    With a ‘lower’ air flow (IP) I would expect a later ‘kick in’ of the Venturi assist which could cause harder breathing .
    Apart from that I think could a pretty low dynamic IP cause problems in an emergency if in greater depths for example a panicking diver has to be provided with quite more air than normal. This should cause a further dynamic IP drop.

    I’m not sure which ‘low’ dynamic IP could be really a problem in general, but when a first stage in our flow tests shows a dynamic IP of less than 100psi, we automatically change the main spring.

    That almost always solves the problem.

    I think there are some good reasons to adjust the 1[SUP]st[/SUP] stages accordingly to their specs.:)
    halocline likes this.
  5. Bigd2722

    Bigd2722 Solo Diver

    # of Dives: 500 - 999
    Location: Winter Park, fl
    Nope not mine

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