Unbalanced Piston

[avocadogirl]

Could anyone tell me what is meant by unbalance piston, referring to a first stage? And, when I am told that it's a suitable first stage for warm water diving at limited depths, could someone explain what is generally considered to be "warm water" diving? Realistically, would that be tropical waters, or say, all diving around Texas: anything for which I wouldn't need a dry suit?'

Any feedback is greatly appreciated. I'd really appreciate opinions either in favor of or against this design, personal experiences, and the like. Thank you.

 

[redacted]

Start here for different designs of 1st stages: Scuba Regulators

Unbalanced piston 1sts tend to be somewhat lower performance because their intermediate pressure is effected by tank pressure. To minimize this effect, they tend to flow less gas than other designs. But they also tend to be more reliable than other designs because of fewer parts and less parts exposed to high pressures.

 

[Akimbo]

In short, Texas is no problem. Warm water would be a gross overstatement.

The unbalanced first stage piston is the oldest volume production design. "Old" should not be taken as derogatory because it is extremely reliable and simple. There is a slightly greater risk of a balanced or unbalanced piston regulator without an anti-freeze kit to freeze up in very cold water. I have personally used one under 28° sea ice in the 1970s without incident, but with extra precautions.

Anything above the 40° F water temperature and above a 0° F air temperature is fine. Basically, you have a piston with two O-rings exposed to the water. The refrigeration effect of expanding gas across the seat cools the temperature on any regulator. Open a Scuba valve to drain most of the air out and you can observe cooling and condensation occur.

It is possible for water to freeze inside the ambient pressure sensing chamber of the first stage and prevent the piston from freely moving. I have never heard of a regulator freezing closed, even in extreme environmental chamber testing. They freeze open because of design characteristics and that is when the greatest refrigeration action occurs.

You also get second stage freezing, also in the free-flow or open position, for the same reasons. In my experience, it is actually more common because thin layers of fresh water from rinsing and expiration condense on metal parts — at least with the regulators and procedures we used then.

 

[GrumpyOldGuy]

Unbalanced piston regulators generally have nothing in place to prevent freezing. They are a KISS (Keep it simple, stupid) design that is robust and good enough most of the time. If you plan on diving in temps lower than 50F or much deeper than 100' then their limits may affect you. Within these limits, they are a viable and low cost option.

 

[halocline]

Could anyone tell me what is meant by unbalance piston, referring to a first stage? And, when I am told that it's a suitable first stage for warm water diving at limited depths, could someone explain what is generally considered to be "warm water" diving? Realistically, would that be tropical waters, or say, all diving around Texas: anything for which I wouldn't need a dry suit?'

Any feedback is greatly appreciated. I'd really appreciate opinions either in favor of or against this design, personal experiences, and the like. Thank you.

Some unbalanced piston design regs have great cold water reputations, like the SP MK2. They're actually less likely to freeze in sub 40F water than their balanced piston cousins. "Limited depth" is a little misleading, the truth is that the MK2 will provide more air than virtually any 2nd stage can handle under full purge. IP drop during inhalation, which is an indicator of performance, is slight with my MK2 as I'm able to measure it. (That's good)

I would feel perfectly comfortable taking my MK2 to any depth that I'm likely to dive to. I've had it at 130 ft and there was absolutely no problem with performance.

Much of the lower performance reputation for this type of 1st stage is actually due to the 2nd stages that are frequently paired with it. If you were to pair a MK2 with a G250, (high performance 2nd stage) you'd have a very hard time telling the difference between it and a much higher flow 1st stage. An exception to this would be if several divers at extreme depth were huffing and puffing off the same 1st stage.

Lots of dealers confuse, either deliberately or out of ignorance, the effects of balancing with the reg's performance at depth. Balanced/unbalanced has nothing to do with depth compensation; it is solely an issue of how the 1st stage responds to lower tank pressures. In reality, I can tell you that a MK2/R190 will begin to perceptibly breathe a little harder under 500 PSI, but until the tank reaches 500, you'll have a very very difficult time noticing any change in performance.

 

[herman]

The rest have covered it pretty well. I will just add that while they are an older design they are still the work horse regs of diving. They are simple and very reliable, perfect for the new diver starting out. Sure some of the more expensive models will breath a LITTLE bit better.....assume the reg is set up by a reg tech who knows what he/she is doing and actually cares... but the vast majority of divers can not tell the difference. Certainly nothing wrong with them unless you are doing some serious tech or cold diving, otherwise an all around good deal in preformance and price.

 

[AquaNSun]

For an unbalanced piston, the HP (high pressure) gas coming from your tank is directly acting on the HP seat. As the tank pressure drops, the HP force acting on the seat is also drops. For example, at a full tank of 3000 psi, you will read an IP (intermediate pressure) of 145psi in your first stage and then at the end of a dive, 500 psi, you will read an IP of 125psi. The drop in
pressure gave the name "unbalanced" to first stages with this type of design. The effort needed to open the valve is much easier at 145psi, assistant from HP gas, than at 125psi given the same spring tension and thus unbalanced first stage is a little harder to breath below 500psi tank pressure. This difference is usually not significant and a good feature to let you know you're low on air.

For a balanced piston, the HP air comes from the side and does not act directly on the HP seat. So, when you see a first stage that looks like an "L" shape, it's a balanced first stage and when you see a first stage that's straight and looks like an "I", it's an unbalanced first stage. Since the air in the balanced first stage doesn't act directly on the HP seat, no assistant, it always required the same amount of force to open the valve regardless of IP pressure. The tank pressure drops just like the above example but in the intermediate chamber the pressure has to build up to a constant amount, say 145psi, to overcome the spring tension. At the end
of a dive, your tank is down to 300 psi but the IP in your first stage is still built up to 145psi and so it breaths just as smooth as you started. As a result this type of design is called balanced design.

The Mk2 is a great inexpensive reg for divers on budget because there is really nothing that can go wrong with this reg. You can take it deep (130 ft), take it cold (40+ F) and with over 40+ years on the market all bugs had been worked out. And a dumb guy like me can DIY with my mk2, you can do it too in future

Welcome and best wishes.

 

[Akimbo]

The sentiments expressed above are far more accurate than a lot of the regulator hysteria I read on the Net. This post may be reassuring to anyone considering the purchase of an unbalanced first and second stage regulator:

http://www.scubaboard.com/forums/5195415-post51.html

 

[avocadogirl]

The sentiments expressed above are far more accurate than a lot of the regulator hysteria I read on the Net. This post may be reassuring to anyone considering the purchase of an unbalanced first and second stage regulator:

I would agree. Thank you all for your having taken the time to reply so thoughtfully. It's a real help for someone like me, for whom this is all pretty new, to make a more informed decision and become, generally, more knowledgeable of what's out there in the scuba world.