Balanced versus Unbalanced Piston

[rscbrowland]

Was looking at the Oceanic Alpha 8 for my wife (previous Dacor Pacer XL finally gave up the ghost). Ran into the SP5 unbalanced versus the DX3 balanced. Both claim consistent breathing at all depths so I'm sure I am missing something. First post on the board so hope I am doing this correctly.

Thanks

 

[Bubbletrubble]

I interpret "consistent breathing at all depths" to mean depth-compensating. The function of the first stage is to step down the pressure of tank air to 130-150psi above ambient pressure. All first stages are designed to be depth-compensating. I guess what I'm trying to say is that "consistent breathing at all depths" is just marketing language.

In general terms, an unbalanced reg will become increasingly harder to breathe as the tank pressure falls to low levels. Contrastingly, a balanced reg will maintain "easy" breathing until the tank gets down to very low pressures (essentially "empty").

FWIW, it has been argued that an unbalanced reg can be a nice backup indicator to a novice or inattentive diver, since when breathing resistance increases, it will almost certainly prompt the diver to check his/her SPG. It could help such a diver avoid an OOA situation.

All of my comments above refer to balanced/unbalanced first stages. Not to confuse you, but second stages can also be balanced and can be paired with either kind of first stage.

 

[herman]

To add a little to BTs post, the increase in breathing resistance in an unbalanced piston stage is not that great. Most divers will not even notice it until the tank pressure drops below 200psi or so. Even a balanced first stage will do the same thing once the tank pressure gets in the same range and this assumes you are using an unbalanced second stage with an unbalanced first stage. Like BT indicated, second stages come in balanced and unbalanced. Pairing a balanced second stage with an unbalanced first will give you the same effect as having a balanced first stage, having both balanced is OK but it is also redundant because if you balance one stage there is no need to balance the other.
Bottom line is, its a non issue for most divers and if you pair a simple unbalanced first stage with a good balanced second stage you will have a solid dependable reg that will perform darn near as good as any other reg.

 

[chile7236]

i had the old alpha 7 (w/ the SP first stage...i think it was the 5 as well) and it was a very good reg....at all depths (down to 150-160ft at least). never got to the point of getting below 500psi and never had any "breathability" issues.

 

[redacted]

The problem with an unbalanced piston comes in if and when you get to working hard. To minimize the IP drop as tank pressure drops, unbalanced pistons use a fairly small seating surface which limits flow rates. Balanced pistons are capable of higher flow rate than you could ever use. Unbalanced pistons have much lower flow rates and may cause some discomfort in conditions that generate high breathing volumes. It will deliver enough gas to keep you alive but it may force you to reduce activity level and, most importantly, may make you feel like you are not getting all the gas you want. My wife ended up going from a Mk2 to a Mk20. But the Mk2 is great as a pony reg, for backup, or as a tire inflator.

 

[chile7236]

true...and i think fitness level plays in here as well. but for the avg, rec diver, this reg is not a bad buy at all.

 

[AquaNSun]

As you research more on this board, you'll also find that even with the lower flow rates of unbalanced 1st stages, they are still flowing more than most 2nd stages can handle. So, don't be afraid to buy one as they are super reliable in return. They will look after you in many years to come for recreational diving. A common misconception is that most people are given the idea that 'unbalanced' is BAD but thanks to Awap for explaining it clearly that those are just potential discomforts in certain conditions. There aren't bad regulators out there, just different designs for different purposes and people (and obviously prices).

 

[halocline]

Actually the flow rates are more than ALL 2nd stages can handle. The problem with overbreathing a NB piston 1st stage might occur with two divers breathing hard off the first stage at depth with low tank pressures.

The more relevant issue is IP drop and recovery, especially when dealing with unbalanced 2nd stages. A 1st stage that has a big IP drop on inhalation will be sending less pressure to the 2nd stage during inhalation, and that means (theoretically, anyhow) more work to keep an unbalanced 2nd stage open. With a balanced 2nd, the large drop in IP is partially compensated for with a drop in pressure in the balance chamber, so it takes less pressure to keep air flowing through the stage.

My MK2 has a pretty decently low IP drop on inhalation. If I dive with a high performance, well tuned 2nd stage on both the MK2 and my highest flow 1st (MK15) I can't tell any difference. But, if I put a R190 (lower performing, unbalanced 2nd) on each, I can tell a difference.

I can't comment on the oceanic, but I have read in the Vance Harlow book that the MK2 has better IP drop and recovery than some of the other NB piston regs of a similar design. That would not surprise me, although if the oceanic is a fairly new reg, it's possible that it wasn't around when the Harlow book was written.

 

[rscbrowland]

Appreciate all the replies and the education. Ended up getting my wife the Alpha8 with the SP5. Based on lots of input this appears to be a decent lower cost option for getting her back into diving and then if we end up diving more often, we can upgrade.