Question First stage Balance regulator and Performance With Increasing Depth

[Ngel]

Hi, I've been reading some of previous post:

• balanced-reg
• regulator-performance-with-increasing-depth

And after those reading, my understanding about how a first stage balanced regulator impact on WOB at depth is due to the fact that balanced 1st stage keep 'almost' constant IP -> allows manufacturer to increase the orifice size -> allowing greater gas flow -> Better performance -> Less WOB

Please comment on or correct my current understanding of it.

 

[axxel57]

Balanced 1sts keep an almost constant IP over the range from full tank to near empty tank.
If they would provide a constant IP absolute while descending, you could not dive very deep without getting serious problems.
That's why ambient pressure has to be transmitted to the main spring which thereby is increasing the IP absolute, securing that you will have a constant lung volume also at depth.
The WOB is mostly influenced by the performance of the 2nd stage ( balanced-unbalanced, Venturi etc.)

 

[stuartv]

All 1st stage regulators for open circuit scuba use are depth-compensating. That means that they output a specific Intermediate Pressure at the surface, and, as depth increases (which means ambient pressure is increasing), the IP increases by the same amount.

For example: If the 1st IP is 147 psi, that is 10 ATM (atmospheres). That means, at the surface the LP hose to the 2nd stage has 147 psi (10 ATM and 10 ATA) in it. If you then descend to 33' of depth, that means ambient pressure is now 2 ATA (atmospheres absolute). Thus, inside the LP hose from the first stage, the pressure will be 147 psi + another 14.7 psi, or (approximately) 162 psi absolute (AKA 11 ATA) - or still 147 psi above ambient.

Some 1st stages are sold as "over-balanced". That means they are saying as you descend (i.e. as ambient pressure increases), the 1st stage IP output increases by MORE than the increase in ambient pressure.

From the above example, that means that at 33 feet, instead of being 162 psi absolute, it will be something more than that. This is claimed to be a benefit by making the 2nd stage flow even more air the deeper you descend. It is debatable if this is a practical benefit. If your 2nd stage reg is tuned for the absolute best cracking pressure it can handle, then increasing the IP at depth means you are eventually going to make it freeflow.

Many 1st stages are also balanced - like the ScubaPro Mk25. Being balanced is independent of it being "depth compensating".

Some are not balanced - like the ScubaPro Mk2.

For all of them, once the cylinder pressure that is feeding the 1st stage is above some threshold - say, around 200 or 300 psi - then they all will give "almost" constant IP. That is their job.

Being balanced means that the 1st stage will maintain that "constant" IP for longer as the tank gets close to empty. But, again, it only really comes into play when the tank is down geting near empty.

2nd stage regulators also come in balanced and unbalanced varietys.

All the top shelf regulator sets will have a balanced 1st and a balanced 2nd. But, as long as either one is balanced you get almost all the benefit. Having both balanced adds very little improvement in performance over having either one balanced and the other unbalanced.

How much a 1st stage flows it pretty much dependent on the size of the orifice inside the LP port. If you made the orifice really small - like they are inside a HP port - then it would output a fixed IP, but when you inhaled from it, the IP in the LP hose would drop dramatically while the 1st stage tried to catch up.

With a big orifice inside the LP port, more gas can flow and when you inhale the IP will drop much less.

I really do not know if being balanced or unbalanced has anything to do with the size of the orifice inside an LP port. I don't see why it would, but I am not a regulator engineer...

If your purpose is to determine what to buy for yourself, I would expect any namebrand 1st stage on the market to work well enough that, paired with an identical 2nd stage, you would not be able to tell the difference in the different 1st stages. They are all made to be able to flow enough gas for 2 people to breathe off of it and also inflate a wing at the same time. I really doubt one person breathing normally could tell any difference between modern 1st stages.

 

[L13]

Hi, I've been reading some of previous post:

• balanced-reg
• regulator-performance-with-increasing-depth

And after those reading, my understanding about how a first stage balanced regulator impact on WOB at depth is due to the fact that balanced 1st stage keep 'almost' constant IP -> allows manufacturer to increase the orifice size -> allowing greater gas flow -> Better performance -> Less WOB

Please comment on or correct my current understanding of it.

For a 1st stage, balanced just means IP does not drop as tank pressure drops (until the very end).

For a 2nd stage, balanced means tuning (WOB) changes very little as IP changes.

For a balanced 2nd stage, 1st stage balancing provides no (or very little) advantage on orifice design.

For an unbalance 2nd stage, increasing the orifice size increases the area affected by an IP pressure change, which increases the change in WOB as IP changes. This makes balanced 1st stages even more desirable for unbalanced 2nd stages with large orifices than ones with small orifices.

As @stuartv said, as long as at least one of the stages is well balanced, WOB does not change significantly with tank pressure.

 

[eelnoraa]

Some 1st stages are sold as "over-balanced". That means they are saying as you descend (i.e. as ambient pressure increases), the 1st stage IP output increases by MORE than the increase in ambient pressure.

I want to add to this. The so call "over balance" of 1st stage is NOT by design. It is in fact the limitation or the by product of the balancing scheme. Even if they want it to, this scheme cannot NOT over balance. then manufacture figure out a way to advertise the limitation as a feature. I won't pay too much attention t this.

 

[Eric Sedletzky]

I always thought over balanced meant that as the tank pressure dropped the IP would actually go up a bit due to how the secondary spring pressure works on the balance chamber side of a diaphragm reg. So with some diaphragm regs the normal IP would be 140 lets say, but as tank pressure drops and pressure is less on the HP spring, the HP seat spring takes over and allows pressure to rise a few psi to say 145 psi. Only until tank pressure drop below IP then everything goes down.
On a balanced piston reg the IP will actually drop a bit from full tank pressure to low tank pressure. Pistons do not have a balancing chamber, they are balanced by a design configuration, or lets say they side step the problem by not allowing tank pressure to press against a spring like on an unbalanced MK2. Amazingly, balanced pistons also have only one moving part. Fantastically simple piece of engineering.

If you use a balanced 1st stage and an unbalanced second stage, the second stage will breath fine until the tank gets to IP pressure then it will gradually breathe harder until tank is empty. You will have warning because the pressure will lessen on the spring of the second stage making breathing harder and harder with each breath. But you will have warning. This was US Divers strategy with the Conshelf/1085 reg combo.

If you use an unbalanced 1st stage with a balanced second stage it will breathe great all the way down past IP until it pretty much is out. High quality pneumatically balanced second stages take almost nothing to work, like 10 psi, due to how tank pressure is used to shut off the poppet, and as soon as you work the lever it pulls away the poppet from the orifice to allow more air to flow.
Another genius design but they work maybe too good and give no warning when you run OOA.
This genius is seen in the legendary Scubapro G250, the most copied second stage of all time.

If both 1st and 2nd stage are balanced you better have good SPG paying attention skills because you will get no warning whatsoever.