Unbalanced First Stages

[PilotRPI]

I'm trying to understand how these regulator doohickies work. I've found some good diagrams at ukdivers, net, /equipment, /regulators, htm. Sorry I can post a URL yet so I had to butcher it.

For the piston 1st stage, it seems that the HP tank air is exerting an opening force on the valve. So it makes sense that as the tank pressure lowers, it is exerting less opening force, requiring more breathing effort to open the valve.

For the diaphragm regulator, it seems that instead of an opening force, the HP tank air is exerting a closing force on the valve. Does this mean that the opposite happens with a diaphram: that the breathing actually would get easier as the tank pressure decreases? Or is the valve diameter so small that the closing pressure created force is negligible? That would make sense since the air flow is less in diaphragm designs I guess.

Some seem to explain the breathing differences in that the openings are small, and less pressure means less flow, but the balanced designed seem to eliminate the HP affects on forces on the valve, not the opening diamters.

 

[Luis H]

In an unbalanced diaphragm first stage regulator it does get easier to breath as the tank pressure goes down.

The last unbalanced diaphragm regulator that was in production (that I am aware of) is the Poseidon Cyklon 300. I have several of them that I bought in the early 70's. This regulators are still around in the used market.

Another unbalanced diaphragm regulator is the double hose Aqua Master. It is a very popular double hose (there are probably more Aqua Master in existence than most all other DH added together, but this is just a guess).

Both of the regulators mentioned do get easier to breath as the tank pressure goes down, especially the Aqua Master.


To reduce the IP variations in any unbalanced first stage, the designers normally select a relatively small valve orifice.

In a perfectly balanced first stage, the forces due to the tank pressure do not affect the movement of the first stage valve and therefore the IP is not affected (or at least it affects it a lot less). The designer can therefore afford to use a much larger valve orifice in a balanced first stage. This allows much higher gas flow and therefore much quicker IP recovery during the inhalation cycle (or when pressing the purge button).

 

[PilotRPI]

That clears things up quite a bit. Thank you

 

[herman]

If your really interested in how a reg works, pick up a copy of Regulator Savvy. It does a really good job of describing the inner working of pretty much any reg ever built. A must have if you ever intend to do your own repair or just want to know what your reg tech is saying. Scuba Tools - Regulator Savvy Book

 

[halocline]

Actually, you can't quite say that an unbalanced piston gets harder to breathe as the tank pressure lowers. What happens is the IP gradually drops. If your 2nd stage is really sensitive to IP changes then the reg will get harder to breathe. With lots of experience diving an unbalanced piston/unbalanced 2nd stage, I can tell you that my reg does not breathe noticeably different until well under 500PSI.

You're right about the diaphragm; since its an upstream valve, the IP goes up as the tank empties. In the case of the old double hose regs, they do get noticeably easier to breathe as the tank pressure gets low, but so far (I'm a double hose newbie) I have to say I'm surprised at how little the breathing resistance changes on my DA aquamaster throughout normal tank pressures (2700-500 PSI)

Luis has so much experience with them that I suspect he can get a pretty good sense of how much air is in his tank by how his reg is breathing, but most divers couldn't do that.

 

[herman]

I agree with you on the DA. The effects of tank pressure on a Mistral, which is a single stage, is a lot more noticable.
Another consideration is the second stage, if you use a balanced second stage with a non-balanced first stage, the effects of IP shift are a lot less noticable.

 

[Tortuga68]

In an unbalanced diaphragm first stage regulator it does get easier to breath as the tank pressure goes down

I thought unbalanced regs became (slightly) harder to breathe as tank pressure drops and/or depth/ambient pressure increased?

 

[halocline]

I thought unbalanced regs became (slightly) harder to breathe as tank pressure drops and/or depth/ambient pressure increased?

Balanced/unbalanced has nothing to do with changes in ambient pressure. All regulators compensate for ambient pressure; if they did not, they'd only work at extremely shallow depths.

Unbalanced means that the supply pressure (from the tank, or from the 1st stage in the case of unbalanced 2nd stages) has an effect on how the valve opens. Balanced regs are designed so that a portion of the supply air is re-routed and counteracts the pressure on the valve, so that theoretically it has no effect.

Piston 1st stages are "downstream" meaning that the supply pressure is trying to open the valve. This means when the tank is full, there is a lot of pressure forcing the valve open. IP is the amount of pressure it takes to keep the 1st stage valve closed. So, higher supply pressure, higher IP.

Unbalanced diaphragm 1st stages aren't made anymore, which is why you probably assumed that all unbalanced 1sts drop IP throughout the supply range. Diaphragm 1st stages are upstream valves, meaning the supply pressure is trying to keep the valve closed. So, with high supply pressure, the IP necessary to close the valve is lower.

It's easy to confuse balancing and depth compensation because some manufacturers talk about "overbalancing" in which the IP supposedly increases more than changes in ambient pressure as the reg gets deeper. They should be saying "overdepthcompensating" but of course that doesn't have the nice marketing ring.

 

[Tortuga68]

Okay, thanks

So this is wrong?

"An unbalanced regulator suffers from the problem that when tank pressure gets low - around 500 psi, it starts getting harder and harder to breath"

Pneumatically Balanced Regulator

(not to pick on scubatoys specifically, I googled plenty of similar links)

Or am I just missing something and need to stop reading scubaboard at 1am

 

[Luis H]

Okay, thanks

So this is wrong?

"An unbalanced regulator suffers from the problem that when tank pressure gets low - around 500 psi, it starts getting harder and harder to breath"

Pneumatically Balanced Regulator

(not to pick on scubatoys specifically, I googled plenty of similar links)

Or am I just missing something and need to stop reading scubaboard at 1am

What Matt said is correct.

The statement you are referring to only applies to new regulators being sold today.

As I mentioned, the last unbalanced diaphragm first stage was the Poseidon Cyklon 300. I don't know how long was that first stage built, but I have several from the early 70's.

Therefore, the statements you are reading (in Scubatoys, etc.) are correct if you are limiting it to today's market of new regulators.