recomended depth limits for unbalanced regs

[Scubaroo]

Which regulator models have unbalanced diapghragm 1st stages?

I've seen Apeks regulators converted to unbalanced (for use on a rebreather) by taking off the end cap, removing the balance piston and environmental sealing cap and replacing them with a plain delrin disk.

 

[EricQ]

The only unbalanced diaphragm reg I know is the Poseidon 300.
With regards depth, my wife dived the deeper parts of the President Coolidge with an older US Divers unbalanced reg.She had no trouble at all.

 

[Saturation]

The only unbalanced diaphragm reg I know is the Poseidon 300.
With regards depth, my wife dived the deeper parts of the President Coolidge with an older US Divers unbalanced reg.She had no trouble at all.

How far down on the Coolidge? It drops down to > 60 meters or > 200'?

 

[EricQ]

Saturation, We went to the bottom of the swimming pool .For some reason it is not recorded in our dive log ,but was probably about 180'.
For our last dive we were to go to the propellors(>200') with scooters,but we chickened out. Instead we zoomed in and out of the majestic wreck behind Allan Power.An unforgettable dive.

 

[bigredbill]

was that if you should happen to get a hot fill, eg. they let it crank to 3200 instead of 3000, the reg would tend to free flow and end up dumping the extra air anyway. The reg was tuned to breath best at about 2500 and the extra pressure was enough to put it over the edge. For the same reasons that the IP goes down on an unbalanced piston, IP goes higher when you get a hotter fill, and with the non-adjustable 2nd I had on that rig, it was sometimes a bit frustrating to not have that extra air going into my lungs during a dive!!

 

[DA Aquamaster]

The IP on an unbalaced piston reg will increase with tank pressure. So if your reg is finely tuned to give minimum cracking effort at a full tank pressure, the IP will increase and the second stage will freeflow slightly until the tank pressure drops to the pressure where the reg was tuned. When I first started diving the tech that worked on my MK3 would always fine tune it on a full steel 72 at 2250 psi, so consequently it would freeflow everytime when connected to a full AL 80 at 3000 psi.

With an unbalanced diaphragm reg, the IP will increase as tank pressure decreases, so a finely tuned second stage will not begin to leak air until the tank pressure gets below the pressure where the second stage was tuned. For obvious reasons you want this pressure to be as low as possible - 300 psi works well.

On any second stage, the seat will develop a seating groove over time, so to precisely tune a non-adjustable second stage you need to let the reg set several days before making the final adjustment. Or alternatively, you need to leave the reg pressurized 8 hours or so before you make the final adjustment. If you finely tune the reg before the groove in the seat forms, the reg will freeflow.

In most shops this is not really an option either because the reg's owner wants the reg sooner or because the sho[ owner does not fully understand the benefits of precision reg tuning and prefers to just get them in and out. So the tech normally adjusts the orifice in about 1/12th of a turn farther than required for minimum cracking effort to anticipate the effects of the groove that forms on the seat. It's less than perfect but gets you very close.

In any case, both the intermediate pressure and second stage performance should be tested at 3000 and at 300 psi before it leaves the bench.

 

[truva]

The IP on an unbalaced piston reg will increase with tank pressure. So if your reg is finely tuned to give minimum cracking effort at a full tank pressure, the IP will increase and the second stage will freeflow slightly until the tank pressure drops to the pressure where the reg was tuned. When I first started diving the tech that worked on my MK3 would always fine tune it on a full steel 72 at 2250 psi, so consequently it would freeflow everytime when connected to a full AL 80 at 3000 psi.

With an unbalanced diaphragm reg, the IP will increase as tank pressure decreases, so a finely tuned second stage will not begin to leak air until the tank pressure gets below the pressure where the second stage was tuned. For obvious reasons you want this pressure to be as low as possible - 300 psi works well.
.

Hmmm. I guess I have been wrong about what an unbalanced regulator was as it relates to SCUBA for the last 20 years.

All this time I thought that this type of regulator didn’t “measure” depth and was set for a specific pressure for example 145 PSIG. Better to think about that as 154.7 PSIA, the A meaning absolute pressure. This wouldn’t change as a result of depth for example this unbalance regulator at 33-feet would still have an IP of 154.7 PSIA while a balanced regulator would have an IP of 169.4 PSIA. This would explain why it would breath harder at depth than a balanced regulator.

I have heard something like they don’t perform as good at lower tank pressures as well but I always just assumed that this would be because they weren’t performance regulators in the first placed not because the regulator would try to pass more air to maintain its’ set-point of say 154.7 PSIA.

This isn’t so?

They are really so horribly designed that they pressure fluctuates that badly?

Truva

 

[stonedfish]

thank you all for your wise words.

I will keep my current first stage since others have not had significant problems with the unbalanced design to 180ft+. That is enough of a margin for the depths I plan to go to. If anyone had indicated a problem at 150ft, then I would have resticted myself to above 100ft until I had replaced the first stage. Seems to be a non-issue then.

thanks once again,

stonedfish

 

[DA Aquamaster]

Hmmm. I guess I have been wrong about what an unbalanced regulator was as it relates to SCUBA for the last 20 years.

All this time I thought that this type of regulator didn’t “measure” depth and was set for a specific pressure for example 145 PSIG. Better to think about that as 154.7 PSIA, the A meaning absolute pressure. This wouldn’t change as a result of depth for example this unbalance regulator at 33-feet would still have an IP of 154.7 PSIA while a balanced regulator would have an IP of 169.4 PSIA. This would explain why it would breath harder at depth than a balanced regulator.

I have never seen a reg, balanced or unbalanced that did not not have an ambient pressure chamber. As you point out, if that were the case, IP relative to ambient pressure would fall off quickly with depth.

I have heard something like they don’t perform as good at lower tank pressures as well but I always just assumed that this would be because they weren’t performance regulators in the first placed not because the regulator would try to pass more air to maintain its’ set-point of say 154.7 PSIA.

Balanced piston regs do not perform as well at low tank pressures because as tank pressure falls they lose some of the assist that tank pressure provides to open the valve.

In an unbalanced piston design, the air from the tank presses directly against the seat and creates a down stream force that aides in opening the valve. At lower pressure, you have less force.

Basically:
pressure of the mainspring + ambient pressure + pressure from the IP or compression chamber = pressure from the air in the tank

So if tank pressure falls, the only way to equalize things is to reduce the IP and this is what occurs.

In an unbalanced diaphragm reg, the tank pressure actually forces the valve closed tighter, so IP will increase in stead of fall instead as tank pressure decreases.

They are really so horribly designed that they pressure fluctuates that badly?

In an unbalanced design, a larger piston head or diapragm will reduce the change in IP as tank pressure decreases but there are practical limits and 10-15 psi is pretty normal and does not inflict to much of a hit on second stage performance.

The main problem with unbalanced designs is that the high pressure orifices have to be relativelty small to keep the piston and diaphragm sizes within reason. This places an upper limit on how much air can flow through the reg. The air is also forced to "sqeeze" between the orifice and the seat which presents a constriction.

Most unbalanced regs are also very compact and have very small internal spaces (as do many balanced diaphragm regs) so there is also very little IP air reserve to feed the second stage until the valve opens and the airflow gets up to speed. So there can sometimes be some lag and an excessive IP drop under high demand situations.

At depth, the volume of air required eventually outstrips the ability of the reg to deliver it and it becomes possible to over breathe the reg.

 

[truva]

I see what you’re talking about with this.

Also found an interesting link but it is very basic though.

http://www.ukdivers.net/equipment/regulators.htm

My experience with air instrumentation comes from the instrumentation industry, I am not sure you’d ever find something like the unbalanced piston in service anywhere. I don’t even think that there would be any extra cost involved in making it balanced; jeez I can’t believe you’d do something like that in the first place.

Thank you again for explaining that.

But I cannot tell you how sorry a design I think that is.


You’d have to go Way out of your way to make a totally unbalance diaphragm regulator. I do not believe that having IP air behind the valve stem really matters to the design as long as you have isolated the stems’ end from the HP supply it should be fine. This is how pretty much all flow control valves work and they don’t have an issue with initial cracking point to over come or else you’d have to account for that in the control system. That low IP air pressure would only help balance the force so the spring pressure will close the valve is probably not really necessary. If designed and biased correctly the stem will should not have a noticeable cracking resistance. If the Apeks of 12 years ago had the extra balance chamber here I haven’t found it on the DST model, not saying it isn’t there, I just haven’t seen the supply hole. If it is, I would expect it to be really small to limit the amount of HP airflow in the event of a failure of the seal at the valve stem.

Truva