Regulator Performance With Increasing Depth

[LeadTurn_SD]

I read a post in another thread that contradicted my understanding of the effects of depth on regulator performance, specifically regarding balanced and unbalanced regulators (balanced, unbalanced, and "over-balanced??" 2nd stages in particular).

I think this topic comes up often enough that I wanted to start a new thread, so that hopefully more folks will read, "weigh-in", and hopefully illuminate any flaws I may have in my understanding on "How Regulators Really Work".

First, the post from earlier today that got me "confused". It may have been an "ooops" or typo by the author, I might be misunderstanding his point, or I might be wrong about some of my basic understanding, but in any event it got me thinking:

"A balanced design means it maintains the same cracking resistance regardless of depth by using part of the exterior pressure and part of the Intermediate pressures to control the resistance it requires to crack the valve and allow air to flow to your lungs.

With an unbalanced design they become harder to breath at the deeper depths. At depths over 160 feet Work of Breathing (WOB) becomes noticeable and physical conditioning and your efforts or labors begin to become things to consider during the dive planing.

Over balanced 2nd stages by the way become easier to breath the deeper you go to counter exterior forces against the chest cavity."

Ok, now.... MY Understanding Is:

1.) All scuba regulators are depth-compensating . This has nothing to do with "balanced", "unbalanced", or "over-balanced". The basic fact is that as depth increases, the first stage "compensates" by increasing the intermediate pressure (IP) so that air is delivered to the 2nd stage at a constant pressure above ambient pressure (say about 135 psi for the "average" IP).

2.) The 2nd stage will also compensate for increasing depth because it has a flexible diaphragm and flexible exhaust valve. If you were to hold you breath and drop like a stone, the 2nd stage would not be crushed... the diaphragm would press inward, the demand lever would be depressed, and air would flow into the 2nd stage until the internal pressure matched the external pressure, at which point the spring would close the valve.... so the 2nd stage is always filled with air at ambient pressure. If the pressure rises much above ambient, the excess escapes out the exhaust valve.

3.) Balancing of a 1st stage results in a very stable IP supplied to the 2nd stage, independent of tank pressure (until the tank is just about empty). An unbalanced first stage will have an IP that varies a bit with tank pressure; in the case of unbalanced piston 1st stages the IP will drop as the tank pressure gets low, resulting in increased breathing resistance with low tank pressure. So, balancing results in a stable IP, and stable performance of the 2nd stage (until very low tank pressure).

4.) Balancing of the 2nd stage involves using some of the air pressure supplied to the 2nd stage to help "close" the 2nd stage demand valve. This allows a lighter spring to be used than in an unbalanced 2nd stage, which is 100% reliant on spring pressure to close the 2nd stage demand valve. So in theory and probably in practice, the balanced 2nd results in a slightly lower work of breathing.... but this should be constant at all depths. 2nd stages that are called "Balanced" are not really 100% balanced, because then they would lose their ability to act as "relief valves" in the case of a failed high pressure seat in the first stage. So true 100% balancing and in particular "over-balancing" is not used in 2nd stages (to my knowledge).

5.) What regulators do not compensate for with increasing depth is the increasing density of the air delivered to the diver as the ambient pressure rises. To compensate for this, either the hose diameter (and I guess other air passage diameters?) would need to increase, or IP would need to increase more than proportionately to increases in ambient.... I believe that this is what the underlying theory is behind "over-balanced" first stages, but I'm not totally sure...

Ok, please comment on or correct my current understanding of "How Regs Work".

Thanks in advance.

 

[Karl_H]

My understanding was also that Balanced or Unbalanced was always related to the tank pressure and relatively unaffected by depth.

The reason I say relatively unaffected is as a general rule, problems with WOB at depth is the regulator been unable to deliver the amount of air (effectively a higher flow rate in due to the increased density) from the tank as easily as at shallower depths because of the physical constraints of the system. Most unbalanced 1st stages tend to be cheaper versions of the balanced model and hence do not perform as well under high flow demand conditions as the more expensive higher performance models.

In effect this agrees with what was said in the second paragraph of the original post in question I guess! The first paragraph seem to be different to my understanding while I'm not to familiar on the actual working performance of 2nd stages marketed as 'overbalanced' as opposed to 'balanced'.

 

[LeadTurn_SD]

Thanks Karl.

I could just be misreading or misunderstanding the post I quoted in blue above .

The reason for my post is a "reality check" on my own understanding of this stuff.

Best wishes.

 

[Zung]

LTSD: you're right and the blue guy is wrong.

With that said, I'm been only once to 220ft/67m on air, and I was so stoned out of my mind I didn't notice the change in WOB, and I swore NEVER to do that again.

On shallower dives, like 163ft/50m, the difference in WOB is hard to tell: I never notice anything, and that's with my MK20/G250.

 

[spectrum]

You're right. Depth compensation is entirely different than balancing. I fell into the same trap once.

Overbalancing has some quirks that are better explored by others.

Pete

 

[herman]

You have it.
When discussing balancing of regs you have to keep in mind therea are 2 ways of balancing the reg, first and second stage balancing. The overall purpose of balancing is to keep cracking pressure constant as tank pressure changes, not to keep the IP constant. It is true the WAY the first stage accomplishes balancing is to keep the IP constant over the range of tank pressures but still it does so to keep the second stage cracking pressure constant. Balancing the second stage accomplishes the same thing reguardless of what the IP is, within reason, so keeping the IP constant is not the goal but rather the way to accomplish the goal. Balancing both is redundant.

What overbalancing, which is actually "over depth compensating" does is try to keep the WOB constant at depth by compensating for the increased density of the air. It does this by increasing the IP slightly over what is needed to compensate for the increased ambient pressure due to depth. I ran the numbers a while back; the increase over ambient was about an extra 1.5 psi for every 10 feet of depth or at 100 ft an extra 15 psi. I suppose it does work to some extent but at least at rec depths I can't really tell much difference. Overbalancing does add one bug into tuning for the tech. Since it does increase the IP over the amount needed to compensate for depth, it requires the cracking pressure of unbalanced second stages (mostly octos) to be increased over what would be necessary in a "standard depth compensated" reg. Nice marketing ploy to sell you a more expensive balanced ....... or maybe I should say "over balanced compensating .....octo.

 

[DA Aquamaster]

The BS regarding overbalancing has been covered in extreme detail before.

This thread will help:

balanced-vs-over-balanced-regulators

This one might be useful too:

basic-scuba-discussions-balanced reg

This one is less useful, but more fun:

can-first-stage-overpower-cheaper-octo

 

[Karl_H]

LTSD: you're right and the blue guy is wrong.

With that said, I'm been only once to 220ft/67m on air, and I was so stoned out of my mind I didn't notice the change in WOB, and I swore NEVER to do that again.

On shallower dives, like 163ft/50m, the difference in WOB is hard to tell: I never notice anything, and that's with my MK20/G250.

Personally I notice a big increase in WOB anything below 40-45m, depends on a lot of other factors but there's certainly a difference for me using Apeks XTX50s - not seriously hard, but there's definitely a noticeable difference. I guess this comes down to other factors, but SAC Rate and Physical Activity would be the main players.

I regularly dive upto a maximum 60-65m on air and still notice the difference even under the narcosis, breathing becomes more of a lung/diaphragm sensation than the usual throat sensation at shallower depths. If I'm using the right trimix back gas for the depth the WOB increase isn't really noticeable - once again backs up the density argument I think!

 

[halocline]

Leadturn, excellent post about a subject that comes around like a nagging infection every so often. A few added details:

1. 2nd stages depth compensate because the ambient pressure increases force on the poppet keeping the seat on the orifice. This offsets the increase in absolute force (increasing IP w/depth) that's pushing the seat off the orifice against the spring. This is especially clear with unbalanced 2nds; since absolute IP increases, and the only opposing force is the spring, something clearly must assist the spring against the increase in force.

2. "Overbalanced" regs work by having two 1st stage diaphragms of differing sizes; the outer one usually acts as an environmental seal. There's a piston that transmits force from the outer to the inner. If the outer is bigger, the total surface area experiences a larger increase in pressure as depth increases. It then transmits this larger force to the inner diaphragm. It's possible that this whole thing started out as an engineering mistake; someone inadvertently designed a reg with a larger environmental seal, then when it was realized that this would result in IP rising more than the change in ambient, they decided to sell this as a benefit.

 

[keyshunter]

Leadturn,
You are correct. Not that it matters much in the real world, but there are some excellent and very experienced techs who don't have a good handle on this subject.