Using an unbalanced second stage vs. balanced second stage

[tbone1004]

A balanced regulator has tank pressure on both sides of the poppet so it can use a lighter spring then an unbalanced regulator. The unbalanced regulator needs a stronger spring in order to be able to close the valve against the air flow. At least that is the theory. I am sure there are ways to decrease the effort required in practice.

Where are you going with that?

 

[ams511]

Where are you going with that?

A balanced second stage should require less effort than an unbalanced second stage all things equal.

 

[rhwestfall]

there are two measures...

cracking effort

and

work of breathing

 

[Lorenzoid]

If I understood tbone's explanation, the major effect is not so much to reduce cracking effort OR total work of breathing but rather to smooth the work of breathing over the whole inhalation cycle, which would otherwise vary as the IP changes during the inhalation cycle.

 

[tbone1004]

A balanced second stage should require less effort than an unbalanced second stage all things equal.

incorrect.
Think of a seesaw which is essentially what this is. @halocline even more simplified, but trying to get conceptual understanding vs. actual ;-)

In an unbalanced reg
put a 135lb weight on one side, and a 137lb weight on the other
Requires 2lbs to move it because 137-135=2
135 being the IP, and 137 being the spring pressure. When you breathe, you are lifting up on the 137lb side

in a balanced reg
2lb weight on one side, 0lb weight on the other
2-0=2, so still 2lbs to move it, and again, breathing is lifting up on the 2lb side

That is cracking effort, the force required to change the state of the valve from closed *seesaw down*, to open *seesaw up*

now, in an unbalanced reg, when you start to lift that seesaw up, that 135lb weight starts to lose weight and goes to 130lbs. You started the motion with 137-135=2, but to continue to keep it up, you now have 137-130=7lbs. The faster you move it, the more weight is lost. Area under the curve increases as you start to inhale which is why in the graphs posted you see a rapid increase in work of breathing as the RMV goes up because the regulator doesn't have time to rebound properly and as gas density increases, the IP drop also increases.

In a balanced reg, there is theoretically no change, so it stays at 2lbs regardless of how fast or how much you moved the seesaw so the area under the curve is pretty flat.
In an unbalanced servo-assisted upstream reg *i.e. Poseidon Jetstream and Xstream*, the 137lb weight is the one that gets lighter as you pull on it and that is why their breathing curve is so different than a normal downstream regulator

 

[ams511]

As I could not get a new diaphragma cover for my second stage oceanic GT3, I used an Alpha 8 for a dive holiday. I got used to it after some dives, but I thought that my air consumption was much worse particularly on deep dives.
I have dived with rental gear with unbalanced second stages for years, but I haven´t done really deep dives then.
I never used the valve to adjust. Thus, I thought it would not make a difference.
What are the disadvantages of the unbalanced seond stage when it comes to deep dives and air consumption?

As I currently can´t get the spare part for the GT3, I would have to opt fot a new reg.

Cheers,
Liberty

I do not find a GT3 in their catalog but simply scuba still has one in stock so it should not be that long out of production.
GT3 CDX5

 

[tbone1004]

@ams511 I think you are misunderstanding how the regulator functions.

So let's go back a bit because I have explained poorly

unbalanced stays the same. 135 on one side which represents the IP, and 137 on the other side of the seesaw that represents the spring pressure.

What you are doing when you inhale is artificially increasing the pressure from the IP side, so you are pushing down on the 135 side to counteract the spring pressure. You only need to push at some amount greater than 2 because you already have 135 pushing down with you, it just wasn't enough to beat the 137.

On the balanced side, you have 135 on both sides that represent the IP, but you also have a 2 on one side that represents the spring. You still have to push down by 2 to overcome the force on the other side.

On the unbalanced seesaw, when you push, the 135 gets lighter so you have to push harder which represents the IP drop.
On the balanced seesaw when you push, both of the 135's get lighter so you just pushing against the 2.

The ONLY time in an unbalanced reg that you are pushing against the whole 137, is when there is 0 IP which is regulator shut off and purged and you obviously can't breathe against that.

As you get deeper, the IP increases, but the ambient pressure is also pushing with the spring and those two should theoretically go up equally. In a regulator like a sealed Apeks, the IP goes up more than the ambient, and eventually will exceed the spring pressure causing a freeflow

 

[halocline]

What you are doing when you inhale is artificially increasing the pressure from the IP side, so you are pushing down on the 135 side to counteract the spring pressure. You only need to push at some amount greater than 2 because you already have 135 pushing down with you, it just wasn't enough to beat the 137.

On the balanced side, you have 135 on both sides that represent the IP, but you also have a 2 on one side that represents the spring. You still have to push down by 2 to overcome the force on the other side.

No, this is not correct. When you inhale, you are lowering the pressure inside the 2nd stage (ambient pressure), which collapses the diaphragm, which pushes the lever, which mechanically opens the valve. It has nothing to do with increasing the pressure on the IP side. In fact, IP drops as a result of the flow of air from the hose into the 2nd stage. This opens the 1st stage valve, which will stay open until IP is restored.

When you push on the purge button, you are doing exactly the same thing, mechanically opening the 2nd stage valve.

 

[halocline]

If I understood tbone's explanation, the major effect is not so much to reduce cracking effort OR total work of breathing but rather to smooth the work of breathing over the whole inhalation cycle, which would otherwise vary as the IP changes during the inhalation cycle.

The primary difference with an unbalanced 2nd stage is that it can compensate for fluctuations in IP with regards to cracking effort.

The 'smoothness' that I was referring to in the balanced poppet vs the unbalanced poppet of the old SP metal case 2nd stages is due to the fact that IP drops a bit when the 2nd stage opens. As such, the balancing air force also drops a bit, which means less downstream force is required to keep the valve open. This effect is amplified under hard work, because the IP drops more. In this sense, and this sense only, a balanced 2nd stage will work better at extreme depth. 2nd stages are 'constant volume' devices, meaning that regardless of depth, there is an identical volume and pressure ratio of gas being passed through them. So theoretically, there is zero difference between balanced and unbalanced 2nds with regards to depth changes.

But that's not true for first stages; they have to flow more air at great depths, because they are exporting higher pressure to the 2nd stage. As a result, under demand, IP drop is greater under demand, and that's where the balanced 2nd is more tolerant.

As far as WOB numbers throughout the whole breathing cycle, there are many other factors that play a very large role, like venturi assist and exhaust effort. This is why the numbers games don't really matter much in terms of actual regulator performance. When it comes down to it, evaluating regulator performance is really about feel. It's more like wine tasting than math. This tends to explain why everyone has a favorite regulator, and they're all different.

 

[ams511]

@ams511 I think you are misunderstanding how the regulator functions.

I could be wrong but I think I have a pretty good idea how regulators function. I am not an expert like Halo, Couv, Awap, or DA Aquamaster. I too like Lorenzoid got the impression from Regulator Savvy that spring pressure is higher on an unbalanced downstream like an R-190 than with balanced barrel poppet like a S-600. When comparing a G-250 HP (hated on SB) with a R-380 (non-octo version) on a MK-20 first stage the G-250 performed better even at shallow depths.

Your analysis hinges on the spring for the balanced poppet being 2 lbs and the unbalanced classic downstream being 2 lbs over IP. However, remember even with balanced first stage the IP could range from 125 to 145 on a first stage (135 +/- 10 psi) depending on what the tech set it at. On a balanced first stage it should not matter because there is pressure on both sides of the valve. On an unbalanced downstream I would assume the manufacturers would need to spec something like 147 to account for the high range of IP. Maybe even higher to account for a bit of creep or a bad tech. This is ok if the IP is run at the high end of the scale, but if lower it will be harder to breath.

Another factor I thought of (and could be wrong). Is how does the gas density affect spring pressure? As you go deeper does the increased density of the gas place more force on the spring requiring it be stronger than on the surface?