Using an unbalanced second stage vs. balanced second stage

[Lorenzoid]

. . .
The adjustment valve isn't what makes it balanced, it's the internal mechanism itself that makes the regulator balanced. The adjustment knob just allows you fine tuning and it is typically used to make the reg harder to breathe so it is less likely to freeflow. . . .

Backing up a minute to the basics of how a balanced 2nd stage works, it's my understanding (from reading Regulator Savvy) that a balanced 2nd stage eliminates the spring having to work against the downstream force of the supply in addition to the force from the lever arm, and so allows the design to use a smaller or less forceful spring, which in turn causes the valve to open in response to less force, i.e., the valve is more sensitive. The result of all this is that work-of-breathing (WOB) is reduced. (Whether WOB is reduced enough to be perceptible to an individual is another matter--things like gas density on deeper dives probably have a greater impact.) The less forceful spring, and thus greater sensitivity to the lever arm force, requires more careful adjustment of the knob to prevent freeflows.

Did I get that correct?

 

[tbone1004]

@Lorenzoid

normal unbalanced regulator *not including Poseidons because they're strange*, think Scubapro 109
135psi coming from the tank pushing to the left, and a spring exerting some value greater than 135psi pushing to the right.

Work of breathing is determined by the delta between the spring pressure and the IP. This is why it is important to have stable IP with unbalanced regulators.

with a balanced regulator, think Scubapro 156
135psi pushing to the left
135psi passed thru to the balance chamber pushing to the right
spring with some amount of force greater than 0 pushing to the right.

The spring force over 135psi can be, and arguably should be the same for either system as that is what determines your cracking effort. A 109 is just as capable of say 1.0" of H2O cracking effort as a 156. The difference is going to show up when, not if the IP changes.

IP will change for 3 different reasons.
First is over-compensation due to an environmental seal. IP goes up at a greater rate than ambient pressure.
Second is IP fluctuation as a function of tank pressure. Even top end regulators are subject to this. It's usually not that much, but it's still there. Diaphragms tend to have better balancing mechanism than pistons *not sure why, it's just a thing*
Third is most relevant to this point which is IP drop during use. I.e. as you take a breath, the IP drops and it takes a bit for it to come back to stabilize after you stop breathing. It's usually less than a second, but it's still there, and the IP drop can be pretty substantial depending on the first stage. This is tied directly to the flow rate of the first stage fwiw.

That third function is why balanced second stages *with the exception of Poseidon Jetstream/Xstreams* have a smaller area under the curve than their unbalanced mates. The balanced regulators have a constant "X over IP" spring effort during the breathing cycle where the unbalanced have a constant spring pressure and the effort required to keep it open increases as the IP goes down during the breathing cycle.
Poseidon is an exception because they are an upstream regulator and actually need IP to shut off, so a big IP drop during the inhalation cycle is actually better for work of breathing which is part of why they start freeflowing a bit after you stop breathing when the tank pressure and as a result IP gets too low. That last bit is actually how I know it is time to switch stage regulators when I am exiting a cave instead of looking at my SPG. I know when the bubbles start coming after I stop inhaling I'm down to somewhere between 150-200psi and it's time to switch over to the other bottle

 

[fmerkel]

Good explanation. Thanks.

 

[halocline]

with a balanced regulator, think Scubapro 156
135psi pushing to the left
135psi passed thru to the balance chamber pushing to the right
spring with some amount of force greater than 0 pushing to the right.

[COLOR=#000000]This is not quite accurate. The pressure per square inch (PSI) is not really the amount of force "pushing" on the seat. It's the pressure times the area. So, if the area of the seat in contact with the orifice was .1 square inch, and the IP was 150 PSI, the force pushing on the seat would be 15 lbs. The air diverted through the seat into the balance chamber pushes against the end of the poppet, which has a smaller area than the seat, let's say .067 sq in. That would result in right around 10lbs of force, leaving the spring to provide 5 lbs.

This preserves a downstream bias which is important in case of a sudden IP spike. As IP rises, eventually the differences in actual forces on the seat differ enough to allow the valve to open and release the pressure. Otherwise you'd never have a freeflow until the hose burst, then you'd have a really bad one! Virtually all 2nd stages (with some poseidons being the exception) have some downstream bias.

Despite what t-bone said in a prior post, there is a difference in performance between the unbalanced 109 and the same reg with a balanced poppet. I know this because I've owned, used, and worked on dozens of each of them. The balanced poppet is a little smoother and exhibits a bit better flow once the reg is opened. It also holds a tune better over a season or two of diving.[/COLOR]

 

[Lorenzoid]

So tbone and halocline, would a good (but perhaps over-simplified) way of stating the main advantage of balancing a 2nd stage be: it slightly smoothes the WOB over the inhalation cycle?

 

[tbone1004]

@halocline correct, but I figured it would be easier to explain that way to get the overall concept across in terms of the pressure. The imbalance is definitely required to allow the regs to freeflow.
the math essentially works out to
planned ip*area of seat=planned ip*area of balance chamber+spring force
freeflow ip*area of seat>freeflow ip*area of balance chamber+spring force

Doesn't have to be much of a difference to get it to freeflow but the larger the difference, the more sensitive the reg will be to freeflow due to higher IP. In over-compensating regs like the Apeks design and clones, you want that to be closer to 1, but

I did comment on the breathing curve being different on the 156 vs the 109 due to the balancing mechanism though.

Navy results below. First stage is essentially the same *the 9 is a 10 without the turret* and shouldn't have changed much of anything. For recreational divers you are looking at the square and diamond lines on the bottom. Square line represents about .8cfm SAC rate, and the diamond is about 1.4cfm SAC rate, both well above what you should be breathing in normal use. This is the total area under the curve that they are measuring for reference.
Now, what is unfortunately not disclosed is which poppets they were using. The 109 has had several poppet changes over the years, as have the balanced adjustables/G series. If you had an original 109 poppet vs. the duro poppet, it may change. Same with the original BA poppet vs. an S-wing poppet. I am assuming the 109 had the original eraser poppet, the BA had the original balanced poppet *eraser with a balance chamber*, and the G250 had the s-wing poppet. That difference is probably going to be noticeable.

 

[fmerkel]

FWIW, I had a old 109 with the old poppet. Overhauled it with everything original re-used. Worked fine for 6 months, then went back to my G250.
Tried out the 109 again and it didn't feel good, though it worked. Poppet had a huge set. On a whim I put the G250 guts in the 109 and surprisingly it worked fine. Used it that way for a year.
Then bought an up to date poppet assembly from VDH and put the 109 back to work with that installed so I could re-assemble the G250.
Can I tell the difference on the same dive between the 109-popped vs G250-balanced? In an A/B comparison, yes, a little. In water diving it really doesn't make any real difference as long as it's tuned well. I like the weight of the G250 better, but appreciate the moisture retention of the 109 a lot more.

 

[rhwestfall]

@tbone1004 : That MK-10/G250 likely was the same poppet as the 156, not the s-wing. Think about the era of these tests... I'd think the difference is due to the venturi assist that was the principal design change with the g250 (and all variants going forward). Maybe too some better flow paths with a moulded body vs. the hand soldered bodies of the metal cased regulators.

 

[ams511]

Actually, I could not find it on Ebay this morning.
The Oceanic dealer for Europe is in the UK now, in mid-August the cover was not available. Getting spare parts in Europe was not possible before my holiday. Even on ebay they were out of stock. Incredible. I bought the regulator in 2008, I never had a problem with it.

Best regards,
Liberty

I do find it incredible but Oceanic has been recently purchased, so maybe that is the problem. Have you tried posting in the Oceanic Manufacturers forum on Scubaboard to ask what other covers may fit. Oceanic makes good gear, I would not expect any problems with it.

 

[ams511]

if you noticed an air consumption or perceived work of breathing difference, it was due to the way the regulator was adjusted, not the mechanism of delivery

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.