Using an unbalanced second stage vs. balanced second stage

[halocline]

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.

I think you do understand how regulators function. I think t-bone does too, he's just not doing a great job explaining things. The stuff in bold from your post is pretty much the main issue with unbalanced 2nds, although there is some wiggle room with regards to IP, and the manufacturer doesn't spec a specific IP, the tech sets the 2nd stage to work with the IP he has from the 1st stage. But if that IP drops quite a bit, the 2nd stage will be harder to breathe. That's exactly what happens with the MK2/R190 at low tank pressures.

Mechanical spring pressure is higher on an unbalanced 2nd, because there is no assistance from air pressure to keep the valve closed. Theoretically, the combination of mechanical and pneumatic pressure on a balanced 2nd is identical to the mechanical spring pressure alone on an unbalanced. But, practically speaking, it's usually more accurate with a balanced 2nd because the balancing air pressure automatically matches IP.

Interestingly, both the balanced poppet (like G250) and unbalanced downstream (R190-not the barrel poppet) SP 2nds use the same spring; the part number is the same. However, the spring is much more compressed in the unbalanced classic downstream models, so it exerts much more force. And, according to DAaquamaster, SP hand selects springs that more closely match tolerances for the balanced regs, leaving the less 'accurate' springs for the unbalanced models.

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?

2nd stages theoretically face the exact same pressure differentials at varying depth, because IP and ambient pressure should rise at the same rate. Don't forget, in an unbalanced 2nd stage, ambient pressure is pushing on the back of the seat like IP does in a balanced 2nd stage. So the mechanical spring will always face the same net forces.

The density (or viscosity) of air does rise with depth, but it only impacts the performance of the 2nd stage in terms of friction, and apparently it's not much of an effect until way beyond any reasonable depth, at which point any sane diver would be using a helium mix which has much lower viscosity.

The advantage that a balanced 2nd stage has at depth, as I mentioned before, is that 1st stages work harder at depth, and as a result IP drops more during demand, and balanced 2nds handle the increased IP drop better.

 

[tbone1004]

@halocline I know the physical mechanisms. That physical opening of the valve is force being applied in the direction of the IP so in the see-saw example it is increasing the weight on the IP side to "help it" open the valve. That's why I said artificially increasing because you are applying a "helper force" to the IP in order to counteract the opposing forces. Apparently shouldn't explain physics while cooking dinner...

@ams511
think back to see saw. On the unbalanced reg you have one force being applied against the IP which is the spring pressure. This is some value over the IP to keep the valve closed. That higher that value is, the higher the cracking pressure.
On a balanced reg, you have two forces being applied against the IP. One of those forces is some percentage of the IP *has to be some percentage to allow the feed side to exceed the balance+spring side at some value in order to freeflow. The total force pushing against the IP is the same. That percentage also does impact IP extremes which is why a reg tuned for 125 may well freeflow at 145, and one tuned at 145 will be stiffer at 125. Just less of a difference than the unbalanced design.

On the range of IP. There is something called Hooke's Law which is what allows us to adjust things like cracking pressure and IP. Basically what it says is that the pressure exerted by a spring increases as the spring is compressed. So at rest it puts out 0, at full compression it puts out say 100, and throughout the middle range, it increases linearly. The spring compression is what you are adjusting with shims or the screw cap in a first stage, and what you are adjusting with the adjustment knob and the valve seat in a second stage. This is what allows you to dial in the cracking effort as long as the IP is in the range of the spring. There are extremes, so you can only get the spring pressure so low or so high *eventually the spring will not be engaged on the low side, and it will eventually be fully compressed on the high side*


picture of all of this which may help. Note, the friggen media upload on this forum is horrific, so off to facebook it goes. Note that with the .9:1 balance ratio and the regulator set up to function at 135psi and using a spring exerting 16psi, the regulator will start to freeflow at 136psi. These numbers are completely random and I have no idea or interest in the balance ratios or spring pressures used. @cerich can probably lend a lot more specific insight to the details of what the manufacturers choose. The farther from 1:1 the balance ratio is, the closer to the setpoint the regulator will freeflow, and vice versa

 

[Lorenzoid]

I just want to say that between tbone and halocline, this is the best explanation of 2nd stage balancing I have yet seen. The explanation in Regulator Savvy is good enough for someone learning to service regulators, but falls short of the kind of intuitive explanation you guys are giving. Thanks!

 

[MichaelMc]

Thanks @tbone1004. On Hooke's Law, it might be a small effect but it seems that the spring tension increases as the valve opens, just when the counter IP force drops in a balanced reg. And this is worse if the spring has a larger range than is needed for a balanced reg. As you are using a smaller part of its compression length, so the valve opening travel is a larger fraction of the springs range than it needs to be.

Said differently: There is travel involved on the part you are keeping closed and opening it is in the direction of shortening the spring. So using a spring suited for a large effective range, such as for an unbalanced reg, in an application that only needed a smaller effective range, balanced, would exacerbate this effect. Such as @halocline 's "balanced poppet (like G250) and unbalanced downstream (R190-not the barrel poppet) SP 2nds use the same spring" example.

This ignores that we want the spring to have a bit larger range to allow adjustment of the reg tuning point to the first stages IP set point. Unless we want to deliver different springs for each of those.

But I guess mechanically on your seesaw, the spring tension should go up a hair when the valve is open. But this is just from the apparent mechanics, all I know of regs has been from here. This has been a great thread, thanks.

Attempt at text based diagram, made up numbers and not exactly proportional.
Heavy spring. Best used in large range needed with unbalanced reg.
|+++++++++++++++++++++| 20 lb. spring with 1” of effective travel
...|++++|.............. 5 lb. range we need against free flow
...|+|................. .1” valve open distance
-> 1.0 lb. extra spring pressure when valve is open

Lighter spring. Best used in smaller range needed with balanced reg.
|+++++++++++++++++++++| 10 lb. spring with 1” of effective travel
...|+++++++++|......... 5 lb. range we need against free flow
...|+|................. .1” valve open distance
-> 0.5 lb. extra spring pressure when valve is open

 

[tbone1004]

@MichaelMc the spring pressure would go up a bit, but if using the same spring, would go up the same for each regulator and would negate itself in the comparison. Point was more to explain how the balance function works to decrease the total work of breathing while the valve is open by keeping the "stay open" pressure lower even though the cracking effort is the same

 

[MichaelMc]

@MichaelMc the spring pressure would go up a bit, but if using the same spring, would go up the same for each regulator and would negate itself in the comparison. Point was more to explain how the balance function works to decrease the total work of breathing while the valve is open by keeping the "stay open" pressure lower even though the cracking effort is the same

Yes. It does not affect the balanced vs. unbalanced understanding, or its big picture mechanics. So a side point whose explanation would have complicated your diagram. And I guess there are other small changes in forces as the valve opens, like the lever arm distance changing. Thanks.

 

[jwd]

I've always wondered this. If "balanced" refers to the opening and closing of the value not being affected by tank pressure, then the IP in the reg hose should remain constant. What is the point of a balanced 2nd stage if you're using a balanced first stage then?
One of my best breathing regs is an old Conshelf 22 unbalanced second stage (attached to an Apek's 1st stage).

 

[reefrat]

In a recently competently serviced and tuned reg that is unbalanced the difference between breathing it and a pneumatically balanced unit should be negligible. However the unbalanced second stage applies a lot more spring pressure to the closed low pressure seat and this will cause it to “set” over time resulting in a slow degregation of the cracking pressure while a balanced second stage performance will stay good for a longer period.
Your Conshelf 22 may be a great breather but that is not really typical of the 22’s (I had a few), in every make and model of regs there are individual units that stand out as really good and others - not so! I have had four Atomic reps over the years from the base to the top titanium model and none of them breathed as well as my wife’s beat up old B2- in fact I don’t think Ive ever tried a reg of any brand that does, its just a freaky good one!

 

[ams511]

What is the point of a balanced 2nd stage if you're using a balanced first stage then?

The intermediate pressure from a balanced first stage is not perfectly constant throughout the dive. The IP does change due to tank pressure and for some regulators (overbalanced) depth, the balanced second stage will smooth out the differences.

 

[ams511]

Your Conshelf 22 may be a great breather but that is not really typical of the 22’s (I had a few), in every make and model of regs there are individual units that stand out as really good and others - not so! I have had four Atomic reps over the years from the base to the top titanium model and none of them breathed as well as my wife’s beat up old B2- in fact I don’t think Ive ever tried a reg of any brand that does, its just a freaky good one!

DA Aquamaster had several good posts talking about the stacking of tolerances. If they stack up well then you get a great breathing regulator, if not then you an average breather, or even poor if they all stack up bad.