Balanced VS Over-Balanced Regulators

[sus9nr]

I have been looking fro a new regulator. My old regulator is a Balanced diaphragm type (hybrid Apollo first stage Atomic Second Stage). I had recently rented an Apeks which was an Over-balanced regulator and felt as if air was being push into me. I am not sure if it was just because the regulator was tuned improperly or if that is what is meant by an over-balanced regulator. Can anyone explain the difference to me?

Thanks

Susan

 

[paulwlee]

Over balanced means the IP increases as you go deeper. It supposedly offsets the increased flow resistance as the gas becomes denser under higher pressure.

Force-fed air is I think more of a function of second stage design. Some makes are said to be like this.

 

[DA Aquamaster]

The definition of "overbalanced" depends on the company.

Scubapro for example "overbalances" their current balanced piston regulators by designing the seating edge of the piston stem to be slightly larger in diameter than the portion of the piston stem that passes through the high pressure o-ring. This ensures that actual area of the seating edge itself does not affect the balancing. The result is that the IP is indeed very stable and does not drop at all as the tank pressure drops. In an otherwise "balanced" piston regulator (like the older Mk 5 or Mk 10 "balanced" piston designs) the identical diameter of the middle and seating ends of the piston stem result in a 3 to 4 psi change in IP between 3300 psi and 300 psi.

As indicated in the previous post Apeks defines "overbalancing" differently and implies that as the ambient pressure rises, the difference between ambient pressure and intermediate pressure increases slightly. This would in fact compensate for the effect of increased viscosity reducing the flow rate slightly. The thing is, the effects of increased viscosity are not an issue at all at depths less than 600 feet. Also if the first stage has adequate flow rate in the first place, the very slight reduction in flow rate due to the denser air flowing through the reg will not even be noticed. Plus at depths below 187 feet (and in most cases long before that) a technical diver will be using a mixed gas with helium in the mix and helium mixes flow through a regulator 20 to 30% more efficiently anyway.

The down side of this type of over balancing would be that the increase in IP at depth would make the second stage more prone to freeflow unless one of two things is done. 1. Detune the second stage - which means that it will breathe harder at less than maximum depth, or 2. Redesign the "balanced" second stage poppet so that there is no downstream bias - but this prevents the second stage from acting as an overflow relief valve in the event that the high pressure seat in the first stage begins leaking. In fact there were some reports earlier this year of excessive IP on Apeks first stages that caused the low pressure infaltor to fail and essentially vent the excess pressure by auto inflating the BC or wing.

In short, there is no advantage to overbalancing (per the Apeks definition) a high performance first stage at remotely sane diving depths and I suspect the "overbalanced" effect if present at all on Apeks regs is very minor and is just an advertising gimic. If it were present to any significant degree, it would either cause the reg to breathe like a rock at shallow depths, or require substantial adjustment to the inhalation effort adjustment knob as depth changed and/or it would be unsafe in the event of a HP seat leak.

----

The forced feed effect you describe would be due to the venturi effect of the second stage case. Once air flow is initiated, the air exiting through the mouthpiece reduces the pressure in the case which then further depresses the diaphragm and lever increasing the air flow. On some regulators a flow vane adjustment allows you to increase or decrease this effect. On others the adjustment can be made by a technican to an internal flow vane or port, while on still others the venturi effect is fixed - but in those cases the venturi effect is usually moderate and will just sustain flow rather than increase flow.

 

[Luis H]

My observation is that the so called “overbalanced” regulators (as defined by Apeks and Aqua Lung) is normally a feature on the high end models and therefore offered with a balanced second stage.

Therefore, since a balanced second stage is intentionally design to perform essentially the same, independent of intermediate pressure (within reason); there is not much point for increasing the IP with depth.

As mentioned above, I am aware of at least one brand that the second stage is so well balanced that an increase in IP will not affect its performance (won’t even vent a leaking first stage until the IP is extremely high). That same brand also pushes the overbalanced first stage.

From a technical stand point those two design features (overbalanced first stage and balanced second) are basically incompatible. Or should I say the balanced second stage would totally negate an “overbalanced” or IP increasing first stage design.

I am thinking (based on engineering experience) that the overbalanced feature was just a minor design problem and instead of correcting it, marketing decided it could be a selling feature and ran with it.

The overbalanced design on a diaphragm (environmentally) sealed regulator occurs due to effective area differences from the outer environmental diaphragm and the inner IP diaphragm (they are connected by a rigid rod). IMHO it was just not practical to design both “effective” diameters on both diaphragms to be exactly the same as they can even change with the diaphragm stiffness.


Since viscosity increases with pressure, increasing the intermediate pressure would only increase the viscosity of the working gas in the IP passages. I don’t believe that flow rate of the IP gas can improve by working at a higher pressure (and basically insignificantly higher viscosity). Any increase in potential energy (associated with the higher IP) in the second stage interface is not only minimal, it is not utilized by a balanced second stage.


It is amazing the stuff that marketing can get away with.

 

[sus9nr]

So if I am understanding this correctly. There are at least 2 different definitions of Overbalanced. The way that Scubapro does it seems like an improvement to the over all piston design but the way aqualung does it seems like a design flaw that they market. Are there other vendors that have different definitions. I have used the same rig for 10 years and it seemed so much easier back then lol.

Thanks

Susan

 

[paulwlee]

As mentioned above, I am aware of at least one brand that the second stage is so well balanced that an increase in IP will not affect its performance (won’t even vent a leaking first stage until the IP is extremely high). That same brand also pushes the overbalanced first stage.

This isn't true, balanced second stages are not 100% balanced, only partially balanced to reduce the force the spring needs to provide to close the valve.
Apeks second stages not venting under very high IP I believe is only true when the cracking adjustment dial is turned down, and you might say there the problem is that they allow the spring tension to be adjusted too high, not that they are too well balanced.

However, I have heard about the over-balancing being a side effect of the dry chamber ambient seal, which seems like a reasonable conclusion. And I too am skeptical whether there will be any noticeable advantage from over-balancing, (other than the dry seal, which is indeed nice) although it seems that at least there is apparently no performance detriment.

 

[paulwlee]

Scubapro for example "overbalances" their current balanced piston regulators by designing the seating edge of the piston stem to be slightly larger in diameter than the portion of the piston stem that passes through the high pressure o-ring. This ensures that actual area of the seating edge itself does not affect the balancing. The result is that the IP is indeed very stable and does not drop at all as the tank pressure drops.

This sounds like just better balanced, not over-balanced. (Meaning they were under-balanced in the MK5/10.) Does Scubapro really use the term overbalanced? I've never seen them use it, but then I wasn't in the market when they came up with the MK15.

The only instances I've seen of the term over-balanced were of the Apeks/Aqualung kind, for balanced diaphragm first stages with the dry seal chamber.

 

[Luis H]

Quote:

Originally Posted by Luis H
As mentioned above, I am aware of at least one brand that the second stage is so well balanced that an increase in IP will not affect its performance (won’t even vent a leaking first stage until the IP is extremely high). That same brand also pushes the overbalanced first stage

This isn't true, balanced second stages are not 100% balanced, only partially balanced to reduce the force the spring needs to provide to close the valve.
Apeks second stages not venting under very high IP I believe is only true when the cracking adjustment dial is turned down, and you might say there the problem is that they allow the spring tension to be adjusted too high, not that they are too well balanced.

However, I have heard about the over-balancing being a side effect of the dry chamber ambient seal, which seems like a reasonable conclusion. And I too am skeptical whether there will be any noticeable advantage from over-balancing, (other than the dry seal, which is indeed nice) although it seems that at least there is apparently no performance detriment.

I am sorry if I wasn’t clear (or perhaps not specific enough), but I never said it was 100% balanced. If it was truly 100% balanced it would never vent any increase in intermediate pressure.

What I said is: “that the second stage is so well balanced that an increase in IP will not affect its performance”. “So well balanced” is a relative subjective statement, but IMO it is not a description of absolute balanced such as “100% balanced”. And in this portion of the statement is related to the regulator performance.

I also said: “(won’t even vent a leaking first stage until the IP is extremely high)”. This portion of the statement should have made it clear that the second stage is not 100% balanced, since if it was it would never vent.

I have witness a leaking Apeks first stage and the inflator was venting the excess pressure. The second stage never did.

The term “extremely high” IP is also very subjective, but when I witness that an inflator will vent before a second stage, that is what comes to mind. I do fill that adding an overpressure relive valve to the first stage would improve that situation (or use an unbalanced octopus).

 

[DA Aquamaster]

Traditionally, second stages have been either:

1. downstream in design (where the valve opens to the downstream side and where consequently any excessive increase in intermediate pressure will force the valve open and vent the excess pressure through the second stage well before a hose or other part of the system fails, or,

2. Balanced with a donwstream bias. This downstream bias is important as it has the same effect as an unbalanced downstream valve - it will safely vent any excess intermediate pressure before something breaks. This is easily achieved by making the area of the poppet on the balance chamber end of the poppet slightly smaller than the area on the orifice end of the poppet. But...it also means the second stage is then not 100% perfectly balanced as an increase in IP will increase the force trying to open the valve (decreasing inhalation effort and in the extreme causing a freeflow to vent the excess pressure) while a decrease in IP will reduce the force acting to open the valve and increasing inhalation effort.

The difference then between a balanced poppet and an unbalanced poppet is a matter of degree (with the balanced poppet being affected by changes in Ip to a much smaller degree than an unbalanced poppet) as both must have a downstream bias to operate safely as high pressure seats can and do leak.

In short, there is no free lunch in terms of "perfect" balancing and some compromise in second stage balancing has to be made to ensure safety or else other means need to be taken to ensure that excess pressure is safely vented.

Scubapro's approach to more completely balance the piston in their Mk 20 and 25 first stages was done to eliminate the 4 to 6 psi error in the previous balancing of the Mk 5, 10 and 15, to ensure that there was no impact, even a small one, on second stage performance.

The Apeks/Aqualung approach however makes me very nervous as there is in effect no way to safely vent excess IP under some circumstances. This is ironic as the fault is most likely to occur in technical diving and tecnical divers seem to regard Apeks regs with near religious devotion. In technical diving, a back up reg may not be used at all during the dive, giving ample time for a leaking seat to build up enough pressure to cause an inflator to fail open and inflate the diver's wing or drysuit at a very innoportune time such as when inside a wreck or at a deco stop. In fact, when the diver is using his or here deco bottle(s) the primary is then not being used and also has time to build up excess pressure and fail an inflator - essentially doubling the risk. One thing you'd rather not have during a manadatory deco stop is a run away inflator.

In this same scenario, using a balanced inflator valve that would in turn be less likely to fail under excess IP, it is possible that the intermediate pressure could increase to the point where a hose ruptures which woudl result in a rapid and catastrophic loss of gas.

If I were using an Apeks reg for technical diving, I'd add a separate OPV to the first stage. In some situations I use an FFM with doubles, two Scubapro Mk 17's and a gas block feeding a single second stage. Since it is possible for each first stage to be isolated from the downstream second stage during portions of the dive, I have an OPV on each first stage to vent pressure in the event of a HP seat leak.

This is in fact what is in effect done with poseiden regulators where the second stage uses an upstream design that cannot vent excess pressure. In the case of most poseiden regs, the OPV is incorporated in the hose and in cases where a thrid party hose is used, an OPV would need to be added to the first stage.

Personally, I think it is extremely important that divers treat their scuba systems as exaclty that - systems. And in any system the traits of a component or the change in one component in the system can have a significant impact on other components in the system. This requires that divers have enough knowledge about how things really work to ensure that traditional and essential design safety features are not compromised. Marketing BS and potentially ignorant (however well meaning) dive shop staff or instructors don't help things.

Practically speaking, most second stage will begin venting excess intermediate pressure at around 170-175 psi and nearly all of them (except the less than adequately downstream biased second stages that we are concerned about) will begin venting by 190 psi. This works as even the lowest pressure rated hoses used in diving are service rated to 240 psi and have a burst pressure at least 150% of that amount (360 psi minimum) and a power inflator will not normally fail until past this same 240 psi service pressure.

 

[paulwlee]

I have witness a leaking Apeks first stage and the inflator was venting the excess pressure. The second stage never did.

I know, one of the local divers here blew his stops due to this exact issue and went to the chamber. He then sold all his Apeks and got Scubapros.
But this was on the left post of his doubles setup, with one second stage that was turned down all the way.
Another of the local divers did an experiment with Apeks second stages after the above incident, varying the adjustment dial and IP.
With the cracking effort dial in the open position, the second stage vented fine.

So it's not true that the second stage is so well balanced that the IP won't affect the way the second stage behaves. It does, and the fact that the second stage won't vent when the dial is turned down really isn't relevant to whether it does or not when it is adjusted fully open.
The effect would be subtle, but these days differences in breathing effort in high performance regulators are subtle, so that's all it takes, if it indeed works.

Please bear in mind I'm not arguing for Apeks or their overbalanced design. And I agree that it is a problem that the second stage can be dialed down such that it won't vent within reasonable IP. I just don't agree that we can dismiss the overbalancing as complete BS due to those particular arguments.

Of course, I use Atomics and also sold my two Apeks a couple years ago to get more Atomics, so I guess I don't care too much. Uhh.. except I guess that I have a number of buddies using Apeks.