Diaphragm 1st stage - unbalanced?

[miketsp]

Well, I just completed an equipment course and it was quite interesting dismantling, cleaning and rebuilding various types of regs. But we got into quite a heated discussion about balanced and unbalanced diaphragm 1st stages.
One of the regs we worked on was the Aqualung Titan and by its construction (left attachment) I considered it was an unbalanced diaphragm, whereas the instructor insisted it was balanced and that what I drew as balanced (see right attachment, attachments borrowed from UKDivers.net) he considered to be a high-performance balanced diaphragm.
So we checked the Aqualung catalog and it really is sold as a balanced diaphragm 1st stage.
We then checked the Internet and some sites and manufacturers do claim that all diaphragm 1st stages are inherently balanced.
This sounds like BS to me.
Can anyone clarify?

 

[Greg Barlow]

We then checked the Internet and some sites and manufacturers do claim that all diaphragm 1st stages are inherently balanced.
This sounds like BS to me.
Can anyone clarify?[/QUOTE]

I don't know of any currently produced unbalanced diaphragm first stages. When you consider the intrinsic design of a diaphragm first stage as compared to an unbalanced piston design, there is no sound reason not to balance the HP poppet assembly.

Unbalanced piston designs are very simple in mechanical terms, with only one moving part needed to transfer ambient pressure to the mechanism.

Greg Barlow
Former Science Editor for Scuba Diving Magazine

 

[fmw625]

I was under the openion that balanced ment that the diffrential across the diaphragm was constant at any depth. Ports 6 and 7 on the Titan are exposed to ambient pressure therefore the diffrential on the spring is unaffected by depth. ie balanced.

 

[redacted]

Unless there is a balance chamber hiding inside #9 in the LH drawing, it is unbalanced. The "balance" in a diaphram compensates for the IP force on the downstream side of the seat. All regulators are balanced for ambient pressure.

I don't know of anyone making an unbalanced diaphram.

 

[miketsp]

As AWAP states, all regulators are balanced for ambient pressure.

The problem is that if the cross-sectional area of the hp seat (red part) is not negligible then there will be a variation of force with tank pressure in the left diagram.
In the right diagram both ends of the red pin are exposed to Intermediate Pressure and the HP comes in from the side. The red part is then truly balanced. As HP varies the reg performance does not change.
So it seems to me that two standards are being applied and that some manufacturers are selling regs advertised as "balanced diaphragm" when they are not fully balanced both for ambient pressure AND tank pressure.

Just to be clear as I think some readers may have confused the ports.
In the left diagram (non-balanced diaphragm), 1 is HP, 6 is ambient, 7 is an IP port. Chamber 2 is at HP.
In the right diagram (balanced diaphragm), 8 is HP, 6 is ambient, 1 is IP.

 

[rmannix]

The only unbalanced diaphragm first stage I know of is on the old USD 2 hose DA Aquamaster, I have 2 of them. Tank pressure holds the HP seat shut. Intermediate pressure RISES as tank pressure decreases. Mfr calls for 110 PSI at 2400 psi and not to exceed 135 PSI on a low tank.

 

[wedivebc]

The reason diaphragm regs are balanced and some piston regs not is that it is perfectly safe to expose one dide of the diaphragm to open water (ambient ) pressure. The only moving part is the diaphragm itself. A piston reg, inorder to be balanced must either expose a very precision machined component to water pressure or make some kind of balance chamber to let the piston sense ambient pressure without coming in contact with the water.
So to answer the question, both regs in the example are balanced.

 

[DA Aquamaster]

The Conshelf, Titan and Legend all use the same basic design.

In this design the seat incorporates a stem that is in turn inserted into the seat carrier (what Aqualung calls a spring block). The hole through the seat/stem carries intermediate pressure air to the other end of the stem where it is then trapped in the end of the seat carrier by the small o-ring and backup ring in the seat carrier sealing the seat carrier to the stem. The surface area of the end of the stem is by design the same as the area of the orifice so that IP air presses equally in both direction which means it's balanced as the net force acting upon it will be the same regardless of supply pressure.

The larger area of the seat itself that extends past the orifice is also acted upon both ways by high pressure air so the balance is preserved.

Given that the only difference between an unbalanced diaphragm and balanced diaphragm first stage is whether the seat carrier is balanced or not, and given how simple it is to balance the seat carrier in this design it makes no sense not to balance one and there is no benefit in not balancing one.

One unique thing about diaphragm first stages is that the seat carrier is on the upstream side of the orifice, so it is an "upstream" design. This also means that if it were not balanced, the intermediate pressure would increase as tank pressure decreased, which is exactly opposite of what occurs on an unbalanced piston first stage. This in turn would mean that an unbalanced diaphragm first stage would breathe easier as tank pressure fell until it reached a point where it could no longer deliver adequate air. In contrast, with an unbalanced piston design, IP falls and inhalation effort increases as tank pressure falls which gives the diver warning of low tank pressure and offers a unique advantage over other balanced and unbalanced designs.

In short, grasshopper, your instructor is correct and you owe him a beer for annoying him.

---

In reference to a couple other posts in this thread, some people are confusing the balancing of the valve poppet (which is balanced on "balanced" first stages and unbalanced on "unbalanced" first stages) with the use of ambient water pressure acting on either a dipahragm or piston to maintain the relative difference between ambient pressure and intermediate pressure.

In most regs intermediate pressure is about 140 psi, and if you are at 100 ft and have about 50 psi of water pressure, this is important as if you did not use ambient pressure to increase the IP, your IP, relative to the ambient pressure, would only be 90 psi and second stage performance would suffer. However using water pressure to asist the piston or diahpragm in opening the poppet allows the regulator to maintain an IP 140 psi over the ambient pressure regardless of how deep you go.

You folks can PM me for the address to send ME a beer.