Function of Dry-Air Bleed System

[Marek K]

I'm trying to understand how our Sherwood regulators (four in the family now) work. Or rather, how the features work that make Sherwoods unique.

I took two regulators in to the LDS for service today -- a 1985-vintage SRB2100 Brut that we've been using occasionally, and a newer SRB7100 Brut that I just bought used.

I asked the service guy about the dry-air bleed system, and what happens when it malfunctions -- I'd heard that breathing becomes very difficult.

He said it wasn't a problem; that the dry-air bleed system is what makes Sherwood regulators balanced, and all that would happen if it malfunctioned is the reg would act like an unbalanced one.

Hmmm... I told him I thought the dry-air bleed system was to keep water out of the ambient-pressure part of the first stage. Besides, Bruts had the dry-air bleed system too, but weren't balanced.

He stuck to his story.

I must say that he seemed pretty knowledgeable about Sherwood regulators in general... He knew the SRB2100 had originally come with a crimped-on piston seat that wasn't replaceable, but that the original piston would have been replaced with a replaceable-seat piston if a service had been done since the early 1990s. He also knew about the flow restrictor, and that the SRB2100 Brut didn't have one -- the air for the dry-air bleed system moved through the piston.

So... Does the dry-air bleed system have anything to do with a regulator being balanced?

[spectrum]

The dry bleed system allows the first stage to sense ambient pressure and adjust the IP while providing an environmental seal without the need for diaphragms and gobs of silicone goo.

There are 2 main components to the system.
1) A bleed "orifice" that meters a small stream of air (2-3 breaths of the time of a dive) Older models had a sintered media in the piston. Newer models use a plug with a laser burned hole that screws into the body. Either device provides a small air flow into the spring chamber
2) A check valve. This is the black rubber plug you see on the outside and where the air "leaks" out.

What you see "leaking" is the air that would pressurize the chamber above ambient. The air that remains in the chamber is equal to ambient and bolsters the spring force to adjust the IP.

The failure mode I have seen is for the element to plug and the flow be lost. It's a small laser cut hole in the plug (mine are all of the new style). When this happens depth compensation is lost. The lack of bleed supplied pressure will cause the check valve to close. With no force being added to the piston the IP will not adjust. This did not amuse my wife at the bottom of Paradise Spring or on the Hilma Hooker since her IP was not climbing to provide easy air delivery. We now check the function every dive day.

The other thing is that the check valve is a rubber plug that is seated shut by external pressure but it's not a positive preloaded check valve. As long as there is a pressure differential it is a good gate keeper. If there is no air flow it can relax and water will seep into the piston chamber leading to ugly results.

I have had a good success rate boiling a plugged bleed plug and restoring the flow.

Pete

[Marek K]

The dry bleed system allows the first stage to sense ambient pressure and adjust the IP while providing an environmental seal without the need for diaphragms and gobs of silicone goo.

OK, your whole explanation corresponds with what I understand. The system is to keep the ambient pressure chamber dry; and doesn't have anything to do with balance, as the LDS guy said.

The failure mode I have seen is for the element to plug and the flow be lost. It's a small laser cut hole in the plug (mine are all of the new style). When this happens depth compensation is lost. The lack of bleed supplied pressure will cause the check valve to close. With no force being added to the piston the IP will not adjust.

Could this be what he meant by balance? But it's not really a matter of depth compensation... Without the extra air pressure, seems like the spring would be too weak to hold the piston seat out from the orifice as far as it should; and that's what causes the lower breathing pressure... at any depth. ??? Or are we saying the same thing?

Wait... There's no other way for "ambient" pressure from outside to get inside the spring chamber, except through the flow restrictor... The pressure doesn't come in from outside around the rubber check valve, since the valve only bubbles excess pressure out...

[spectrum]

Balance refers to a constant IP regardless of cylinder pressure. I have heard of how Sherwood's do it but I'm not comfortable describing it. I'll leave that to someone smart.

Pete

[Marek K]

Balance refers to a constant IP regardless of cylinder pressure. I have heard of how Sherwood's do it but I'm not comfortable describing it.

Yeah, the balancing is done through the moving orifice which is a feature in every Sherwood reg except the Bruts. That's the next thing I'm going to try to understand.

But my first priority is understanding this dry-air bleed system... for the exact same reason you're interested in it. Because of what happened to your wife.

I wonder whether the failure -- clogging of the flow restrictor -- happens suddenly? And if gradually... I know the flow rate is supposed to be tested during each service, but if it's not... I don't have tanks to be able to test it at home.

[spectrum]

I have caught some in gradual decline so it's on usually a sudden failure.

FWIW, If it's discovered during a dive the check valve can be pried out with a knife to restore depth compensation. The regulator is now 100% unsealed and that opens anther can of worms.

Pete

[spectrum]

1) You can check it at the start of a five, like any other bubble check.

2) After the dive you can usually see it bubbling while still wet.

3) While pressurize press on the check valve for 5 seconds. If you listen close you can hear a psssst when you release the valve and the buildup releases

[Marek K]

Thanks, Pete... But there's no relationship of the system to a reg being balanced, is there?