Sherwood's Moving Orfice operaion in "vintage" First Stages ..

[MR12111]

.. Hello .. I searched for this .. can't find any information on either this Forum or Google .. I have been thru 2 Classes .. Certified to service/repair shrwoods back in the 80's .. Serviced tons of these .. I just cannot remember or picture how the moving orifice controls the intermediade pressure (when the tank gets lower ?) I just remember the Brut didn't have this feature .. Can someone please explain it to me ..again ..it's driving me crazy .. Thanks .. Don J .. PADI OWSI 28096

 

[tmassey]

The very brief version is: a fully pressurized tank squishes the Belleville washers, pushing the orifice toward the seat on the end of the piston. That slightly squishes the piston spring, allowing it to push back a little harder. That decreases the IP lower than it would be if the orifice hadn’t moved. That lowers the IP pressure you would have with a full tank relative to a system without the moving orifice.

As the tanks pressure decreases, that effect lessens, meaning that the IP would be slightly higher. But that is offset by the fact that the pressure of the tank is lower, which means it’s pushing against the seat a little less, which would normally cause the IP pressure to drop a little. That keeps the IP closer to the same pressure it was when it was a full tank. Theoretically, that continues the entire way down as the tank pressure drops.

It’s not perfect, but it greatly decreases the amount of IP change between a full tank and an empty tank.

So instead of keeping the tank force and the piston force perpendicular to each other, with full tank pressure on all sides of the piston and very carefully machining everything to minimize imbalances like you have in a truly balanced first stage, you have a traditional unbalanced first stage that mechanically offsets the pressure shift inherent in an unbalanced first stage. Simple, clever, easy to produce… Not as good. But it has a certain appeal of its own.

 

[CG43]

Here is an attempt at another explanation

The force of the tank pressure and the spring force act in the same direction against the piston .
Their sum must be independent of the cylinder pressure, so if the tank pressure is high,
the spring force must be lower.
This is achieved by the tank pressure pushing the nozzle in the direction of the piston.
As a result, the piston closes the orivice in a position where the force of the IP spring is lower
because the spring is not compressed as far.
This compensation works very well with my two SHerwood first stages.

With
A = area of the compensation piston
O = area of the nozzle opening
R1 = spring rate of the compensation piston (disk springs)
R2 = spring rate IP spring

Compensation condition results in : A / O = R2 / R1

 

[MR12111]

Here is an attempt at another explanation

The force of the tank pressure and the spring force act in the same direction against the piston .
Their sum must be independent of the cylinder pressure, so if the tank pressure is high,
the spring force must be lower.
This is achieved by the tank pressure pushing the nozzle in the direction of the piston.
As a result, the piston closes the orivice in a position where the force of the IP spring is lower
because the spring is not compressed as far.
This compensation works very well with my two SHerwood first stages.

With
A = area of the compensation piston
O = area of the nozzle opening
R1 = spring rate of the compensation piston (disk springs)
R2 = spring rate IP spring

Compensation condition results in : A / O = R2 / R1

... Thank You .. Got it .. Good Refresher Course ..

 

[MR12111]

The very brief version is: a fully pressurized tank squishes the Belleville washers, pushing the orifice toward the seat on the end of the piston. That slightly squishes the piston spring, allowing it to push back a little harder. That decreases the IP lower than it would be if the orifice hadn’t moved. That lowers the IP pressure you would have with a full tank relative to a system without the moving orifice.

As the tanks pressure decreases, that effect lessens, meaning that the IP would be slightly higher. But that is offset by the fact that the pressure of the tank is lower, which means it’s pushing against the seat a little less, which would normally cause the IP pressure to drop a little. That keeps the IP closer to the same pressure it was when it was a full tank. Theoretically, that continues the entire way down as the tank pressure drops.

It’s not perfect, but it greatly decreases the amount of IP change between a full tank and an empty tank.

So instead of keeping the tank force and the piston force perpendicular to each other, with full tank pressure on all sides of the piston and very carefully machining everything to minimize imbalances like you have in a truly balanced first stage, you have a traditional unbalanced first stage that mechanically offsets the pressure shift inherent in an unbalanced first stage. Simple, clever, easy to produce… Not as good. But it has a certain appeal of its own.

Thank You .. Good Explanation .. Refreshed my memory .. Those Engineers are Genuis .. I wonder if they use that syatem in any other products .. Enery now and then I got one with some bellville washers broken .. Thanks Again ..

 

[rsingler]

I'm trying to talk a company into building a Mk2 with Belleville washers, but without that silly sintered filter in the piston head that Sherwood had.
I think WMD in Taiwan actually builds one, but I haven't seen it yet.
Can you imagine? A bulletproof BALANCED Mk2?

 

[tmassey]

I'm trying to talk a company into building a Mk2 with Belleville washers, but without that silly sintered filter in the piston head that Sherwood had.

I hope you’re not eliminating the entire dry air bleed: the laser drilled version is pretty bulletproof (and cleanable) too. I hate – hate! — unsealed first stages.

Can you imagine? A bulletproof BALANCED Mk2?

Sure: my mid-1990’s Blizzards! (And I even prefer the early 90’s round unbalanced second stages.)

 

[rsingler]

I hope you’re not eliminating the entire dry air bleed: the laser drilled version is pretty bulletproof (and cleanable) too. I hate – hate! — unsealed first stages.

Which versions/models are laser drilled? You have a point, apart from having to tell EVERYBODY on the boat that your first stage isn't really leaking...

 

[tmassey]

The SRB 5300/5700/5900. The one that inspired this thread about cleaning the laser-drilled orifice: Cleaning Sherwood First Stage Laser Drilled Flow Control

You have a point, apart from having to tell EVERYBODY on the boat that your first stage isn't really leaking...

Hey, it’s a great conversation-starter!


And I prefer the SRB 3200/3400 second stages. The round ones with concentric grooves, with an actual purge button. The next generation (3700/3900) had the soft face/purge cover which I dislike (and they were ugly) and the 5000’s had oval diaphragms, which turned out to be worse than round (and still ugly).

I have a set of doubles regs made up of 5900 first stages paired with 3200 seconds. They are my favorite swift-water regs. I’ve had zero reg failures with those regs, something I *cannot* say about clone DST/XTX regs (among others). Bulletproof indeed.


But yes, they bubble.

 

[MR12111]

I'm trying to talk a company into building a Mk2 with Belleville washers, but without that silly sintered filter in the piston head that Sherwood had.
I think WMD in Taiwan actually builds one, but I haven't seen it yet.
Can you imagine? A bulletproof BALANCED Mk2?

That was the 1 problem with them .. "Back in the Day" Pistons were cheap and we changed them all the time .. The seats in the sherwoods were crimped in back then so you had to change the piston ..I was a full time Divemaster using a Mares MR12 .. The H.P. seats kept blowinng .. I finally found a "spare" first stage from when Sherwood had that side draft "odin?" copy .. It had a bigger piston with Changeable H.P. Seats .. Used that thing for years .. zero problems ... Think it might have been the 3300 model 1st stage ..