Understanding the Intermediate Pressure Gauge

[halocline]

Great thread! I think one of the coolest things about being a reg tech is seeing all the different ways of accomplishing the same thing. I would confidently say that both systems being compared are balanced (defined by the end function, not how you get there) but the real beauty lies in how they take 2 completely different routes to accomplish the same task. Sherwood definitely employs a few unique methods to keep their regs simple while still providing the same function (dry air bleed screw and rubber bleed valve for example).

I don't see the floating orifice flow-by piston regs from sherwood as particularly simple, especially compared to a simple balanced piston design like the MK5. That's balancing at it's most simple; the design simply removes any significant downstream bias from supply. There's no need to counter the downstream force with either a pneumatic or mechanical opposing force.

What sherwood has done with the old NB flow-by design is add some complexity to it with the floating orifice and the dry bleed valve. I'm not knocking them, as these additions have an extremely long and proven track record.

 

[halocline]

The area behind the volcano orifice is just a different looking balancing chamber......snip.....

What do you mean by “you can’t believe you are arguing with me”… It is not the first time… and I am sure it won’t be last time either … and I surely don’t mind. I am very capable of making mistakes or overlooking something. At the same time I am aware that sometimes my explanations may not be always as clear as I wish they were.

My comment about the 'arguing' was just a good natured statement about how you are pretty much right all the time, but I do learn quite a bit from posing questions to you and challenging your explanations, which by the way, are almost always very clear at least to me.

Now calling the area behind the sherwood orifice a balance chamber is IMO a little misleading. That's because a balance chamber is a closed area specifically designed to provide for counteracting pneumatic force. The area behind the orifice in this reg is simply exposed to supply pressure because it's in line with the flow of air from the tank. The "balancing" comes from the action of the spring washers which mechanically move the orifice back further away from the seat, which causes the mainspring to compress more as the tank empties and the downstream force on the seat lessons.

But, I understand your point that the area functions sort of in the way that a balance chamber does in that it's a changing pneumatic force counteracted by a spring force. It's just not designed specifically for pneumatic balancing in the way that a balance chamber for a BD 1st stage (removing HP force on the base of the seat stem) or for a barrel poppet 2nd (providing opposing IP force on the poppet base) works.

 

[Luis H]

Well, maybe I am not being very clear.

The “floating volcano orifice" has two pneumatic forces of consideration; in addition it has the spring force and the contact force of the volcano orifice. The area of the O-ring sealing the HP pressure chamber from the LP pressure chamber (minus the small area of the volcano orifice itself) times the differential pneumatic pressures provides the pneumatic forces. When you have two pneumatic forces actuating a moving cylinder, you have a pneumatic piston. The designer then added a spring to bias the differential pressure to make that piston do what it is designed to do; in this case it is to provide the required contact force for the volcano orifice to seal at the designed location.

I am not sure how else to explain it, but by definition, the volcano orifice is mounted on its own pneumatic piston.

You are used to seeing regulator pneumatic pistons with two O-rings because they are working with three pressures simultaneously: tank HP, IP, and ambient pressure. They are dealing with three pressure chambers, not two.

It may help if you think of the system in its static condition… forget about flow, downstream or upstream forces… just think of the static forces.

 

[PhilEllis]

This thread is great! Just shows that there is a WEALTH of knowledge on this board. Bravo!

Phil Ellis
www.divesports.com

 

[LeadTurn_SD]

This thread is great! Just shows that there is a WEALTH of knowledge on this board. Bravo!

Phil Ellis
Discount Scuba Gear at DiveSports.com - Buy Scuba Diving Equipment & Snorkeling Equipment

1+

I'm really just beginning the "journey" on understanding regulator function and repair, so threads like this are terrific and get "bookmarked".

Thanks all.

 

[halocline]

Well, maybe I am not being very clear.

The “floating volcano orifice" has two pneumatic forces of consideration; in addition it has the spring force and the contact force of the volcano orifice. The area of the O-ring sealing the HP pressure chamber from the LP pressure chamber (minus the small area of the volcano orifice itself) times the differential pneumatic pressures provides the pneumatic forces. When you have two pneumatic forces actuating a moving cylinder, you have a pneumatic piston. The designer then added a spring to bias the differential pressure to make that piston do what it is designed to do; in this case it is to provide the required contact force for the volcano orifice to seal at the designed location.

I am not sure how else to explain it, but by definition, the volcano orifice is mounted on its own pneumatic piston.

You are used to seeing regulator pneumatic pistons with two O-rings because they are working with three pressures simultaneously: tank HP, IP, and ambient pressure. They are dealing with three pressure chambers, not two.

It may help if you think of the system in its static condition… forget about flow, downstream or upstream forces… just think of the static forces.

No, I think I understand what's happening with the orifice, and I understand why you would choose to call it a pneumatic piston, based on it's movement. In fact, there's a certain symmetry to the whole design if you consider the orifice one piston, and the (main) piston the other, each with mechanical spring pressure pushing them apart and pneumatic pressure pushing them together.

I just think it's misleading to call the floating orifice a balance chamber because the pneumatic pressure that makes the orifice act as a piston is not there to provide air balancing for the reg, it's just there because that's where supply pressure comes from, and the exact same pressure is there in the fixed orifices if I'm not mistaken. The o-ring on the orifice does not isolate HP air from IP, correct? As soon as air flows through the orifice, raising IP closes the piston, with IP on one side of the seat/orifice seal and HP on the other.

The beauty of the flow-by system is that the piston stem o-ring is not subjected to HP air, and if I'm correct, the o-ring on the floating orifice has no pressure differential. If IP were on one side, the orifice would move with each breath in the same manner that the piston does. That's not what happens, is it?

 

[Luis H]

The volcano orifice piston O-ring does see full pressure differential all the time. On one side is full tank pressure and on the other side is IP. That piston also moves with every breath just like the primary piston.

The description on your first paragraph was very good, about having two pistons working together to produced the designed results. And yes the symmetry is was makes the balance of forces possible. The primary piston uses IP with a large area, the secondary piston (volcano orifice piston) uses HP with a small area. The HP didn't just happen to be there, it was designed to be there, behind the volcano orifice piston.

The non-balanced (fixed volcano orifice) has no piston or piston chamber. The HP only pushes on the seat of the one (and only) primary piston.

 

[halocline]

Okay, if the o-ring on the orifice separates HP from IP, which is what thought at first, then is there no pressure gradient at the seat, except for the area inside the orifice itself? This does make sense now that I think about it, and would explain why there is an o-ring on the fixed orifice as well. Because the piston o-ring is behind the small hole in the piston, the IP chamber extends from the area on the other side of the piston head, through the piston stem, out the small hole, and includes the area all around the seat/orifice to the o-ring on the orifice. Right?

If this is correct, that o-ring is a dynamic o-ring with a big pressure gradient. Does it have the same problems with extrusion as the HP o-ring on the MK5/10? And this also means that the system loses one of the benefits of the MK2, which is the absence of any dynamic o-rings subject to HP air.

 

[Luis H]

Okay, if the o-ring on the orifice separates HP from IP, which is what thought at first, then is there no pressure gradient at the seat, except for the area inside the orifice itself? The inside of the volcano orifice has HP the outside is IP.
This does make sense now that I think about it, and would explain why there is an o-ring on the fixed orifice as well. See comment below.
Because the piston o-ring is behind the small hole in the piston, the IP chamber extends from the area on the other side of the piston head, through the piston stem, out the small hole, and includes the area all around the seat/orifice to the o-ring on the orifice. Right? Correct

If this is correct, that o-ring is a dynamic o-ring with a big pressure gradient. Yes, it is a dynamic O-ring with a large pressure gradient.
Does it have the same problems with extrusion as the HP o-ring on the MK5/10? It has the same design issues, but it is not a problem if it was design with that in mind from the beginning. The main issue with the Mk-5 and Mk-10 is that the design dates to lower pressure tanks.
And this also means that the system loses one of the benefits of the MK2, which is the absence of any dynamic o-rings subject to HP air.

Comment:
I am assuming that you are referring about the Brut. I am not familiar with that first stage… actually the truth is that I am not that familiar with any of the Sherwood, I have only serviced a few. I don’t know the geometry of the Brut, but if they wanted to reuse most of the parts, one way to do it is to replace the Belleville spring with some solid spacers. Is that what they are doing? If that is the case they would require the same O-ring to isolate the HP chamber from the LP chamber, but in this case it would be a static O-ring (still the same type of gland seal, but it doesn’t move).


The balanced design does contain a dynamic O-ring with high differential pressure and it seems a lot more complicated than the Mk-5, but IMO it really isn’t that complicated. All the O-rings work in a very clean environment and if the tolerances on the O-ring gap are design properly it is possible to design against O-ring extrusion.

IMO what is far more critical is the controlled leak or dry vent as they call it. That is a very clever idea and when it works, it works great (and it seems like it works most of the time). But if it stops leaking the ambient pressure does not compensate at depth and diving below about 60ft it get to be like sucking molasses with a straw (I haven’t personally experienced it but I have witnessed it).

I have serviced just a few Sherwood and when I opened the environmental chamber they were all immaculate. The dry vent (I prefer to call it the controlled leak) does a great job at keeping any contaminants and corrosion out of the chamber. Therefore, all the O-rings are sliding on a clean surface.

Even if the ambient chamber flooded, the two O-rings exposed to water are the LP O-rings similar to the Mk-2. The HP dynamic O-ring is always in a clean internal environment (as opposed to the one in a Mk-5) and the pressure differential is a bit lower (HP to IP, versus HP to ambient), but truly this last point is very minor.

 

[DaleC]

1+

I'm really just beginning the "journey" on understanding regulator function and repair, so threads like this are terrific and get "bookmarked".

Thanks all.

I'm in the same boat. Just starting to look at the guts. Most of this thread is going over my head but in a year or two I'm sure it will all make sense