Tuning PCP regulator designs

[Nietzsche]

I'm trying to wrap my head around Huma's regulator design & I don't mean to call out Huma specifically. I own their products & believe their machining quality/finish is 2nd to none. In reality my questions could be applied to all PCP regulated air rifles.

In reference to the regulators in pcp guns they all seem to use spring washers are providing the resistance to the flow of air from a high pressure to a low pressure. As that pressure difference increases the resistance to flow weakens & the flow increases. As the difference lowers the resistance to flow increases & the transfer lowers. this process never completes. the transfer tapers off to infinity & thus never completing the transfer. The amount of transfer can be so small that our tiny gauges would never be able to show the difference, nor could we hear the common hiss of that tiny amount of air passing thru the regulators. This process begins & continues once we place air into the gun & never completes until the gun is void of any pressure.

Is this how all pcp regulators are designed or is there any regulators that incorporate some sort hysteresis in order to achieve a complete seal? Do they all gradually taper off the transfer between high & low chambers to such a small amount that we just view it as a "complete seal"?

Nietzsche

 

[sb327]

Just for the record

The springs provide an OPENING force for the flow. The reservoir pressure also provides a smaller opening force as well but is reasonably negligible compared to the spring force in a well designed reg.

The CLOSING force of the reg is provided by the downstream/plenum pressure.

If the seal is in good working order, the downstream pressure will, after x amount of time, close the valve completely. This is due to the increasing closing force as flow (pressure rise in plenum) occurs.

You reference infinity in your description but I see it more like approaching zero. I do understand (I think) what you were trying to say though.

Dave

 

[bchannell]

YT has some good videos showing regs in action.

 

[Nietzsche]

Okay so there is a point in time "reason point in time" where the regulation of the two pressures actually completes & seals until there is a difference in pressure greater than what the washers can hold back such as "shooting the gun"?

 

[nervoustrig]

Yes, at some point the output pressure rises sufficiently high to completely squeeze the valve seat closed.

Depending on the nature of the valve seat (geometry, size, materials, and surface quality), the pressure may plateau very quickly or very slowly. But it definitely will not continue to rise indefinitely toward the reservoir pressure unless there is a seal failure of some kind.

 

[SilentSquirrel]

No insult implied or intended @Nietzsche
simply a little fun

Your reasoning suits your username

just my t00 sense
Edward

 

[corny]

In my many years of messing with regs.

I like to see them maintain air pressure even when the air source is removed.

That tells me, it is in fact sealed.

 

[Nietzsche]

No insult implied or intended @Nietzsche
simply a little fun

Your reasoning suits your username

just my t00 sense
Edward

Hahaha!!! No insult taken...

I hadn't thought about performing some sort mental gymnastic routine that results in depressing the hell outta everyone on the forums.

Nietzsche must was been that friend that everyone has that was pasty white & always dressed in black, was either getting over being sick or coming down with some sort of rare virus or disorder. All his friends must have asked each other, 'what's wrong with Nietzsche now?'

Nietzsche

 

[sb327]

Okay so there is a point in time "reason point in time" where the regulation of the two pressures actually completes & seals until there is a difference in pressure greater than what the washers can hold back such as "shooting the gun"?

“””until there is a difference in pressure greater than what the washers can hold back such as "shooting the gun"?”””quoted from above

Again, the washers (spring) do not hold the regulator closed. They provide the force to open the regulator.

The pressure in the plenum (regulated pressure) pushes against a piston to provide the closing force.

So ….basically two forces acting in opposition.

When plenum pressure drops (shooting), the pressure on piston drops…lowering the force acting against the spring.

Springs force regulator open until pressure rises in plenum to effectively provide enough force to overcome spring (regulator closes again).

You mentioned hysteresis in your original post. Some does exist, sort of, but it happens from set point pressure down. Meaning, it reaches setpoint quickly and remains there. A small pressure drop doesn’t always induce the valve OPENING. This is evidenced in low power/large plenum guns that don’t drop plenum pressure, per shot, enough to make the regulator cycle. This is probably due to the friction (stiction) created by the orings being static at this time. @Nervoustrigger can speak better to this than I.

Dave

 

[stovepipe]

I'm trying to wrap my head around Huma's regulator design & I don't mean to call out Huma specifically. I own their products & believe their machining quality/finish is 2nd to none. In reality my questions could be applied to all PCP regulated air rifles.

In reference to the regulators in pcp guns they all seem to use spring washers are providing the resistance to the flow of air from a high pressure to a low pressure. As that pressure difference increases the resistance to flow weakens & the flow increases. As the difference lowers the resistance to flow increases & the transfer lowers. this process never completes. the transfer tapers off to infinity & thus never completing the transfer. The amount of transfer can be so small that our tiny gauges would never be able to show the difference, nor could we hear the common hiss of that tiny amount of air passing thru the regulators. This process begins & continues once we place air into the gun & never completes until the gun is void of any pressure.

Is this how all pcp regulators are designed or is there any regulators that incorporate some sort hysteresis in order to achieve a complete seal? Do they all gradually taper off the transfer between high & low chambers to such a small amount that we just view it as a "complete seal"?

Nietzsche

I understand your observations and your question. I've thought about this stuff many times when trying to think of a regulator design that would very quickly refresh the plenum and then just slam shut. I'm only familiar with the type of regulator you described -- the type that fills the plenum slower and slower as the desired plenum pressure is approached. Theoretically, if there was absolutely no friction and the springs provided a perfectly smooth resistance curve and there was no such thing as thermal expansion/contraction and all surfaces were perfectly smooth, then the regulator would never truly stop allowing air into the plenum -- oh, and atoms would have to be infinitely small too. In the real world, friction keeps the closing mechanism (piston) from moving in direct proportion to the plenum pressure increase. Near the end of the refresh cycle, pressure is building in the plenum slowly and friction keeps the piston from moving until finally the plenum has enough pressure to make the "stuck" piston move with more than enough pressure to completely stop the flow of air. So, thanks to friction, a complete seal will eventually happen. Unfortunately, thanks to friction, the amount of pressure in the plenum won't always be exactly the same after the flow of air has stopped. So I guess friction (stiction) is indirectly the hysteresis that allows for a complete seal.

I sound smart, but I'm just wingin' it.

stovepipe

 

[Nietzsche]

Stovepipe,

First thanks for the reply & everyone else's. I'm really taken back & at a loss for words, this is "hands down" the most courteous forum I ever joined (& I've joined more than I should..). The thought that there's a forum available that not only shares ideas but elevates new members to a higher level of knowledge without the verbal assaults is priceless & rare. So for those reasons alone thanks guys, it means alot to me!

I'm glad you understood what I was trying to ask & Sb327 "Dave" is right, I'm always get formulas mixed up in my head. You could give me the 27 glossy photos with circles & arrows & somehow I'd still come out of the bathroom with the beans above the frank, jammed up in the zipper!?

As far as the regulator design, I've only found slight variations but they ALL use the bellevue washers. So I guess this is for now the tried & true mechanism for regulating hpa tanks for airgun use. My reasons for asking were mainly because of my lack of understanding of the mechanical design. I've been working on an M3 .357 trying to get it to seal up. It's taken about 3-4 times breaking it down, using a set of magnifiers glasses & really looking at the orings for damage & over all condition, cleaning the threads on everything & treating it like you were splitting a hydrostat tranny. Anyway, I finally got the M3 sealed it's holding almost 250bar for 3 days now with just a small drop from temp.

Nietzsche

 

[Troy Bakel]

I understand your observations and your question. I've thought about this stuff many times when trying to think of a regulator design that would very quickly refresh the plenum and then just slam shut. I'm only familiar with the type of regulator you described -- the type that fills the plenum slower and slower as the desired plenum pressure is approached. Theoretically, if there was absolutely no friction and the springs provided a perfectly smooth resistance curve and there was no such thing as thermal expansion/contraction and all surfaces were perfectly smooth, then the regulator would never truly stop allowing air into the plenum -- oh, and atoms would have to be infinitely small too. In the real world, friction keeps the closing mechanism (piston) from moving in direct proportion to the plenum pressure increase. Near the end of the refresh cycle, pressure is building in the plenum slowly and friction keeps the piston from moving until finally the plenum has enough pressure to make the "stuck" piston move with more than enough pressure to completely stop the flow of air. So, thanks to friction, a complete seal will eventually happen. Unfortunately, thanks to friction, the amount of pressure in the plenum won't always be exactly the same after the flow of air has stopped. So I guess friction (stiction) is indirectly the hysteresis that allows for a complete seal.

I sound smart, but I'm just wingin' it.

stovepipe

For winging it you are pretty much right on!

The only property you did not include is that each disc does not physically expand or compress at a linear rate nor at the same rate as any of the other disc's in the stack.

As such, when you are measuring the force exerted by the stack as it is slowly compressed, the output is no where near linear, but is a sawtooth with varying peak heights and valley depths.

This is why the stack may close off flow at a slightly lower than setpoint pressure (valley) or higher (peak).

Disc type regulators are vary sensitive to the differential pressure across the stack. As your input pressure decreases to that of the regulator's output pressure the regulator will begin to " hunt" and may not fully close when input pressure is within 10 bar of the regulator's set pressure.

Add your "stiction" and you have the full picture.

I apologize @stovepipe ! I read your "springs" literally when you actually meant disc.

So not bad for "winging it"!!!!

 

[stovepipe]

As such, when you are measuring the force exerted by the stack as it is slowly compressed, the output is no where near linear, but is a sawtooth with varying peak heights and valley depths.

This is why the stack may close off flow at a slightly lower than setpoint pressure (valley) or higher (peak).

I imagine if someone recorded the sound of a regulator cycle -- like with a direct to metal high-sensitivity and high dynamic range microphone recorded at 192K samples per second and 32-bits per sample (typical digital audio workstation setup on a home computer) and worked some audio magic on it, they'd be able to isolate the sound of the piston "eeeking" past the o-rings and also isolate the sound of the belleville washers making whatever sounds they make as they get compressed. I imagine that, near the end of the cycle, the bellevilles might exhibit "ticking" sounds.

That might be a fun project for a really, really, really bored audio engineer / musician that happens to have a PCP rifle.

stovepipe