First Regulator Use after Service

[DA Aquamaster]

HP seat failure would be a bad case scenario; freeflow would be an understatement if the 2nds got a blast of 3000PSI air. Maybe "worst" case would be total loss of flow, but I don't see how that could happen other than a main spring failure, which I've never heard of.

A high pressure seat failure would be no worse than the first stage freezing open on an ice dive - the flow is more or less unrestricted but the pressure drops significantly as it leaves the valve and the pressure will never be anywhere near 3000 psi past the first stage.

Mainspring failure is a very rare occurrence primarily associated with over shimming a spring, causing it to function outside it's designed working range. Popular opinion is that this would in most cases not result in a loss of flow in a piston reg as the piston is downsteam of the orifice and would float between the open and closed position without spring pressure and would probably freeflow. Given the upstream location of the seat carrier in a diaphragm reg, the odds are higher that a mainspring failure could result in a failure to deliver air.

 

[halocline]

Mainspring failure is a very rare occurrence primarily associated with over shimming a spring, causing it to function outside it's designed working range. Popular opinion is that this would in most cases not result in a loss of flow in a piston reg as the piston is downsteam of the orifice and would float between the open and closed position without spring pressure and would probably freeflow. Given the upstream location of the seat carrier in a diaphragm reg, the odds are higher that a mainspring failure could result in a failure to deliver air.

I was wondering about that; on an unbalanced piston it's clear that downstream pressure would tend to push the seat off the orifice, but what about in a balanced flow-through piston? I suppose, if nothing else, drawing on a 2nd stage would eventually lower the pressure on the IP side of the piston head below ambient, which would allow pressure on the ambient side to "push" it off the seat. Does that seem right?

I don't understand why it would freeflow.

 

[dave4868]

A high pressure seat failure would be no worse than the first stage freezing open on an ice dive - the flow is more or less unrestricted but the pressure drops significantly as it leaves the valve and the pressure will never be anywhere near 3000 psi past the first stage.

Mainspring failure is a very rare occurrence primarily associated with over shimming a spring, causing it to function outside it's designed working range. Popular opinion is that this would in most cases not result in a loss of flow in a piston reg as the piston is downsteam of the orifice and would float between the open and closed position without spring pressure and would probably freeflow. Given the upstream location of the seat carrier in a diaphragm reg, the odds are higher that a mainspring failure could result in a failure to deliver air.

People would laugh watching my efforts this morning to "get my head around" these various failure modes. "It's jess shad", to quote Ross Perot....

Anyway, I've looked at several schematics and just now removed the mainspring and pressurized a piston 1st stage and was surprised to find an IP of 25 psi, rather than something close to 0 (zero). :11:

Apparently 25 psi differential is what it takes to completely stop the flow at the HP seat.

Anyhoo, if removing the mainspring simulates "mainspring failure", it appears to result in very low IP, high breathing effort and marginal flow, not freeflow, at least for a piston 1st.

I don't want to pull a muscle in my brain, but I'll definitely need to exercise some gray matter to get a better grip on this.

I'll also have to look at the design of the diaphragm 1st stage, but I'll need to pace myself....

I was wondering about that; on an unbalanced piston it's clear that downstream pressure would tend to push the seat off the orifice, but what about in a balanced flow-through piston? I suppose, if nothing else, drawing on a 2nd stage would eventually lower the pressure on the IP side of the piston head below ambient, which would allow pressure on the ambient side to "push" it off the seat. Does that seem right?

I don't understand why it would freeflow.

I just pulled a brain muscle, thanks a lot Matt!

I'll have to get back to this later....(after somebody explains it)

It still looks like problems due to servicing are low incidence and/or low risk, at least for regs with piston 1sts.

Dave C

 

[redacted]

Anyway, I've looked at several schematics and just now removed the mainspring and pressurized a piston 1st stage and was surprised to find an IP of 25 psi, rather than something close to 0 (zero). :11:

Apparently 25 psi differential is what it takes to completely stop the flow at the HP seat.

Anyhoo, if removing the mainspring simulates "mainspring failure", it appears to result in very low IP, high breathing effort and marginal flow, not freeflow, at least for a piston 1st.

I don't want to pull a muscle in my brain, but I'll definitely need to exercise some gray matter to get a better grip on this.

This one is making my brain hurt a bit also. If you have not reassembled that reg yet, try shaking it a bit and holding it with the turret pointing downward so gravity helps open the piston.

What model 1st stage are you using? I would think that the new 1sts (Mk20/25) with the cup HP seats would behave a bit differently than the older models (MK5/7/10) with the flat or cave cone seats.

 

[dave4868]

This one is making my brain hurt a bit also. If you have not reassembled that reg yet, try shaking it a bit and holding it with the turret pointing downward so gravity helps open the piston.

What model 1st stage are you using? I would think that the new 1sts (Mk20/25) with the cup HP seats would behave a bit differently than the older models (MK5/7/10) with the flat or cave cone seats.

Sherwood Blizzard is what I use, similar to an unbalanced 1st, but getting a balancing effect to compensate for changes in tank pressure by having a moveable crown/orifice sitting on cupped spring washers.

With the mainspring removed and regulator pressurized, I don't think shaking or holding the end cap upside down is likely to move the HP seat off the crown/orifice. At least not for more than a split second.

When pressurized, air moves through the piston's hollow shaft and then pushes the piston and its HP seat away from the end cap and toward the crown/orifice.

Apparently, when the pressure on the head of the piston reaches 25 psi, it's enough to shut off the high pressure flow at the crown/HP seat.

25 psi makes sense, now that I think about how the regulator works and the relative size difference in the piston and the crown, but I think I'm getting "brain fever", as they say....

Now, if "mainspring failure" means something other than breakage, perhaps meaning a major change in an intact spring's "rate" of force or range of compressibility, it would have a different effect on IP and performance.

Ow, my head hurts....

Dave C

 

[redacted]

Sherwood Blizzard is what I use, similar to an unbalanced 1st, but getting a balancing effect to compensate for changes in tank pressure by having a moveable crown/orifice sitting on cupped spring washers.

With the mainspring removed and regulator pressurized, I don't think shaking or holding the end cap upside down is likely to move the HP seat off the crown/orifice. At least not for more than a split second.

When pressurized, air moves through the piston's hollow shaft and then pushes the piston and its HP seat away from the end cap and toward the crown/orifice.

Apparently, when the pressure on the head of the piston reaches 25 psi, it's enough to shut off the high pressure flow at the crown/HP seat.

25 psi makes sense, now that I think about how the regulator works and the relative size difference in the piston and the crown, but I think I'm getting "brain fever", as they say....

Now, if "mainspring failure" means something other than breakage, perhaps meaning a major change in an intact spring's "rate" of force or range of compressibility, it would have a different effect on IP and performance.

Ow, my head hurts....

Dave C

Too many boards and too many threads. The poster with SP regs is on Scubatoy's forum.

I had to dig out Harlow. So it's an unbalanced piston with a floating orifice to compensate for the IP drop - intresting. I suspect my scubapro Mk2/5/7/10s would behave the same. Not so sure about the Mk20 with the distinctly cupped seat as the piston has to move a bit more to get good flow rates. Well, maybe that really is not going to matter much.

I believe we are talking about a broken/cracked spring which would result in the spring applying less force to push the piston away from the seat. Even damaged, the pieces should provide some force resulting in an IP a bit higher than you got with no spring. Plus, with a pneumatically balance 2nd stage, the effects of the reduced IP would not be as degrading as they were with your classic downstream design Sherwood 2nd.

 

[dave4868]

Too many boards and too many threads. The poster with SP regs is on Scubatoy's forum.

Thanks for explaining the mixup! Had me wondering....

I had to dig out Harlow. So it's an unbalanced piston with a floating orifice to compensate for the IP drop - intresting. I suspect my scubapro Mk2/5/7/10s would behave the same. Not so sure about the Mk20 with the distinctly cupped seat as the piston has to move a bit more to get good flow rates. Well, maybe that really is not going to matter much.

I believe we are talking about a broken/cracked spring which would result in the spring applying less force to push the piston away from the seat. Even damaged, the pieces should provide some force resulting in an IP a bit higher than you got with no spring. Plus, with a pneumatically balance 2nd stage, the effects of the reduced IP would not be as degrading as they were with your classic downstream design Sherwood 2nd.

Interesting! If I had a rare mainspring failure, I have a feeling there would still be adequate output from my 2nd stage to abort the dive without a hurried CESA.

Dave C

 

[DA Aquamaster]

I got bored today and took the spring out of a MK 5 (balanced piston first stage). It delivers air regardless of position and from the sounds of it, the piston vibrates quite rapidly as it has a whistling sound to it. It does not deliver an enormous amount of air and the inhalation effort is a little higher, but I suspect it would be enough to get you to the surface. It was interesting that it never free flowed at all.

The wild card in the deck would be how the parts of the spring oriented themselves in the reg. If they prevented the piston from moving all the way back, it would cause a freeflow, but it should not prevent air from flowing.

 

[dave4868]

I got bored today and took the spring out of a MK 5 (balanced piston first stage). It delivers air regardless of position and from the sounds of it, the piston vibrates quite rapidly as it has a whistling sound to it. It does not deliver an enormous amount of air and the inhalation effort is a little higher, but I suspect it would be enough to get you to the surface. It was interesting that it never free flowed at all.

The wild card in the deck would be how the parts of the spring oriented themselves in the reg. If they prevented the piston from moving all the way back, it would cause a freeflow, but it should not prevent air from flowing.

With the mainspring removed, rather than freeflowing, it locked right up when pressurized, just as it would normally, right?

You didn't happen to check IP while it was locked up, did you?

I'm looking at a schematic for the MK5 and it appears the crown contact area might be bigger relative to the piston diameter than in my Sherwood.

If it is, I'd bet your IP with the mainspring missing is higher than the 25 psi I got with my Sherwood.

Just curious.

Dave C