To purge or not.

[Greg Barlow]

When a first stage (piston or diaprhagm) is in an unpressurized state the hard seat (volcano shaped orifice) is separated from the soft seat. This is caused by an internal spring forcing the two seats apart. At this point, the two parts are separated by a distance that is typically greater than what occurs during a normal drop in intermediate pressure during inhalation.

As the reg is pressurized the two seats are forced together until the internal pressure slightly exceeds the ambient pressure, and the mechanical forces that were separating the two seats. Think of increasing or decreasing the external spring tension that is acting on a diaprhagm first stage, and how that impacts the reg's intermediate pressure setting.

Pushing the purge button, and/or slowly opening the cylinder valve allows the two seats to make their initial contact in a more controlled (slowed) manner. Remember, the two are at maximum separation prior to this action.

Given this thought, you can see how this should lessen the impact that the soft seat undergoes. Not to mention lessening the adiabatic compression issue that Akimbo clearly explained.

Me? I press the purge button, and slowly open the valve on all my cylinders. I can think of no logical reason not to do so.

Greg Barlow

 

[Thalassamania]

What I was taught was this:

Open the second stage with the purge valve, then slowly crack open the tank valve. As pressure builds and air starts to flow, release the purge valve. This avoids unnecessary wear on the second stage's needle seat.

If you've not seen the inside workings of a second stage I recommend you arrange to do so at a local shop. It's a quite simple mechanism and you'll see immediately what I'm talking about.

-Bryan

I can't see the logic here. All modern second stages are downstream valves, the pressure serves to open the valve (or at least reduce the force on the seat).

When a first stage (piston or diaprhagm) is in an unpressurized state the hard seat (volcano shaped orifice) is separated from the soft seat. This is caused by an internal spring forcing the two seats apart. At this point, the two parts are separated by a distance that is typically greater than what occurs during a normal drop in intermediate pressure during inhalation.

As the reg is pressurized the two seats are forced together until the internal pressure slightly exceeds the ambient pressure, and the mechanical forces that were separating the two seats. Think of increasing or decreasing the external spring tension that is acting on a diaprhagm first stage, and how that impacts the reg's intermediate pressure setting.

Pushing the purge button, and/or slowly opening the cylinder valve allows the two seats to make their initial contact in a more controlled (slowed) manner. Remember, the two are at maximum separation prior to this action.

Given this thought, you can see how this should lessen the impact that the soft seat undergoes. Not to mention lessening the adiabatic compression issue that Akimbo clearly explained.

Me? I press the purge button, and slowly open the valve on all my cylinders. I can think of no logical reason not to do so.

Greg Barlow

I also can not see any reason not to, but it is clearly not indicated with a downstream first stage.

 

[eponym]

I can't see the logic here. All modern second stages are downstream valves, the pressure serves to open the valve (or at least reduce the force on the seat).

You're quite right. The first stage seat is the question.

-Bryan

 

[knowone]

Press your thumb against your yoke outlet or stick it in your din hole.
Crack your valve and burn your thumb.
Fast gas especially oxygen, combined with most lubricants makes sparks fire and boom.
Non adiabatic.

 

[halocline]

I can't see the logic here. All modern second stages are downstream valves, the pressure serves to open the valve (or at least reduce the force on the seat).


I also can not see any reason not to, but it is clearly not indicated with a downstream first stage.

The exception with 2nd stages are the ones that have a 'seat saver' feature that typically uses cupped washers to keep the orifice off the seat unless the reg is pressurized.

You're right about this not being applicable to downstream 1st stages, but then again I've never seen a downstream 1st stage. I don't know if one's ever been made.

The idea of purging and/or opening the valve slowly is to protect the 1st stage, though I'm not sure it does much. It's IP that closes the valve, not HP air. I suppose the practice could reduce stress on o-rings that separate HP air, like the piston stem o-ring or the diaphragm balance chamber o-ring. The HP seat is designed to open and close with every breath, for hundreds of dives. It's pretty tough, and quick pressurization is almost certainly not going harm it. As far as 2nd stages go, I don't think there's any appreciable effect. As many people have said, it can't hurt.

 

[herman]

The exception with 2nd stages are the ones that have a 'seat saver' feature that typically uses cupped washers to keep the orifice off the seat unless the reg is pressurized.

You're right about this not being applicable to downstream 1st stages, but then again I've never seen a downstream 1st stage. I don't know if one's ever been made. The idea of purging and/or opening the valve slowly is to protect the 1st stage, though I'm not sure it does much. It's IP that closes the valve, not HP air. I suppose the practice could reduce stress on o-rings that separate HP air, like the piston stem o-ring or the diaphragm balance chamber o-ring. The HP seat is designed to open and close with every breath, for hundreds of dives. It's pretty tough, and quick pressurization is almost certainly not going harm it. As far as 2nd stages go, I don't think there's any appreciable effect. As many people have said, it can't hurt.

Go get some coffee Matt, you have got to still be asleep.
How about the SP MK-5 and MK-2, both are down stream first stages as are all piston regs- HP tries to push the valve open which makes them down stream valves. Down stream first stages are the ones that benefit most from slow pressurization. While I agree that the seat can take it, why subject it to any more harsh impact than is necessary. On a breathing cycle the piston is moving a relatively short distance and the diver tends to decrease the flow of the inhale towards the end of the breath so the IP can gradually increase and the piston more gently comes in contact with the seat. Contrast that to a quickly opened valve on an unpressurized reg. The piston is at its most distant point from the seat and is quickly and continually accelerated until it hits the seat. It's moving fast and has had more time to gain speed since it's at its maximum distance from the seat and being subjected to the maximum differential pressure it will see. It may not do much damage but IMO it's never good to slam parts together if you can avoid it. Doing yearly service it’s not likely that slamming the seats will make much difference but on the other hand if you want to keep the same seat in service for several years, then being gentle on them can not be a bad thing.