Opening Tank Valve

[DavHug]

Hi All - my recently purchased Scubapro Mark25 Evo reg still says in the manual to gently purge the second stage when opening the cylinder. I thought that practice was now obsolete - would appreciate any guidance. Thanks

 

[rsingler]

You'll get lots of advice here on SB about a thing called IP, or Intermediate Pressure. It's the pressure delivered to your second stage by your first stage, which steps down 3000 psi to about 135 psi.
Checking IP stability can help you see some problems in advance. Since most divers don't check their IP regularly, they don't have advance warning of an oring failure which would allow IP to go way past safe limits for your hoses and second stage mechanism.

That being the case, Scubapro's advice vents the first stage, so that if there were a catastrophic internal failure, it would be suggested by abnormal air flow during those initial moments of venting, allowing you to turn the tank back off before you blew a hose or broke apart the parts inside the second stage.
Pretty conservative advice, given the general safety of well maintained gear. But not bad advice.
Better advice? Read and follow @couv 's checklist here:
Regulator Inspection and Checklist (Rev-8)

 

[tursiops]

You'll get lots of advice here on SB about a thing called IP, or Intermediate Pressure. It's the pressure delivered to your second stage by your first stage, which steps down 3000 psi to about 135 psi.
Since most divers don't check their IP regularly, they don't have advance warning of an oring failure which would allow IP to go way past safe limits for your hoses and second stage mechanism. Checking IP stability can help you see some problems in advance.

That being the case, Scubapro's advice vents the first stage, so that if there were a catastrophic internal failure, it would be suggested by abnormal air flow during those initial moments of venting, allowing you to turn the tank back off before you blew a hose or broke apart the parts inside the second stage.
Pretty conservative advice, given the general safety of well maintained gear. But not bad advice.
Better advice? Read and follow @couv 's checklist here:
Regulator Inspection and Checklist (Rev-8)

I thought the reason to gently purge while turning on the gas was to prevent a pressure wave (like a water hammer) from harming the seat.

 

[rsingler]

Yeah that's the original advice when you could theoretically damage a HP seat. If that's why it's still in the manual then it's obsolete. Folks have been slam opening the valve for decades to pre-indent a new seat. You can call it bad practice, but every new polymer should be able to handle it. The physics of it are that increasing IP should already be decelerating the rate of valve closure. Slam opening depends on inertia, and most diaphragm poppets are very light these days, compared with the old Dacor stems. And for pistons, where mass is indeed significant, there are no more true knife edges.
For the readers: keep in mind that it's not 140 psi vs 0 on the seat side. It's actually the opposite: the piston spring and the diaphragm spring are both already placing almost that much pressure in the opposite direction, opposing valve closure. It's not a freight train hitting a wall, it's a seaman in a submarine movie pushing a hatch closed against a flood: it goes easily only at the beginning.
But @tursiops is right. In the oldest days, the Mk5 piston seat wasn't supposed to be flipped. The sharp knife edge of an unshouldered piston coring the seat could actually go right through the weak spot, if a seat was flipped for a fresh surface instead of being replaced.

 

[TMHeimer]

I learned to do it from the instructor when I took Rescue. Still do it every time. He said "Gives the air somewhere (easy?) to go". Then again, I still turn my tank valve back a tiny bit after turning air on (not a quarter turn). I'm just one of those guys.

 

[PfcAJ]

Pointless. Right up there with quarter turn back on the valve.

 

[OTF]

never heard of this practice; am still alive

 

[mwerle]

There's a small point when turning on pure O2 or very high percentage mixes - to have slightly less adiabatic heating and thus reducing the chances of an oxygen fire. But firstly you should be turning the tank on very VERY gently with those anyway and secondly at IP (9-10 bar vs 200-300 bar) the risks are significantly reduced already. Still, if there is a first stage issue letting significantly higher IP through it could get "interesting".

As for hoses bursting, that should never happen - the second stage (unless it is ALSO faulty) is designed to freeflow not much above regular IP thus venting any excess pressure.

Of course, this is all with regards to modern regulators..

 

[DavHug]

Hi All - my recently purchased Scubapro Mark25 Evo reg still says in the manual to gently purge the second stage when opening the cylinder. I thought that practice was now obsolete - would appreciate any guidance. Thanks

Many thanks all

 

[Bob DBF]

I thought the reason to gently purge while turning on the gas was to prevent a pressure wave (like a water hammer) from harming the seat.

I was told to open the valve slowly to avoid that issue, with any high pressure gas, slow is good. A number of Os' could be added to slowly to emphasize the point.