Valves "full open" or not?

[lamont]

I brought this question up at the LDS last week. they said they recommend turning a half turn back after opening because leaving the valve full-open compresses the teflon (or nylon) seal in the valve. the longer it is compressed, the less life you get from it. by closing the valve the half turn, you relieve the compression from that seal.

Anyone actually ever encountered this problem in real life? (And on halfway modern scuba gear made since the 1980s or so, since I really don't care about valves made back in the 60's?).

 

[rjack321]

I brought this question up at the LDS last week. they said they recommend turning a half turn back after opening because leaving the valve full-open compresses the teflon (or nylon) seal in the valve. the longer it is compressed, the less life you get from it. by closing the valve the half turn, you relieve the compression from that seal.

Exactly which part is supposely breaking down?
A modern valve looks like this (from here: Thermo Valves)

Part number 23, is what closes the orifice allowing the gas to flow or not. Its impossible to compress that by OPENING the valve. In the pic the nylon seat portion of that plug is on the right hand side of it.

There are two parts you MIGHT be able to wear out by forcefully and excessively opening a valve all the way. Those are parts hold the gas in but allow the stem to still turn, numbered 5 and 17 in the pic. If you do wear those out the valve will start to leak from "underneath" the knob. Although valve can also leak there when any lube gets crusty and salty even though the nylon washer and o-ring are fine.

I have never heard of anyone having a catastrophic stem failure. Yes you have to replace those parts every few hundred dives more for salt and rotational wear than excessive compression.

Edit: the stem can break off but the oring doesn't catatrophically leak. linked direct to pic too

 

[wreckchick]

I have had valves stick open on occasion, but this was probably due to lax maintenance and jamming the valve open. The only way to fix it is to crack the burst disk and drain it that way which is a PITA.

R

 

[whoji]

Ive had valves 'Jam' ( for want of a better word) shut.
Kitting up on a pontoon I turn my air on whilst the tanks are on my back, kind of tells my arms they can reach them. On occasion they have been too 'closed' to open.

Therefore as I have found there is a possibility of jamming, all be it in the reverse direction my preference is to open then slightly turn back.

 

[Dive-aholic]

I have had valves stick open on occasion, but this was probably due to lax maintenance and jamming the valve open. The only way to fix it is to crack the burst disk and drain it that way which is a PITA.

R

You can always depress the purge on your second stage...

 

[Arizona]

For recreational diving it seems open all the way, and just a hair back makes most sense from the majority of posters.

 

[Capt Jim Wyatt]

AND if the valve gets bumped it reduces the risk of it jamming open

It will??? I think a millimeter will make no difference, Rick.