Viton 75 or EPDM 80 for valve stem O-ring?

[Pao]

I'm fresh out of 010 Viton 90 o-rings and I need the valve in about 12 hrs. Can I use a 75 durometer Viton or an 80 durometer EPDM o-ring for the valve stem? It's the o-ring that pairs with the teflon packing ring. It's part # 17 on the picture. It's not a Thermo valve, I just used the picture for reference.

Thanks!

Exploded View - Thermo K

 

[tbone1004]

you'll be fine with either of them. if it's not being used with O2 then it doesn't matter which one you pick

 

[Pao]

you'll be fine with either of them. if it's not being used with O2 then it doesn't matter which one you pick

I'm more afraid of failure/extrusion than O2 compatibility since that O-ring is seeing 3000 PSI and is dynamic.

 

[tbone1004]

I'm more afraid of failure/extrusion than O2 compatibility since that O-ring is seeing 3000 PSI and is dynamic.

it'll be fine. It can't really extrude if you think about how it works. It goes around the stem with the pressure pushing it inwards. Very different than a yoke or tank O-ring where the pressure is pushing it outwards and it can extrude easily.
Failure isn't going to happen, certainly not in a few dives.

 

[guruboy]

What is that oring for?

 

[tbone1004]

What is that oring for?

it keeps the air in the tank. If you've ever seen a tank where it holds air but when you open the tank valve it leaks, then that o-ring is bad.

The stem, part 8 in that diagram is what connects to your tank knob allows you to turn the seat *2,3*
The seat seals the tank valve when it is closed, but when you open it, the stem actually shoots out towards part 4.
Part 4 seals against the valve with the copper crush gasket *16*, and then the outside sealing is done by the o-ring *17*
Part 5 is a PTFE washer that allows the o-ring to slide around part 4 without shredding since it is under a lot of pressure *whatever tank pressure is* and you have to turn the knob to open it all the way.

 

[guruboy]

How come you said it gets pushed inward?

 

[Pao]

That o-ring sees a lot of pressure. You put in that teflon packing nice and flat when new, but come service time it has a groove where the o-ring snugs up against it.

 

[axxel57]

it keeps the air in the tank. If you've ever seen a tank where it holds air but when you open the tank valve it leaks, then that o-ring is bad.

The stem, part 8 in that diagram is what connects to your tank knob allows you to turn the seat *2,3*
The seat seals the tank valve when it is closed, but when you open it, the stem actually shoots out towards part 4.
Part 4 seals against the valve with the copper crush gasket *16*, and then the outside sealing is done by the o-ring *17*
Part 5 is a PTFE washer that allows the o-ring to slide around part 4 without shredding since it is under a lot of pressure *whatever tank pressure is* and you have to turn the knob to open it all the way.

The o-ring in that configuration does some sort of sealing, but a second teflon washer would do the same job as it works with the Sherwood valves.
The main sealing work is done by the Copper Gasket or o-ring and Bonnet nut, in more than 90% of the cases, if the valve leaks when opening the valve, the Bonnet Nut has to be tightened a bit more.
If there is still a small leak, the chrome in the Bonnet Nut might be worn off and gives so some space for leaking and must be replaced.
The washers (Sherwood) and the o-ring (Thermo etc.) help to reduce the friction when opening and closing the valve, depending on how strong the hand wheel has been tightened to the stem or how strong the handwheel is closed.
This is why the o-ring should be Buna 90, but of course it works also with Buna 75 for a while.
The stem is of course pushed outwards........