Lessons Lost Mav button... how is it even possible....

[HeorhiiP]

How is this possible?

After coming back home I see oxygen mav knob is gone

Some parts are missing, probably these ones

Quite dangerous at depth

That mav can be stuck open and I would have to shut off the tank immediately or face the danger of oxygen toxicity...

 

[-JD-]

It looks to me that that button is threaded onto the valve plunger shaft which passes through the metal valve body from the inside.
My guesses:

• It was unscrewed by repeated off-center presses that rotated it a little at a time. Something then bumped it in transit and was the last straw. Threadlocker may be the only practical fix for that design - it is not immediately obvious how it could be pinned.
• It is also possible that a strong hit broke the internal threaded boss in the button and it came apart. The pictures don't give detail of the inside of the button, so a bit more of a guess

From the arrows on the body it would appear that that it is acting as a downstream valve and relys on the spring under the button to hold it closed. So yes to needing to shutdown at either an inline shutoff or cylimder valve.

Finding and inspecting that button would be worth a bit of searching IMO.

 

[grantctobin]

I'd also add, most people don't seem have the correct tool MAV Tool - SubGravity to maintain and service those valves. They're rather a pain in the arse to service without. Is there a chance you or a previous owner managed to not tighten it sufficiently?

 

[HeorhiiP]

I'd also add, most people don't seem have the correct tool MAV Tool - SubGravity to maintain and service those valves. They're rather a pain in the arse to service without. Is there a chance you or a previous owner managed to not tighten it sufficiently?

I'm first owner... never tightened it since the beginning and was actively using it... probably that is the case. Need to include that into predive checklist ..

 

[HeorhiiP]

It looks to me that that button is threaded onto the valve plunger shaft which passes through the metal valve body from the inside.
My guesses:

• It was unscrewed by repeated off-center presses that rotated it a little at a time. Something then bumped it in transit and was the last straw. Threadlocker may be the only practical fix for that design - it is not immediately obvious how it could be pinned.
• It is also possible that a strong hit broke the internal threaded boss in the button and it came apart. The pictures don't give detail of the inside of the button, so a bit more of a guess

From the arrows on the body it would appear that that it is acting as a downstream valve and relys on the spring under the button to hold it closed. So yes to needing to shutdown at either an inline shutoff or cylimder valve.

Finding and inspecting that button would be worth a bit of searching

It holds the pressure without button until you press it.
Then it is impossible to pull back - nothing to catch onto, and doesn't come back on its own, no matter inlet is pressurized or not, as a result it just free-flows until pulled back for example by tweezers

 

[-JD-]

It holds the pressure without button until you press it.
Then it is impossible to pull back - nothing to catch onto, and doesn't come back on its own, no matter inlet is pressurized or not, as a result it just free-flows until pulled back for example by tweezers

Somewhat surprising to me that it doesn't blow open. Was that test at full IP, connected to a reg or just blowing into it?

 

[HeorhiiP]

Somewhat surprising to me that it doesn't blow open. Was that test at full IP, connected to a reg or just blowing into it?

Connected to 1st stage and pressurized
Tank was closed
I pushed that pin and it released all the pressure into the loop

 

[Jona Silverstein]

this is another reason to have mavs that are designed to fail closed ie have the upstream side opposing the direction you push the mav so that in the instance you lose the button or have the spring fail, the IP will always keep the button closed instead of forcing it to stay open.