One Way MDE Twinset Valve?

[Neil R]

Possible explanations are invited for this one please:

This is on my manifolded 12L twinset, with the previous generation of MDE valves (the ones with the flat-face O-rings at each end of the manifold). On a recent trip, the valve that normally sits behind my left shoulder worked fully as expected during the diving, but it completely refused to pass gas inwards when re-filling from that side. Not even a dribble; the compressor would only fill from the other side. Yes the valve was fully open, and always felt normal to turn, and this behaviour was fully repeatable over a couple of days.

For right or wrong I still dived with it like this, after checking that the problem side would always deliver gas, both to the necklaced reg and SPG (with the manifold closed), and as normal to the primary reg via the manifold. I obviously kept a close eye on the SPG throughout.

I have just drained the affected cylinder, disassembled the valve parts, and expected to find either debris else a loose plastic insert in the threaded brass part. But nothing obviously wrong spotted.

I've given it a good blow through (from both the manifold side and with a decanting whip), installed a new brass insert, and all now appears to be working normally. I just wish I knew what might have caused this so I can have some confidence it's been properly fixed!

In case this might be relevant, that cylinder did have a very short free flow when the valve was inadvertently operated as it was unloaded from the car early in the trip.

[Steve Clark]

How odd.

Could this be a human error? It's a left hand valve and everything is opposite. If the fill whip is connected first and then the habit is to turn the valve a particular way, with a particular hand and they have to use the other hand because of the tank position, could they actually be trying to fill against a closed valve that they think is open? Or using a wrong hand and opening the valve on the cylinder, but using another opposite motion and closing the valve on the whip?

[Dave1w]

I can’t remember the insides of those, but have had it on a similar age normal
MDE valve. The jobby between the knob and the seat thing is pushed out by pressure and didn’t engage. Long time ago so don’t remember exactly


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[BcDiver]

I have a similar manifold on a set of twin 12s that does the same thing when filling - if the compressor is pressurised to blow off pressure then I open the cylinder valves no air will pass from the compressor side to the cylinder side, I need to bleed the compressor down so that it is less than the cylinder side and when I do this and then open the cylinder valves the air will then flow from compressor to cylinder. Only seems to happen with this set of valves as has not happened with any other mde valves. There's never been an issue when in use so has never really bothered me.

[Energy58]

I can’t remember the insides of those, but have had it on a similar age normal
MDE valve. The jobby between the knob and the seat thing is pushed out by pressure and didn’t engage. Long time ago so don’t remember exactly


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Is that the white plastic thingy? I seem to remember an issue with that in the isolation valve on my MDE manifold which meant it didn't flow.

It only happened once and I can't remember exactly what the issue was. I serviced the valve with new bits from MDE and it didn't recur

[Neil R]

Thanks all, I eventually replaced *all* the service parts, plus a few more bits from a spare. All working normally. The only possible culprit eventually spotted was the plastic insert was perhaps more indented than normal (presumably over-tightened), maybe that can cause it to stick. They are so lovely to use when serviced, in this case with the once fabled Sherwood knobs!