Sp mk5 piston

[DA Aquamaster]

I also handwrote a note after looking at the handouts I was given and it states that when the old pistons stabbed the seat it could possibly extract the seat from the cap. Another remote possibility I suppose.

If it extracted the seat from the seat retainer it would in essence be pulling the seat upstream against whatever the tank pressure is at the moment, and with the substantial area of the seat, this would be a force in the hundreds of pounds at medium to high tank pressures. I suppose it could happen with a near empty tank, but I really doubt it.

There is also the issue of the flat orangy-pink ring that sets between the seat and the reg body creating a physical barrier to moving the seat as well as providing an additional seal.

So...I suspect the real chance of the seat being lifted out of the carrier ranges from extremely slim to none.

I get the impression that one goal of the so called expert course is more or less to promote some scare tactics to get people to trade in their "unsafe" Mk 5's for newer regs. And you have to ask if the reg is so fundamentally unsafe, why it is one of the most cloned regs in history being marketed by several major companies and even more minor ones.

I also see Mk 5's way back in caves on stage bottles, where in some cases they have sat for months and yet can still be relied upon to work. Hell, I use a Mk 8 (Mk 5 with 4 port non swiveling cap) on a deco bottle.

And then there is the problem that the Mk 10 uses the same basic piston and seat design with the same too short to really stop the piston shoulder.

It really sounds like anti-Mk 5 BS to me.

 

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I think I managed to cause such a seat extraction once. I was trying to lower the IP well below scuba specs (to power an air tool) by shimming the seat to make it higher. Somewhere around an IP of about 90 psi the seat must have bypassed the seal (red washer ring in modern kits but I was still using an o-ring which probably made it easier) and lock up even when IP was relieved. Live and learn. I ended up adding another simple dialed regulator to get pressure down to 25 psi.

I suspect that the additional width of the current red washer would tend to resist such an extraction much better than the old purple o-ring (-013 I believe).

 

[redacted]

I realize this is a pretty old thread but I recently ran into a failed Mk5 that demonstrated the need for the piston upgrade. The Mk5 was being use to power an UW air tool. The failure mode was uncontrolled free flow. When I disassembled it I found one of the older red cave cone seats. My understanding is this was an early model of the current seat (you might also find some in blue) and the material had some problem. This seat was breaking up and in 3 or 4 pieces - obviously not able to seal. But the old piston gave it a good try, pressing so far into the body that the very narrow shoulder was actually wedged into the body and did not want to move. It took a good bit of twisting and pulling to finally separate the piston from the body. The good news is there was no serious damage and a new set of soft parts put that old Mk5 back to work.

So, it looks like the new large shoulder also limits piston travel (beyond where the reg could possibly be functioning properly anyway) preventing the piston from lodging in the body in the event of a catastrophic HP seat failure. And it looks like Scubapro carried the lesson into the Mk10s also. But I have never heard of such catastrophic HP seat failures with the current version of the cave cone seats.

 

[halocline]

Its always good to see an old post by DA Aquamaster.