Balanced Second Stages

[b1gcountry]

We've been having a lot of talk on upstream second stages, and needing an OPV.

Is this the case with Balanced second stages as well? If the balancing chamber on a second stage compensates for whatever IP, then won't it just compensate for a rising IP until something else pops?

Tom

 

[DA Aquamaster]

Nearly all balanced second stages have a downstream bias so they will still freeflow somewhere between 170 and 190 psi.

There were reports of an Apeks reg where the LPI would fail and auto inflate prior to the second stage releasing excess pressure. Consequently there was some doubt whether that particular model had any downsteam bias.

 

[b1gcountry]

I've overhauled a couple LPI, but I can't picture the insides right now. I always thought they were more or less upstream designs. I guess not?

Tom

 

[rjack321]

Nearly all balanced second stages have a downstream bias so they will still freeflow somewhere between 170 and 190 psi.

There were reports of an Apeks reg where the LPI would fail and auto inflate prior to the second stage releasing excess pressure. Consequently there was some doubt whether that particular model had any downsteam bias.

In that particular incident the 2nd stage adjustment knob was cranked all the way closed after a gas switch. Apparently the 2nd didn't act as an OPV and since it wasn't being breathed at that point the IP crept up until the gas decided to go vent through the wing inflator. While wing inflators are balanced apparently the innner oring let loose first and poof - runaway wing. This was in MoCal about 4-5 yrs ago IIRC.

 

[DA Aquamaster]

It should not make a difference. There needs to be enough donwstreeam bias to overcome the spring pressure acting against the poppet. Either the spring needs to be lighter or the areas of trhe orifice and balance chambers need to be changed, either way it is a design flaw that defeats the OPV/safety function of the second stage.

 

[rjack321]

It should not make a difference. There needs to be enough donwstreeam bias to overcome the spring pressure acting against the poppet. Either the spring needs to be lighter or the areas of trhe orifice and balance chambers need to be changed, either way it is a design flaw that defeats the OPV/safety function of the second stage.

Well the thought was at the time that when the 2nd was in use, the IP was kept from creeping up. Once it was stowed the IP crept up and would have vented via the 2nd except that the knob had so detuned the 2nd wouldn't vent relative to the wing inflator.

Kinda like cranking the vent pressure up on an argon reg OPV, eventually its no longer serving the purpose.

I wasn't there, never saw the gear, and I don't really know, that's just what the gist mill was talking at the time.

 

[DA Aquamaster]

I understand the concept - if the seat were leaking slightly and the reg was in use, it would never be an issue as the IP would drop back to normal on every inhalation. On the back up however, it can sit there slowly creeping up for long periods if the DS inflator is not used. Worse, if the diver is dry or has an argon inflator bottle along with no inflator hose, there is potentially no way to vent the excess pressure if the second stage does not relieve it.

Consequently, the reg has to be designed so that it releases excess pressure before it reaches unsafe levels regardless of what how the diver may adjust it.

I used to have a bailout plumbed into a gas block on commercial dives with a FFM and in that configuration there most of the time the first stage on the bailout was not connected to anything other than the unselected port in the gas block, so an OPV on the second stage was essential in the event of a leaking HP seat.