rEvo ADV Shutoff

Background

Many rEvo divers I know have a love/hate relationship with the Automatic Diluent Valve (ADV). It is nice to have especially when you are trying to do a quick descent and might be task loaded. However, it always seems to fire at the most inopportune time and then adds diluent to your loop when you are trying to maintain a PO2 on deco AND adds buoyancy. Or you are navigating through a wreck, move into a slightly odd position which then shifts the gas in your counterlungs and the ADV fires — once again affecting your buoyancy and your loop PO2.

Many other rebreathers actually have this feature built-in. The rEvo has the ADV inside the unit in the exhale counter lung so it isn’t so easy to add a shutoff valve.

My ideal situation would be to have the ADV “on” during my descent and then “off” during the dive and the ascent.

“Normal” Solutions

The first solution, and the one that I know quite a few rEvo divers use, is to simply turn off the ADV before the dive. Again, because the ADV valve is inside the unit and not accessible during a dive, this is a “black and white” configuration option and cannot be changed during a dive so it doesn’t really meet my needs.

The second solution is to “tune” the ADV valve such that it only fires when you really bottom out the lung and pull in a large breath. I’ve tried tuning my ADV for probably 20-30 dives and couldn’t ever get it the way I wanted such that it fired on the descent but not thereafter.

I went in search of other solutions that involved adding a shutoff valve that I could manually control during the dive.

ADV Shutoff – Version 0

My first “solution” to this problem was simply to shut off the diluent tank valve!

This actually worked reasonably well but had a couple downsides:

  1. If I needed to inflate my BC for whatever reason, it required me to first remember to turn on the diluent tank valve
  2. If I did need to dilute my loop gas, once again, I would have to first turn on the diluent tank valve

This really wasn’t a big problem, but was far from idea. Ideally, I would be able to isolate JUST the ADV supply and not everything connected to the diluent tank.

ADV Shutoff – Version 1

A standard rEvo supplies the ADV valve from a hose that goes from the diluent first stage to a coupling in the housing (which then goes to the ADV valve in the exhale counterlung). The first stage (and the ADV feed hose) is at the bottom of the frame on the back of the unit.

My first attempt at a ADV involved simply putting a shutoff valve inline:

ADV Shutoff, Version 1

Shutoff valve inline

This was a great concept. It was simple, easy to reverse, cheap to implement, etc. However, it had the big disadvantage of not being able to be changed mid-dive easily. I tried for 4 or 5 dives to reach back and change the shutoff valve. Either because of the diluent tank itself, my bailout tank or something else, I really couldn’t easily operate the shutoff when it was in this location.

Back to the drawing board….

ADV Shutoff – Version 2

I have to admit, I was not the first to come up with this idea. There were various posts to Rebreatherworld about similar setups but it wasn’t clear how people did them. This is the way my unit is currently configured.

First, it might help to have a “line diagram” of how the rEvo gas supply works from the diluent supply:

The solution I implemented involves splitting the gas supply after the T-Connector as it flows into the BC. Here is what the modified line diagram looks like:

Parts List

Here are a few pictures that show how the system looks during assemble and when finished:

Conclusion

I like the new system and it works well. Sometimes I forget to “reset” the shutoff to “on” between dives so I have to either use the MAV or push the shutoff down to turn it back on. I also have incorporated testing the ADV shutoff into my build process.

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