[*]Start with AIR diluent & ADV shut-off open so ADV can supply breathable gas to loop in shallows
Why would you want even air available "in the shallows"?
The Meg training procedure of starting on the strap-in with something in the loop "less than setpoint" and then letting the solenoid bring the system up to 70% and then jumping in with the isolator off was selected for a few reasons:
(1): It checks to see that your 02 bottle is actually on.
(2): It validates your solenoid and controller function.
(3): 0.7 is a gas that you cannot "breath down to hypoxic" while still having a breathable volume of gas in the loop, even at the surface. (leaner "jump" PP02 selections, such as 0.40, may go hypoxic while being metabolized, yet leave a breathable volume in the lungs).
(4): If you have an "optimal loop volume" you can descend to 6 meters /20 feet with that volume in the lungs, adding 02 using your manual bypass valve "if needed" to have lung volume. You will pretty quickly learn to set the lung volume on-deck so you do not even need to do this.
(5): If you have correct diluents for depths, you will not generally spike your PP02 on descent if you start at 6 meters with "what you end up with there" using this procedure.
(6): There is no possibility of adding diluent either by diver error or by technical malfunction because the ADV is off until descent from 6 meters is initiated.
This is THE STANDARD for Meg training, and is a procedure that has been well proven to be robust and error-detecting. It checks 02 bottle, controller, solenoid, and precludes shallow water hypoxia.
Back to the basics:
Shallow water hypoxia is caused by one of two things:
(1): 02 supply not operating.
(2): Uncommanded diluent being added.
Remediations:
(1): Let the 02 system keep you alive for 5 minutes on the surface to make sure it'll keep you alive underwater. mCCR or eCCR, keep yourself:
Recite this:
"Breathe for Five to Survive the Dive" Live by it.
(2) Prevent diluent from being added until you need to add diluent for keeping a breathable lung volume. You do not need to do this above 20 feet. The method you select to accomplish this is your business. Just do it.
Dave
ADV Tech Notes:
There are two types of ADV's 8in addition to the normal characterizations of "upstream and downstream"*:
(1): Suction triggered (like a second stage)
(2): Mechanically triggered (like the foot of the Meg ADV bottoming out on the counterlung when the loop collapses).
The rEvo ADV is a scuba second stage valve: It breathes great, but is difficult for some divers to sense firing, and is more susceptable to inadvertant diluent addition into the loop than any other that I have worked with. Putting in a slider is possible if you think it thru. PM if you want info.
Mark-15 ADV is a Schraeder valve, and is VERY slow to add gas and VERY hard to trigger. It's also impossible to isolate without serious rig modifications. You cut a stainless steel line and run the ends to a Whitey valve located on the side of the chassis.
The USN Mark-VI started off the type of ADV that was used on the Cis Mk-5P, then the Prism, and Meg, then the HH, etc. It's an upstream valve also using a Schraeder valve and is very simple and reliable. Inadvertant triggering in the shallows is usually mechanical, IE: triggering it with your arms as you snap on stages, etc. They should always have an isolator on them. They are the gold standard.
The ADV's in the inhale Tee fittings have all been downstream scuba regulator type valves AFAIK, bearing in mind that I have not handled an Inspo one. These have all of the potentials of a second stage regulator and should have isolators on them.
The KISS ADV is upstream and might as well not exist. I always needed to use my BOV to add diluent. .
