I thought I had posted about this modification a long time ago but was recently made aware that I had not.
This post is about how I have configured the rEvo to use Air Integration pressure sensors instead of traditional Submersible Pressure Gauge (SPG) for the Diluent and Oxygen tanks. I’m not the first to do this nor the last and my configuration isn’t anything really unique. However, hopefully by documenting the configuration and, more importantly, the “why” it will help others.
At the end, I also have a story about how this recently benefitted me in a real world situation on a dive in Vis, Croatia.
I have been using wireless transmitters for a very long time (at least 12+ years) and started using them when I was diving open circuit. During that time, I found them useful to predict dive time remaining, to monitor my SAC/RMV rate, and to have an easy way to check my pressure on my dive computer.
The rEvo comes default with SPGs for both tanks that come up and over the unit and are secure on the shoulder straps. By using AI transmitters, these can be eliminated which clears up the chest area a little bit and is one less thing in the way (especially when using cameras, scooters, bailout tanks, etc.).
They are definitely not necessary and many, many divers use the standard SPGs. I prefer the AI method and I’ll detail one of the reasons “why” further on in the post.
My first configuration was to put the AI transmitters directly on the first stage of both tanks without a high pressure hose. They can be wedged in and they do fit; however, I found that I could not get a consistent pressure reading at the NERD2 which is up by my eyes on the rebreather breathing loop.
I then moved them to sit under the steel weight pouch plate which is where they are now. I’ve conducted hundreds of dives with this configuration and only once had a problem with signals from the transmitter getting disrupted.
This problem occurred when I zip-tied the excess cable from my Petrel cable under that same steel shelf. I believe that signals on the DiveCan bus on the cable interfered with the AI transmissions. As soon as I moved that cable, everything started working well again.
Here are some pictures of the setup with some detailed photos showing how the transmitters sit (I have the gray transmitter connected to the Diluent tank and the yellow transmitter connected to the Oxygen tank).
I know some people that route the AI transmitters up and over the shoulder which also works (longer high pressure hose required obviously). I’ve also seen another configuration that uses slightly longer hoses than I have and routes the transmitters across the top to the other side using 55cm/22″ high pressure hoses.
I tried to measure my hoses without taking them off of the 1st stage and cutting the zip ties that hold my hoses to the frame. I don’t know how accurate the measurement is, but they seem to be somewhere in the 16-18″ range. Unfortunately, I do not know where I bought them and can’t find that length of hose readily on-line.
I have configured my NERD2 and my Teric to read the pressure from the transmitters. I chose to put the Dil on the “top” and the O2 on the “bottom” simply because I think of the “left” tank as “top” and the “right” tank as “bottom” in my brain. Maybe that is because we read left to right, top to bottom? Here is what it looks like:
A quick note about transmitter types/colors:
I haven’t tried the newest Shearwater transmitters. For a long time, I used two of the grey Pelagic Pressure Systems (PPS) transmitters without an issue. There are rare cases where using two transmitters with the same transmit period can result in clashes. I never had this happen but it can. At one point, I had a problem with a transmitter and bought a new one so now I have one grey one and one yellow one which shouldn’t conflict.
One other quick and important note: When you are done with your dive, turn off AND de-pressurize your tanks. The AI transmitters will keep transmitting as long as there is pressure in the hose. This will use battery and your batteries will die quicker than expected.
Is it worth the expense and hassle to configure AI transmitters and have another device that requires batteries, etc.? Well, for me it is but everybody needs to make their own decision on that.
There are the obvious benefits like being able to read pressures at a glance (this usually isn’t really a big requirement for rebreather diving), the ability to track and record your Dil/O2 usage on dives, etc.
I recently had a situation whereby the AI definitely benefitted me. I will note before I start this story that it is probably not a situation that should normally occur, but life happens…
I was diving on the Ursus in Vis, Croatia which is a reasonably deep dive (200 feet). It was essentially a solo dive since I got separated from Alesh during the descent and there was a lot of current. Therefore, I couldn’t do a “bubble check” that is normal procedure for buddies to conduct as you start your descent to ensure that there aren’t any leaks. I will also note that diving solo is common for me and I don’t always have a buddy to do a bubble check (I have bought a wrist mirror to help me do it myself). I won’t debate whether or not solo diving is “smart.”
I was focussed on the wreck and taking pictures (again, we can debate that) and, as mentioned, there was a current. At one point, my NERD2 flashed either yellow or red to indicate that one of my tanks was below the threshold for an alarm. It immediately (and proactively) caught my attention. Could I have checked my SPGs on a periodic basis? Sure. Do most rebreather divers check their SPGs on a regular basis? I’m guessing not.
I quickly realized my Diluent pressure was low and I shut off the tank and switched to an off board bailout tank. In reality, this isn’t a big deal at all. You generally don’t need more diluent once you are at the bottom of a wreck dive with a reasonably square profile and I had plenty of gas in my bailout to use for diluent if I needed it. The leak was also slow and I could have easily kept using my current Diluent tank by turning on/off the valve when I needed diluent.
However, it was nice to get the proactive alert so that I could do something about it and not have it turn into a potential panic situation.
Arguments can be made for whether I should have been monitoring my pressures more closely, whether SPGs are better because they don’t fail, whether I should have been taking pictures on a dive at 200′ with current, etc. For me personally, I like having the pressure right in my face where it is easy to see and I like the proactive alert if the pressure is dropping below where I expect it to be.
I never definitively found out what was leaking, but I do believe it was a slow leak from where the hose from my ADV shutoff system is connected to the wing. It is strange that I didn’t hear the leak because normally I would. I think the reason I didn’t see the bubbles is due to the current.
By switching my diluent to the bailout tank, I didn’t “fix” the problem but I had an AL80 worth of gas and I was already on deco. I believe I ended up switching back to my on-board diluent and just keeping the valve closed until if and when I needed it.
Here is a graph of the pressure of my tanks and you can see the Diluent slowly dropping until about 50 minutes into the dive when I shut it off and switched to my off board.
I personally prefer the AI integration as opposed to SPGs. Whether or not they are right for you really has a lot of variables. For anybody new to rebreather diving, I would not recommend it because you should probably get used to diving a “standard” unit before making any changes.