rEvo O2 Delivery System Modifications (HMHS Britannic Prep)

NB 1: This post is probably only interesting to those who conduct deep rebreather dives OR anybody who just wants more information about how the rEvo rebreather works.

NB2: Most rebreather divers should never need to do this. Please don’t modify your system unless you know how (and why). I’m supplying this for any rEvo diver who regularly dives deeper than 90m and wants an “easier” way to do that.


One of the two primary functions of a rebreather is to provide oxygen to the diver (the other is to remove exhaled carbon dioxide).

There are a few different variations on how to provide that oxygen. One very important fact to keep in mind is that amount of oxygen that a person metabolizes DOES NOT change with depth.

The rEvo rebreather has something called a “Constant Mass Flow” (CMF) feature (also sometimes nicknamed a “leaky valve”). I personally really like this feature while some others do not. The CMF constantly puts oxygen into the system at a specific and fixed rate which ideally equals the rate at which your body metabolizes it while at rest (usually about 0.5 or 0.6 liters per minute).

Below is a white paper written by the creator of of the rEvo rebreather which provides a lot of details on this system.

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One of the challenges to overcome is that, as mentioned, oxygen metabolism does not change with depth.

Most regulators are designed to increase gas flow as the diver goes deeper to ensure that the pressure of the gas you are breathing equals the pressure of the water (otherwise your lungs would collapse). Therefore, modifications are needed to the “1st stage” of the regulator to create a constant pressure. Otherwise, you would get less oxygen at shallow depths and more at deeper depths and this would be a very big problem.

The way that the rEvo (and some other rebreathers) achieve this is that the first stage regulator is “plugged” such that it does NOT sense the depth & pressure of the water; therefore, it provides a constant intermediate pressure (IP). It is then paired with an orifice such that no matter what depth you are at, you get the same amount of oxygen.

Below is a picture of the steel blanking plug on the rEvo regulator:

Pretty cool, huh? Once you are at a fixed depth in the water and get your oxygen level set correctly, for the most part, it stays there since the CMF system is leaking in as much oxygen as your body is consuming.


If you look closely on the blanking plug in the picture above, it specifies that the max output pressure is 12 bar. Practically, it is generally set to about 11 or 11.5 bar at the factory.

In most cases, this is just fine. The “problem” is when you dive deeper than the intermediate pressure (less the one ATM of pressure at the surface) less the pressure it takes to activate the orifice or manual oxygen addition.

Practically, this means that you start to lose oxygen delivery below about 280-300 feet.

There are generally three ways that rEvo divers work around this when they go deeper than the “normal” limit:

MethodDescriptionPros / Cons / Notes
Remove the “blanking plug” and make the regulator a “normal” pressure sensing regulator and then plug the orifice to disable the CMF flow This essentially turns the rEvo into a “eCCR” that only supplies oxygen via the solenoid or via manual add and disables the CMF feature.The only aspect of this that I don’t like is that it requires you to fiddle with your rebreather right before a deep dive. Well, and that if you are on long deco stops, either the solenoid fires or you need to manually keep adding oxygen.
Add oxygen manually with a source connected to the “external” port on the manual add gas blockThis also works well but requires you to connect and carry a cylinder with a “rich” gas (this would normally be the case anyway on a deep dive).It is a little risky if you connect a tank of pure oxygen because at very deep depths, you only need a TINY bit of oxygen and if you push the add button a little too much, you can definitely risk oxygen toxicity. Some divers connect a tank with 50% oxygen (which is usually carried for bailout anyway).
Connect a diluent gas to the external port and then “flush” the loop when you are deeper. For example, if you have a bailout tank with 10/70, the 10% oxygen will provide a PPO2 of 1.1 at 330 feet (10 ATM). This isn’t really an ideal PPO2 but will suffice for short periods of time but will increase your deco obligation.This isn’t ideal for two reasons: (1) you need to tailor the deep bailout to provide a PPO2 close to what you would normally want (for me that is 1.3) and (2) if you are on a dive that varies greatly in depth, you would not have an ideal gas at the shallower depths of the dive.

I’ve used both methods (2) and (3) in the past and it has worked well but I really wanted something a little more seamless.

Below is a picture of the small O2 tank I rigged up to be installed on the side of the rEvoo and connected to the external port of the MAV off board port when using method 2 above.

Also, on the “con” of Method 3, keep in mind that I am getting ready to dive the HMHS Britannic which has a depth change of 100 feet between the floor (385 fsw) and the top of the wreck (285 fsw) so there really isn’t an ideal deep bailout for that entire range of depths!

Alternative Method

A while ago, I was reading about the Fathom rebreather which also has a similar system but also has a “needle valve” that allows the user to adjust the flow of O2 in the CMF system. What really intrigued me though was that they had a modified first stage with an intermediate pressure of 14 bar and they use the same Apeks DS4 first stage as a rEvo (well, for units built before the Mares acquisition).

However, the rEvo doesn’t have a needle valve (I could probably adapt one) so at that pressure and with the Fathom DS4 first stage, I would need a smaller orifice. Otherwise, at a higher pressure, the standard rEvo orifice would deliver a higher rate / flow of oxygen.

I emailed my friends at rEvo and asked about a smaller orifice and to get their thoughts on this plan.

They mentioned that it was possible but the intermediate pressure should be limited to 13.5 bar in order to not apply too much pressure to the rEvo Manual Addition Valve (MAV) system. They also offered to build a “one-off” new 1st stage with an IP of 13.5 bar and smaller orifice for my upcoming Britannic trip!


Installing the new O2 System

A while later, my new 1st stage and orifice showed up and I went to work replacing the existing one. Keep in mind that I’m generally good about maintenance on my rebreather but it has seen a lot of use over the years and I’ve never done a full system maintenance.

Below is the new Mares 1st stage:

Removing the old first stage is easy enough, you just need to make sure you remove the hoses in the “right” order since you cannot easily rotate the hose that goes from the first stage to the MAV (the yellow one in this picture).

My general approach is to remove the SPG first, then unscrew the nut from the hose into the case (which goes to the solenoid/orifice), then remove that hose from the first stage, then finally rotate the first stage to free it from the O2 / yellow hose. I’ve numbered this in the diagram below:

I then connected the new first stage to a tank and checked the intermediate pressure and it was “bang on” 13.5 bar. Nice job, rEvo!

Now the job was about to get a little tricky. My orifice and solenoid had been neglected over the years and had some rust which made removal a bit tricky, but not impossible. Below are some pictures. I suggest treating your system a little better. 🙂

I have a workbench in the garage but it is a bit small to fit the entire rebreather. Here is a fun shot of my garage floor in the middle of the project. Not exactly an “O2 clean” environment, but…

And, finally, here is the new solenoid and orifice installed and ready for testing (the solenoid is the black box by “Jaksa” and the orifice is the part with the red tape on it next to it):


Anytime you install a new or different oxygen delivery system, you really need to test that everything is working since it is a critical life support system.

I bench tested the new orifice (CMF) and solenoid and then did some shallow dives to ensure that the system was working before doing a 300 foot dive locally.

One key parameter to check is whether the CMF flow rate matches your metabolism.

The best way to do that is to hover at a fixed depth, slow down your breathing, change the set point to low so the solenoid doesn’t fire and then see what the PPO2 level does. If it matches your metabolism at rest, then the PPO2 should stay constant. If not, the system may need to be adjusted. Mine matched perfectly.

I ended up diving the Britannic recently (more on that in future posts) and the system worked great down to 360-ish feet. The engineer at rEvo mentioned that they had also modified their units similarly and had done some deeper dives with them as well.

I really like the new setup but it isn’t for everybody.

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