Oxygen CCR from SCR (**)

The article describes how to convert your semi closed Dolphin rebreather into a closed circuit oxygen rebreather. You will end up with a similar unit to what the military combat divers use where you can continously dive for three hours without releasing any bubbles but you are limited to a mximum depth of five to ten meters.

You got your semi closed rebreather certificate and head out for your first dives without any bubbling open systems around you. You expect to see just the occasional bubble but instead you see to your dissappointment that the unit actually bubbles quite a bit. It is much better than the Darth Vaderish feeling of an open circuit but it is still not the silent bubbel free experience promised. You also note that the air is actually not lasting that much longer than a normal dive. The bottle is smaller but the space saved is taken up by some new crap like breathing bags and scrubber canisters. So you replaced a very bubbling unit that is 100% safe all the time with a slightly less bubbeling unit that potentially can kill you without a warning. Basically trading some bubbles for some extra risk.

You start to think about the gas theory you learned in your SCR course and realize that the Dolpin unit is actually not that economical when it comes to gas usage. You think about when you fill your 5 liter tank to 200 bar with a 40% Nitrox mix and calculate that that will give you 5 x 200 x 0.40 = 400 liters of oxygen. If you have some swimming to do, your body will consume about 2 liters of oxygen per minute. That means that if we could use the gas in an optimal fashion we would get 400 / 2 = 200 minutes or more than 3 hours of dive time!! However, when we use our Dolphin with the 40% jet it will inject gas at a rate of 10.4 liter per minute we only get 5 X 200 / 10.4 = 96 minutes dive time. Unfortunately we are not that lucky to really get the 96 minutes divetime since we won't have 200 bars left after gas analysis, positive pressure tests and flow rate checks. Usually we end up with something like 150 bar giving us 5 x 150 / 10.4 = 72 minutes divetime. Then we jump into the water and realize that we actually need to clear our mask a few times and purge the system when ascending and we are down to about an hours dive time.

So how can we use our gas in a more economical fashion? First we need to look at what is actually spoiling all our divetime which is the constant mass flow valve. It lets our gas leak out into the water at far to high rate. So what happens if we simply block it off? We would still be able to breath in our unit, if the inhalation bag is running out of gas the demand valve will give us more gas from the tank and the scrubber will still remove the carbon dioxide from the exhaled gas. The only problem is that as we breath we will consume the oxygen in the loop but not the nitrogen so our breathing loop will soon be filled with mostly nitrogen. Since the nitrogen is taking up the volume in the breathing bags the demand valve will not be triggered and no new oxygen rich nitrox will be added from the tank. When the oxygen percentage falls below 16% we will start to notice the same problems as when you climb a 3000 meter high mountain a phenomenon called hypoxia or lack of oxygen. We can solve this problem quite easily though. If you watched when someone is mixing nitrox you know that they start out with an empty tank which they add a certain amount of 100% medical oxygen to depending on what percentage of oxygen is desired. Finally the tank is topped up with air to the final 200 bar. What we simply can do is to skip the last step of filling up the tank with air and just use the 100% oxygen in the tank.

We would of course not have 200 bar with 100% oxygen but we would on the other hand not need that either if we look at some simple calculation. Any decent dive center that can fill nitrox should at least be able to provide you with a 36% nitrox mix and any dive center that caters for rebreathers should be able to provide the top oxygen mix for the Dolphin unit which is 60%. Dalton's gas law of partial pressures gives us that a dive center that can provide us with EAN36 should be able to give us at least 38 bar of 100% oxygen:

1.0 * X bar 02 + 0.21 * (200 - X) bar air = 0.36 * 200 bar nitrox

0.79 X = 72 - 42 = 30

X = 30 / 0.79 = 38 bar 100% O2 -> 38 * 5 liter tank / 2 liter O2 per min = 95 min dive time

If we could get a 60% mix they should be able to provide

(120 - 42) / 0.79 = 99 bar 100 % O2 -> 99 * 5 liter tank / 2 liter O2 per min = 248 min dive time

This is great news. This means that any nitrox dive center should be able to provide you with enough oxygen for at least a short bubble free dive and any divecenter supporting rebreathers should be able to give you a very long bubblefree dive.

There is one important thing to remember though. Breathing 100% oxygen can be dangerous at too high pressure which will limit your depth to approximately 5 meters. Maximum exposure for oxygen can be seen in the table below. Note that the depths are calculated with 80% oxygen in the rebreather. The reason for this is that even if you purge the bag when entering the water with your closed circuit oxygen rebreater there will always be a small amount of air trapped in the system that you won't be able to get out. This is actually a good thing since it will lower the partial pressure of the oxygen making it possible for you to stay 20% deeper than if the system was completely purged.

Modifying the unit
There are many ways to modify the unit in order to shut off the constant flow jet. For instance it would be possible to remove the entire hose or plug the hose with epoxy glue but those alternatives would not make the modification reversible. Instead a better way to do it would be to use a plug that can be inserted when we want to dive closed circuit and removed when we want to dive it semi closed. The plug will be used as a selector for closed circuit (CCR) or semi closed (SCR) mode This is the same way as Dr䧥rs dive unit LAR VII works but it is unfortunately only sold to the military.

Getting the raw material
The parts you need to create the CCR/SCR selector are very cheap and should be available at your local hardware store or gas station.

The parts are:
1 Screw, 20 mm long, thread 3 mm (M3), 5 mm head
2 Washers, 1 mm thick, 12 mm outside, 4 mm inside diameter
2 Nuts, 2 mm thick, thread 3 mm (M3), 5 mm outside diameter
1 O-Ring, 2 mm thick, 12.5 mm outside, 6 mm inside diameter
1 Kit of two part hardening epoxy glue

Build the CC/SCR selector

Start by inserting the screw in one of the washers and fit it tight with one of the screws as show in the picture below.

On the top you can see the head of the screw a half millimeter above the washer.

Since the pressure is rather high (around 17 bar) in the hose the gas will leak thru this opening so it needs to be sealed with epoxy glue.

A convenient way to seal the top with glue is to put it into the opening of a small bottle.

When the glue hardened you can attach the other parts. Simply put the O-ring over the first nut. Put the second washer over the O-ring. Finally seal the oring with the second nut hard enough that it won't come loose.

Install the CC/SCR selector

Find the constant mass jet and remove the small rubber foam that is inserted inside it.

Insert the CC/SCR selector into the jet.

Push it all the way into the bottom.

Screw the constant mass flow hose back onto the jet.

Trim the second stage

Now you have complete most of the work and it is time to test that your CC/SCR selector is working. Make sure both the constant flow hose and the demand hose is connected to the second stage. Attach the first stage to your tank and turn on the gas. In this test phase you probably want to use cheap air instead of the more expensive pure oxygen. If you are really lucky no gas will come out of the second stage. You can test it by holding your hand over the big opening or by covering it with a plastic bag. Most likely you won't be so lucky though and there will be a small leak somewhere. A very small leak doesn't hurt since your body will absorb it and the demand valve will just be triggered more seldom. If the leak is bigger than your oxygen consumption (1-2 liter per minute) the system will bubble and spoil your gas. There are two places where it can leak from; the demand valve and the constant mass flow jet. If your CC/SCR selector is leaking you simply have to rework it to fit better into the jet. If the demand valve is leaking you have to trim it manually to shut off any free flowing gas. You do that by first unscrewing the big black lid that covers one side of the second stage.

You then use a monkey wrench or spanner to remove the demand valve from the second stage. On top of the demand valve there is a small nut that you can use to trim it. You want to loosen it enough so that the spring on the inside fully closes the demand valve when the lever is not touched. This should be the normal way it works but in practice the demand valve is usually leaking a small amount of oxygen but it is not noticable since the flowrate of the jet is much higher. When trimming the nut you will probably end up with a slightly less sensitive but fully closed demand valve when not triggered. Trimming the demand valve usually takes a few test rounds before you are totally happy with the result.

Final steps before diving
You now have a fully working closed circuit rebreather. Remeber to only use the unit with 100% oxygen to prevent builup of nitrogen. Also make sure that you close the over pressure valve as much as possible to avoid any unnecessary gas from leaving the rebreather. You close the over pressure valve as much as possible by turning it clockwise. You are now ready to go diving but before you jump into the water you should read the article on how to dive with closed circuit oxygen rebreathers in order to avoid to make any mistakes.