Diving an oxygen CCR (**)

The article tells you how to dive a closed circuit rebreather. PADI only holds courses in semi closed rebreathers or computer controlled rebreathers. If you want want to dive on 100% oxygen this information is essential to keep you alive unless you have had 100% oxygen CCR training somewhere else.

You have turned your semi closed Dräger Dolphin into a fully closed oxygen rebreather and you are now ready to go for long bubble free dives. But there are a few things that are essential to know if you want to get the most out of the experience and more importantly, stay alive. We will walk thru all the steps that are specific to closed circuit oxygen rebreathers that you may not think about.

Fill your tanks
When you fill your tanks you have to make sure that you get 100% medical clean oxygen. If you get less than 100% of oxygen there will be a buildup of nitrogen in the breathingbag and you will go hypoxic. However, if the percentage is close to 100 you can frequently purge the purge the bag and still keep a decent amount of oxygen in the mix. This not recommended and should only be used if you have a OXYgauge attached to your system so you can monitor the decrease of the oxygen partial pressure.

Close the over pressure valve
You want to avoid any accidental leakage from the system. Real closed circuit oxygen rebreathers like the Dr䧥r LAR V does not have an over pressure valve and there is no problem with accidental leakage. The Dolphin still has an over pressure valve and you need to close it as much as possible. You do that by turning it as much as possible in the clockwise direction. Think about how you close the lid of a jar and you will remember how to turn it. The over pressure valve is not only bad since it will prevent you from rupturing the breathing bags when ascending.

Purge your breathing bags
Before leaving the surface it is essential to purge the rebreather from nitrogen. If the bag is still containing air with 79% nitrogen when you start your dive you will consume the oxygen but the nitrogen will still occupy so much space that the demand valve will not be triggered. If the demand valve is not triggered you will get to little oxygen and go hypoxic. To purge the rebreather fully do the following at least TWO times. Some instruction recommends you to do it THREE times.
- Turn off the gas supply on the cylinder
- Inhale all the gas in the system with your mouth
- Exhale all gas thru your nose
- Keep inhaling thru the mouth and exhaling thru the nose until you can not inhale any more
- Turn on the gas and take a deep breath and breath a few times
- Repeat the steps above TWO or THREE times

The purging procedure will not remove all nitrogen from the system since there are some dead spaces that you can not empty by sucking. After you purged the bag it will contain approximately 75% - 80% oxygen. Note that it is not preferrable to overly purge the bag since that will increase the risk of CNS (oxygen intoxination).

Loosen your straps
When you entered the water and feel comfortable you can loosen the BCD straps a bit which will leave more room for the breathing bag and minimize the leakage thru the over pressure valve.

Bubble check
Do the normal bubble check as you are used to with a SCR. Make sure that no bubbles escape at all. Let your buddy check you or lay on your back and face the surface if you are diving alone.

Mask clearing
During your dive you may have to clear your mask with some gas. Note that this is even more uneconomical use of gas compared to doing it in your SCR. Make sure that you minimize the need for mask clearing as much as possible.

Use of bailout
If you need to use your bailout you will breath air again just like on the surface. When you switch back to your rebreather you will have to purge it again.

Fast ascends = bubbles
If you want to avoid bubbles when you reach the surface you will need to do it slowly. To avoid letting any bubbles out you need to absorb the expaning gas with your body. One way to do this is to start taking smaller breaths before surfacing to avoid triggering of the demand valve. A simple calculation shows how quickly you can ascend:

4 liter counterlung ascended 5 meters => 2 liter extra gas => 2 minute for body to absorb

After your dive
When you dive with 100% oxygen you also equalize your ears with 100% oxygen. If you don't equalize your ears with air you will still keep a high percentage of oxygen in your middle ear and this can cause a squeeze in the middle ear. Make sure you equalize on the surface using either the Valsalva (pinch your nose) method or the swallowing method.