Physiology of Rebreathers Part 2

By Dr. Sawatzky

One way to do this is to monitor the PO2 and add O2 or diluent as required, but there are other ways to address this problem. One way is to feed a continuous supply of gas into the counterlung, as is done in the Canadian Forces rebreather. It continuously adds gas that has a PO2 of 1.6 ATA. If the diver is not breathing on the set, the PO2 in the counterlung will be 1.6 ATA. There will also be a continuous stream of bubbles from the set equal to the amount of gas being added. If the diver is breathing on the set, his body will absorb O2 at a rate determined by how hard he is working. Therefore, if the diver is at rest, the PO2 will be higher, if he is working hard, the PO2 in the counterlung will be lower. The rate at which gas is added to the counterlung is set so that the PO2 in the counterlung will be between 0.8 and 1.3 ATA, a safe range for bounce dives.

How is the rate set or controlled? Again, there are several possible mechanisms. Some rebreathers use tanks of pure O2 and pure diluent (nitrogen or helium). The size of the O2 orifice is such that as the density of the O2 increases with depth, the flow of O2 is reduced so that the PO2 stays near 1.6 ATA. Other rebreathers use specific mixes of gas (e.g. Nitrox40 or Nitrox60), with each gas requiring a specific orifice. This kind of rebreather is usually designed for nitrox or heliox but not both.

Not all SC rebreathers have a continuous flow of gas, some only add gas periodically, with the addition being either active or passive. However, all SC rebreathers vent a fraction of each breath into the water and replace it with fresh gas. This means that they are far less efficient than CC rebreathers (no gas is vented at depth) but still five to 10 times as efficient as OC, with the efficiency increasing with depth. I can not get into too much detail in this section because there is a wide variety of ways in which these rebreathers work. The important thing is to fully understand the specific rebreather you are diving.

There are several ways other than gas efficiency in which all rebreathers are superior to OC diving. The first is that you are always breathing a fully humidified gas. The first time you breathe the gas, your body adds water to the gas until it is fully saturated at body temperature in the lungs. In OC this happens with every breath and you will loose a significant amount of water during a dive (many years ago I calculated one cup for every 80 cubic feet breathed). With a rebreather, not only are you rebreathing the same gas over and over, the reaction that binds the CO2 to the chemical in the scrubber also releases water. Breathing a humidified gas means no more Ôcotton mouthÕ at the end of a dive. It also reduces your risk of DCS slightly by increasing your hydration. Another advantage of rebreathers is that the gas you are breathing is warm.

The body has to warm the gas you breathe to body temperature in the lungs. In OC diving, this means that you lose a significant amount of heat with every breath. With a rebreather, not only are you rebreathing the same gas over and over, the reaction that binds the CO2 to the chemical in the scrubber also releases heat. This reduced heat loss means that a rebreather diver will be warmer when diving in cold water. Staying warmer, especially during decompression, again reduces your risk of DCS. I had been cave diving for several years when we took a video camera into the cave for the first time. When we were watching the tape after the dive, I was amazed at how NOISY the gas bubbles were. I noticed the same thing the first time I was diving the rebreather with an OC partner. I could find him by listening for him! The difference when diving with marine life is unbelievable. Last June I was diving OC with eight to 10 foot six gill sharks off Vancouver Island. We could get about five feet from the sharks before they slowly swam away. Bill Nadeau was on the Inspiration and he swam right up to a shark, until his eyes were about six inches from the eye of the shark, before the shark slowly moved away!

The next advantage of rebreathers is that the PO2 is fairly constant. Therefore, the ÔpercentageÕ O2 is continuously changing. The implications of this are fairly complex. LetÕs compare a dive to 100 fsw on air and on a CC rebreather. On this dive, we will be diving in a cave where the passage goes up and down between 60 and 100 fsw, with a bottom time of 60 minutes. The air dive is fairly straight forward, We check our air tables and find out that 100/60 means that we have to do one minute deco at 66 fsw, eight minutes at 30 fsw, 20 minutes at 20 fsw and 45 minutes at 10 ft (using Proplanner), a total of 74 minutes decompression. ThatÕs a lot more air deco than I would want to do. Therefore, lets plan the same dive using Nitrox32 for the bottom time and deco. Now we have only one minute at 59 fsw, one minute at 39 fsw, eight minutes at 20 fsw and 20 minutes at 10 fsw, a total of 30 minutes deco. ThatÕs much better but still not optimal because we are spending a lot of our time shallower than 100 fsw but calculating all our decompression as if we were at 100 fsw. If we dive a nitrox computer set to 32% O2, it will give us credit for all the time we spend shallower and we will have less deco for this dive.

So how do we calculate deco on the CC rebreather? We usually use a setpoint of 1.3 ATA so at 100 fsw we will be breathing 1.3/4.0 = 32.5% O2. We could use tables or we could use a program like Proplanner to figure out our deco.

Another option would be to use a nitrox computer set to 32% O2. The computer would give us the same deco as our OC dive partner diving Nitrox32. But what is really happening to the rebreather diver? At 100 fsw he is breathing 32.5% O2, almost the same as his OC partner. But at 60 fsw the rebreather diver is breathing 1.3/2.8 = 46% O2. At in between depths, the percentage O2 will be between 32.5% and 46%. The rebreather diver will be breathing and absorbing much less nitrogen during the bottom time than his OC partner. During the decompression stops, the difference is even greater.

At 20 fsw the OC diver is breathing 32% O2 while the rebreather diver is breathing 1.3/1.6 = 81% O2. At 10 fsw the rebreather diver will be breathing very close to 100% O2 with a PO2 of 1.3 ATA. The rebreather diver will be doing far more decompression than they actually require. Full versions of Proplanner allow us to calculate the decompression requirements for rebreathers. For this dive, using a setpoint of 1.3 ATA, we require the following decompression: one minute at 56 fsw, one minute at 36 fsw, two minutes at 20 fsw, and 11 minutes at 10 fsw, for a total of 15 minutes deco. As can be seen, the rebreather is much more efficient than OC nitrox. But we are still doing much more decompression on the rebreather than we really need because we are still doing the calculation as if we were spending the entire 60 minutes at 100 fsw. The only way to solve this problem is to dive a computer that can be set to partial pressures of O2 instead of percentage O2. At the moment, the only computer I know of that can do this is the VR3 computer, made by Delta P. Technology Ltd. in England. This computer allows the diver to program up to 10 different gases, using either percentage of O2 or partial pressure of O2, nitrogen, helium, or both as inert gases, and to change the gases on the fly during the dive. On the CC rebreather, the above cave dive is most likely a no decompression dive (remember on OC air this dive requires 74 minutes of deco)!

The above, slightly long example, illustrates how the changing percentage of O2 in the breathing gas of a rebreather maximizes the decompression profile. As the dives get longer and deeper with more decompression stops, the CC rebreather becomes even better.

Yet again, I am out of room! Lets just briefly look at one of the negative aspects of diving a rebreather and then in the next column we will discuss the rest of them.

Setting up a rebreather and going through all the pre-dive checks takes longer that putting on your OC dive gear. Furthermore, not doing the pre-dive checks correctly has been the cause of several rebreather fatalities and many close calls. In general, to dive a rebreather safely and to justify owning one, you have to be just a bit obsessive and compulsive. You should be the kind of diver who takes good care of their equipment and always fixes problems, before you put it away. You should also do a lot of diving. To dive a rebreather safely, you need to be diving it on a regular basis. A friend of mine says, "you need to eat rebreathers, think rebreathers, sleep rebreathers, dive rebreathers, and drink Coke"!