Planning a rebreather dive is a little bit more complicated than planning a normal open circuit dive. This articles describes how to do all the steps and why they are done.
To plan or not to plan Do you plan every single open circuit dive in advance? If you have some diving experience you probably know that on any dive shallower than 20 meters you will run out of air before you will experience any problems and you won't bother looking at tables until you might go for a second dive that day. Even dives down to 30 meters are quite safe if you use a normal 12 liter cylinder filled to 200 bar since you will run out of air before you expose yourself to too much nitrogen.
Diving with a semi-closed rebreather is a little bit different. The reason for this is that you have to make a decision on what percentage of oxygen you will dive on. A random decision will most likely give you too little or too much oxygen with fatal consequences. Using 60% oxygen will limit you to 13 meters depth but you will have more than 2 hours of divetime. Using 32% oxygen will let you go down to 33 meters but wil limit your time divetime to less than an hour.
The diagram below shows how divetime decreases as depth increases. The line with orange dimonds shows how your air consumption increases with depth. The line with dark blue diamonds shows how your nitrogen no decompression time limit increases with depth. The line with squares shows the gas consumption for a semi closed rebreather. Note that on depths deeper than 20 meters it is quite easy to stay so long on a single dive that you have to take decompression into account.
Dive time decreasing as depth is increasing
Planning overview Normally you plan a dive based on a certain divesite. The divesite will have something you like to take a look at such as a wreck or some corals at a certain depth. If you are using an open circuit air system you will be limited by either the lack of air or too much exposure to nitrogen (decompression sickness). If you dive with an open circuit nitrox system you will also have to consider how much oxygen you expose your body to. Diving with a semi-closed rebreather adds a forth variable which is your oxygen consumption. Your oxygen consumption depends on factors such as your body size and your rate of work. A big body working hard consumes more oxygen than a small relaxed body. To plan a rebreather dive you will go thru the following steps:
1. Decide your DEPTH based on your DIVESITE 2. Decide your OXYGEN % IN GAS based on the DEPTH 3. Decide the amount of OXYGEN YOU BREATH based on your WORKLOAD and OXYGEN % IN GAS 4. Decide your equivalent EQUIVALENT AIR DEPTH based on the OXYGEN YOU BREATH 5. Decide your NITROGEN EXPOSURE TIME based on the EQUIVALENT AIR DEPTH 6. Decide your CYLINDER DURATION TIME based on jet associated with OXYGEN % IN GAS, CYLINDER VOLUME and PRESSURE 7. Cap the maximum time with scrubber duration and oxygen exposure time
Step 1 This step does not involve any calculations but it does involve some thinking ahead. Quite often you fill your tanks at one place and then go diving with other people the day after. Since you need to know your mix the day before the day you may have to give your dive buddies or the remote divecenter a call to check up the dive site. For instance it would be pretty poor planning if you fill your tanks with a fat 60% mix only to find out at the divesite that the exceptionally cool wreck is located at 30 meters depth. 60% oxygen means a maximum depth of 13 meters which probably won't give you the best view of the awesome wreck.
Step 2 When calculating recreational dives you should avoid an oxygen partial pressure of more than 1.4 bar. Oxygen partial pressure or ppO2 for short, means the percentage of oxygen times the surrounding pressure. For instance 50% oxygen at 2 bar (= 10 meters depth) will give us a partial pressure of 0.50 x 2 = 1.0 bar. 40% oxygen at 40 meters will give us a pp02 of 0.40 * 5 = 2.0 bar. If you allow yourself to be exposed to 1.6 bar ppO2 your exposure time will only be 45 minutes instead of the 150 minutes you are allowed at 1.4 ppO2. It is generally considered bad practice to plan dives with higher pp02 than 1.4 bar and it will not be covered in this article. To calculate the amount of oxygen you should use in your mix to get a maximum ppO2 of 1.4 is calculated with the following formula:
1400 / (depth + 10) = % oxygen in mix
Example for 18 meter:
1400 / (18 + 10) = 1400 / 28 = 50 % in mix
When you have decided what your maximum percentage of oxygen is you may want to round it down to one of the preset mixes for the Dolphin rebreather which are 60%, 50%, 40% or 32%. The reason for rounding it down is to allow you to go a few meters deeper than planned but still maintain enough oxygen in the system for the shallow part of the dive. If you know you will have a heavy workload, like swimming against a current, then it is recomended to maintain a slightly higher oxygen percentage to avoid getting too little oxygen on the shallow part of the dive.
Step 3 This step is what makes rebreather diving different from regular nitrox diving and here is when the question about workload comes in. The second stage will supply the breathing bag with a constant flow of oxygen rich nitrox. It does that using a constant mass flow dosage device also known as a sonic valve. A sonic valve is so called since the gas passes thru it at the speed of sound and has the specific property of delivering the same amount of gas as long as the pressure difference on the sides of the valve is at least 2 bars. The rebreather will loose oxygen when the diver consumes it but will also loose oxygen when gas frequently escapes from the over pressure valve. It sounds quite complicated but can be simplified quite easily using some math:
G02% = Gas oxygen percentage JFR = Jet flowrate B02% = Bag oxygen percentage OPFR = Over pressure valve flow rate D02 = Diver oxygen consumption
oxygen in = oxygen out G02% * JFR = B02% * OPFR + D02
flow in = flow out JFR = OPFR + D02
combined these two gives us G02% * JFR = B02% * (JFR - D02) + D02
B02% = (G02% * JFR - D02) / (JFR - D02)
In order to use this formula we need to know two more things, the flow rate of the jet and the oxygen consumption of the diver. You get these by looking them up in tables
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Example: Recreational diving using 52% nitrox and the 50% jet
(G02% * JFR - D02) / (JFR - D02) = B02%
(0.52 * 7.3 - 1.5) / (7.3 - 1.5) = 0.40 = 40% oxygen in breathing bag
Step 4 Now when we know what oxygen percentage we are breathing we can use it to calculate our equivalent air depth or EAD for short. We calculate EAD in order to be able to use normal air divetables to lookup our nitrogen exposure timeas and avoid decompression sickness. Equivalent air depth is calculated using the following formula:
(1 - oxygen percentage in bag ) * (depth + 10) / 0.79 - 10 = EAD
Example: We continue with our calculations with our depth 18 meters with 40% oxygen in the breathing bag:
Step 5 Now when we know our EAD we can use a standard divetable to look up our maximum time on the depth.
Example: Using our PADI divetable we can see that the closest depth above 11 meters is 12 meters. 12 meters gives us 147 minutes divetime without requiring any decompression stops.
Step 6 Now it is time to estimate how long our cylinder will last. When we are using open circuit scuba we consume more air the deeper we go. With a rebreather we consume the same amount of gas regardless of the depth as long as we use the same jet configuration. Since the gas consumption does not change throughout the dive it is easy to calculate how long the cylinder will last. The cylinder duration can be calculated with the following formula:
Example: The normal Dolphin unit comes with a 5 liter cylinder 200 bar cylinder. Normally you use up about 20 bar for flowrate and over pressure test so you will start the dive with 180 bar. This will give us:
5 * 180 / 7.3 = 123 minutes
Step 7 Now we have seen that our gas will last for 123 minutes and our no deco time is 147 minutes. The final things we have to think about is that we won't exceed our scrubber duration or maximum oxygen exposure time. The scrubber will last for 3 hours filled and 1 hour half filled (using half fill equipment) so we are fine as long as we use the full scrubber canister. We calculated our maximum depth based on 1.4 ppO2 which means a maximum exposure time of 150 minutes. The conclusion of these facts gives us that our divetime will be limited by our cylinder duration of 123 minutes.
