Karl´s Rebreather Page

Decompression

Decompression calculation for a Rebreather is different to open-circuit scuba.

All calculations on this page will be done for Nitrox, Heliox and Trimix are quite similar.
What changes when you look on your deco-table is only that you look up an air-equivalent (EAD) of the maximum depth, calculated with the Information you have about the dive, the rebreather and the used gas. (Sure, I do NOT mean that you can calculate an EAD for Heliox )

Diving an Oxygen-Rebreather you won´t have to think about Decompression, you are breathing 100% Oxygen, so 1) there is no inert-gas to decompress 2) due to Oxygen toxicity you can only make shallow water dives, so that Oxygen toxicity is what you have to watch as rebreather-diver and did´nt had to care about on open-circuit scuba (21% O₂).

Diving an CCCR (Chemical-Mixedgas-Rebreather) like the legendary IDA71 is for the calculations just the same as doing a open-circuit scuba dive on the same gas. (Sure, better add some security, much better: use an Loop-ppO2 measuring Divecomputer that does your Deco-, Paul Bert- and Liorraine Smith- calculations.)
EAD[m]=(Depth[m]+10m)((100%-%O2Tank)/79%)-10m

A Semiclosed-Rebreather (SCR) will bring you some more calculation work, because you breath a gas what has not the same %O2 as in the tank. On a Active Flow-SCR, like the Dräger Atlantis/Dolfin, the %O2 is changing, so if you don´t use a ppO2-measuring deco-computer you have to calculate with worst cases: you have to use the highest ppN2 for deco-calculations (means lowest ppO2) and the highest ppO2 for the O-tox.
FiO2 = the Inspiratorive Fraction that is O2
VvO2 = the O2 your Body needs (from 0,5bar*l/min to 2,5bar*l/min)
Vs = the gas [bar*l/min] your active-flow delivers
FsO2 = the O2 fraction of the gas in your tank (NOAA2=Nitrox-36 means FsO2=0,36)
FiO2=((Vs*FsO2)-VvO2)/(Vs-VvO2)
EAD[m]=(Depth[m]+10m)((1-FiO2)/0,79)-10m
EAD[m]=(Depth[m]+10m)((1-(((Vs*FsO2)-1,5bar*l/min)/(Vs-1,5bar*l/min)))/0,79)-10m
With a Passive Flow-SCR like the Halcyon-Rebreather the O2% does not change that much, but the principle is similar, you can´t simply use the O2% of the tank, you have to calculate what O2%-range you will breath.

Electronic-Closed-Rebreather (CCR) will provide you with a constant ppO2.
EAD[m]=((100/79)*(Depth[m]/(10m/bar)+1bar)-ppO2[bar])*(10m/bar)-10m

On Liquid-Rebreather you do not breath gas, so you don´t have to think about decompression, but who of us saw such a machine in reality and not only in the movie "the abyss".

So, what´s about using a computer ? A good idea, but you always should be able to do it with your own brain. As you noticed above: a computer has to know what ppO2 the Rebreather has, because for the deco calculation it has to know the ppN2 (on Nitrox environment_pressure - ppO2 = ppN2). One solution are computers you can tell what ppO2 or %O2 you dive, but it´s more secure to use a computer that measures the ppO2, so it can also warn you if the oxygen runs out of range. A Computer should not only do the Deco-, but also the Paul Bert- and Liorraine Smith- calculations, demand a detailed description on how it calculates the two Oxygen-Toxic effects before you buy a computer. The Computer on the Picture is a Cochran "LIFEGUARD PO2", it is described to be a full-functioned dive computer for any pricipe of Nitrox-Rebreather, from left to right: Transmitter, Computer, ppO2-Sensor. Using a Nitrox-Computer that was build for OC-dives to do Nitrox-ECCR-Dives is possible too, but only a makeshift.

The most important thing about Decompression and Rebreather is: you can only do the dives your bailout logistic allows. If you can only breath 5 minutes form your Bailout-System when your rebreather has a technical problem you should not do decompression dives ! Dive save, never do dives that need decompression if you don´t have a big enough bailoutsystem/-logistic for it, always calculate to be save in the worst case.

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