Karl´s Rebreather Page

Nitrox %02 vs. ppO2 and depth

Some Informations on the (1998) UWATEC system, I hope it is quite correct

The Uwatec AladinX O2 system consists of three parts:
- the decompression computer (DC, wrist unit)
- the tank unit (tank pressure sensor)
- a ppO2 measuring unit for installation in the breathing loop

The DC can be used alone or either with the tank unit or with the ppO2 unit.
The DC alone (or in combination with the tank unit) works with fixed fiO2 (AladinO2 from 0% - 100%, Nitrox version: 21% - 50%), and cannot be used for a fixed ppO2 ECCR like the Inspiration for a exact calculation of the deco obligation.

The ppO2-Sensor of a Computer what was build SCR and workload-factors in mind will not help you in combination with a constant ppO2-ECCR because it will go mad seeing the fiO2 decrease while you descent.

But a 'Workaround' is possible for Nitrox-computers without Sensor like the Aladin Nitrox or the AladinX O2 used without ppO2-Sensor:
Take the maximum planned depth of the dive (D) with the corresponding ambient pressure (P) and the ppO2 setpoint (O2set) an do the following calculations:

P=(D/(10m/bar)+1bar)
fiO2 = O2set / P * 100%

With this setting the 'real' deco time is much smaller than the time, the computer displays to you.
Of course this happens due to the inability of the computer to take advantage of the higher effective %O2 on the Inspiration during the shallower part of the dive.

The CNS-tox% are higher in 'reality' than on the computer display
(the latter might not be a problem because Paul Bert- and Liorraine Smith- calculation on fixed ppO2 is quite trivial without a computer...).

Yes, this workaround might not be that great, but sometimes it's better than using divetables...

To repeat what happens then using a Nitrox-computer that was not build for ECCR-Dives too in combination with a ECCR:

You can not use the ppO2 sensor (if the deco-computer has one) because the computer will go mad seeing the fiO2-changing.
You can use a fixed fiO2-Mode by the workaround fiO2 = O2set / P * 100%

The workaround is done for the maximum-depth that means:

You can not dive deeper than what you planed as maximum.
The computer is unable to do proper Paul Bert- and Liorraine Smith- calculations, it will display a too low O2-load because it does not know that you breath at a constant ppO2.
The computer will display a shorter NDL-Time and longer decompression-time because it does not know that you breath at a constant ppO2 and so can't use the deco-advantages of that fact.

Several Nitrox-Computers are build for "shower-baths" (bounce-dives) and do no Liorraine Smith- calculations, but as you saw above both calculations of toxic effects have to be done manually in our workaround, so it's just a "care about" when you are going to buy a ECCR-Deco Computer.
Most Nitrox-Computers will not allow you to switch to a bailout-diveplan on the fly.

The %O2 values are rounded DOWN to the nearest "2" (with the exception of 21%) to fit in with the way the Aladin Nitrox computer works. (21, 22, 24, 26, etc). The Aladin Nitrox only goes up to 50% too, so obviously some values in this table don't quite make sense for the Aladin Nitrox computer. (These entries are marked with a "*")

Using these tables with the Aladin Nitrox (or other similarly capable Nitrox computer), your dives should be quite conservative, as you are decompressing with a higher %O2 that the computer expects, but if you like, you can be more conservative by using a ppO2 value less than the Inspiration is set to. (eg. reading the 1.30 column on the table with the Inspiration set to 1.35), or by going down to the next depth row (eg. reading the table down to 32m when you are doing a 30m dive), or by simply setting the Aladin to the next value down (eg. Setting it to 30% when the table gives you 32%)

The tables were generated by Gordon Henderson, using a computer program he wrote in "C", so they should be free from any sort of transcription errors, but I can't give any sort of guarantees to it's accuracy! If you want a copy of the program, drop Gordon an email.

There are 2 tables:

EAD Tables for 1.3 und 0.7bar ppO₂, inspired by Tim Cashman,
for Download and Printout they are delivered in 3 common Formats:
Setpoint.xls (MS-Excel 97, 21KB),
Setpoint.gif (GIF87a, 51KB) and
Setpoint.ps.zip (Zip packed Postscript-Printfile, 11KB).

Rebreather-Deco