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This page offers information how to calculate the inspired oxygen fraction for a diver using a semi closed rebreather. 17 may 2010

The semi-closed equation shows us which mean fraction is being inhaled by the SCR diver. This important formulae is essential for the decompression obligation and for the maximum sustainable depth.
For even better understanding:

O2%=[(O2 flow - O2 consumed) / (Mixture flow - O2 consumed)] * 100


Part in Formulae Meaning Unit
FiO2 inhaled fraction by the diver % oxygen / 100
Qs The volume of breathing gas that is being injected continuously l/min
FSO2 The O2 fraction in the injected breathing gas % oxygen / 100
VO2 The volume of the diver's metabolised oxygen. l/min

VO2 is a difficult concept. The metabolic usage of a SCR diver strongly depends on the workload. In general, this value is approx. 0,5 - 0,8 l/min, however when the diver works under water a usage up to 4 litres is possible. This variable has influence on the inhaled gas fraction.

Since there is a variation in the metabolic use of oxygen during the dive, there will be a variation in the inhaled oxygen fraction. This has a direct impact on the decompression obligation because the nitrogen fraction varies with the oxygenfraction.

To calculate the fraction of the oxygen content the diver is breathing we need to know a couple of things:

1. The gas in the bottle
2. The metabolic oxygen consumption of the diver
3. The gas injection with this particular gas

OK, suppose we dive a Dolphin rebreather fitted with a 50% Nozzle (intended to be used with EAN 50) the flow is fixed at 7,3 ltr/min. This value has to be found in the manual. The 50% nozzle is a orifice that allows a fixed mass of gas when certain parameters are OK. If you want to know more about orifices and how they work search for "gas flow through an orifice" on this website. Also we presume the diver has a metabolic consumption of oxygen of 0,9 ltr/min, we than find the following:

Gas choice = EAN 50
Dolphin injects 7,3 ltr/min with the 50% jet

 = 7,3 ltr/min

= 0,9 ltr/min

 = 0,5

When filled in the formulae we find:




The diver will breath a Nitrox mixture of 43 % 

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