The operating principles of the SCR are in fact similar to the working
principles of the oxygen rebreather. The main difference lies in the
choice of gas. By feeding the loop with Nitrox the depth at which oxygen
becomes toxic will be reached at a substantial greater depth. A
disadvantage though is that Nitrox also contains nitrogen and this one has
to count with the nitrogen uptake within human tissue.
Nitrox is being brought in the loop via a constant
flow injector (commonly a orifice) from a Nitrox cylinder. The gas can
expand in the counter lung(s) and thus becomes available to the diver. By
making use of one way valves the gas flow is being forced through the
scrubber. The gas flows continuously into the loop. The diver uses up
oxygen and produces CO2 as waste gas. As the diver gets deeper the partial
oxygen pressure will increase, the fraction though will
Because the number of litres of gas that is being added is being
determined by the nozzle, this will probably be installed in such a way
that the diver will be provided with sufficient (if the rebreather is well
designed) oxygen. The consumption though depends on the divers exertion.
However, even at maximum exertion oxygen will (should) be supplied
sufficiently. This will cause a small amount of breathing gas to escape
via the overpressure valve every 4 to 5 gasps of air.
Since the diver has a varying use of oxygen during
the dive, and since there is a variation in the speed in the loop, a
different fraction is breathed then the fraction in de cylinder. This
necessitates to calculate the fraction that is being inhaled in order to
define the nitrogen uptake. The calculations of MOD and CNS are based upon
the fraction of the gas in the cylinder.
To calculate the actual fraction inhaled by the diver:
The oxygen fraction of the
The gas flow
The oxygen usage of the
individual diver (metabolic).
Further details about calculations with a SC
rebreather can be found
Note: The oxygen used by a diver has no relation to
the ambient pressure. The variation is only related to factors as
exertion, stress, condition, design of the rebreather. There are certain
situations that the diver has to manually add gas. Therefore a SC
rebreather should always have a manual bypass!
Also it is highly recommended to have facilities to
measure and display the fraction of the inhaled gas. The oxygen content in
the loop should always be known during the dive! Also it is most important
to follow a dedicated course specific for the type of rebreather you are