When we dive an oxygen rebreather one of the
limiting factors of the system is the diving depth. Oxygen
becomes toxic when it is inhaled at 1,6 bar. When we breath pure
oxygen this is the situation when we are at 6 meters of depth.
When we want to dive deeper we need to dilute the gas. Most
common is to do this by adding nitrogen. The gas mix we breath
then is better known as Nitrox. In a semiclosed rebreather (SCR)
the gas addition commonly is constant. A system with constant
gas addition we call "a active gas addition system". Since the
diver only uses the oxygen the surplus nitrogen is vented
through the overpressure valve on the counterlung.
Exact dating of the first SCR is not known.
It is however known that the first model for sports
became available in the 90's. One
of the first SCR from the Dräger company
was known as the Atlantis, After the Atlantis
a improved version was
named the Dolphin. The cost price for a Dolphin apparatus was approx 3400 euro
in 1996. After this introduction, SCR diving
became more and more popular. In the 90's electronic closed
circuit rebreathers where extremely expensive and most
rebreather sports divers dived a Dolphin. In Japan the
Fieno rebreather was introduced and a large number of these
units where sold world wide.
Advantage of using Nitrox
Oxygen enriched air induces a lower
nitrogen rate in the blood. The advantage of diving with Nitrox over OC
diving is mainly to be found in the recycling of the gas, by partially
re-using it. Dive depths, dive tables and nitrogen rates are identical to
the normal Nitrox diving here.
Re-using of gas means that a substantial
smaller volume of
gas needs to be carried. This implies that the construction of the diving
set can be lighter and that a smaller gas container will be sufficient.
Typically a SCR will be about 4 times more
efficient compared to a open circuit diving system.
Partial re-use of the gas is being explained in the
fact that Nitrox (EANx=enriched air
Nitrox of a certain composition) is
constantly being injected. This happens via an injector with a permanent
flow. When thinking of an injector you should think of a perforated piece
of hard metal with an extreme precision that creates a constant flow.
Overpressure and bubbles
The injected gas of course contains nitrogen.
Since the body does not use this gas in its system, it returns in the loop
and has to be dumped to the outside of the system. For this purpose, the counter
lung has a overpressure valve. This valve that has a light working spring
mechanism enables the gas to be dumped when there is over pressure.
Because of this way of functioning this systems is being referred to as a
Gas injectors are used to add a gas flow with
a precise volume (mass). By making the injectors interchangeable we are able
to adapt the flow in relation to the composition of the injected gas. If a richer
oxygen gas is being used, a smaller injection suffices since the human
body has the exact same metabolic use at any depth.
For example the Dolphin has the following flow rates
by the Dräger company):
- Ean 60 5,8 barL/min
during 124 minutes with limitation MDD = 17 mtr
- Ean 50 7,3 barL/min
during 98 minutes with limitation MDD = 22 mtr
- Ean 40 10,4 barL/min during 69
minutes with limitation MDD = 30 mtr
- Ean 32 15,6 barL/min during 46
minutes with limitation MDD = 40 mtr
A Nitrox mixture of 60% oxygen enables a dive of
124 minutes to a depth of 17 meters.
With a Nitrox mixture containing 40 % oxygen a 69
minutes dive to 30 meters can be made.
Semi Closed Equation
In real life: Because the mixture is being recycled and its composition
depends on a metabolic consumption, a dive with 40% Nitrox mixture will
result in an inhaling of 33,62% oxygen. If we dive according to for
example the Dutch Underwater Organisation (NOB) norms, a maximum
partial pressure of 1,5 bar oxygen is allowed. In case we want to
accomplish a dive to a maximum depth of 30 meters within decompression
limits, we will calculate with EAN34, thus resulting in a maximum of 25
minutes dive time. In the section Calculations with a
semi-closed rebreather we will further look at the way how the
actual fraction is calculated.