The following text is written under the assumption that the reader has knowledge about the relation between gas fractions and partial pressures as well as basic knowledge in rebreather design. Also, nitrox concepts like MOD and EAD are assumed to be familiar to the reader.
The following test describes the function of the Draeger SM 1, a self-mixing semi-closed rebreather designed in the 1960’s. The SM 1 is a design where the fresh gas flow and composition is depth dependent. Rebreathers with this kind of fresh gas addition result in a lower partial pressure at depth and a higher one at shallow depths compared to the constant mass-flow semi closed rebreather, CMF SCR.
The full name of the Draeger SM 1 is “Selbstmischendes Mischgas-Schwimmtauchgerät Model SM 1”, or “self-mixing mixed gas swim diving apparatus model SM 1”
The steady-state oxygen fraction in the breathing loop of a semi-closed rebreather is a function of the fresh gas flow, fresh gas composition, and the divers’ oxygen consumption:
The nominator describes the net flow of oxygen flow in the circuit and the denominator the total flow leaving (i.e. venting) the breathing circuit.
The fresh gas addition in the SM 1 self mixing semi-closed rebreather is implemented by the use of a mechanism that adds a constant mass flow of oxygen to the breathing circuit and a depth dependent (i.e. increasing) flow of diluent gas. The result is a rebreather that maintains a lower PO2 at depth than a constant mass flow SCR. In theory it is possible to design a fresh gas addition that maintains a constant P O2 independent of the depth, the Carleton SIVA + in an example of such a design.
A unique failure mode of the self-mixing unit is the loss of one of the supply gases and the resulting erroneous flow and composition of the fresh gas mixture. The Draeger SM 1 utilizes two uncomplicated pressure switches to light a lamp mounted in the divers mask in order to alarm for the loss of one or two of the supply gases.
Oxygen is added using a pressure regulator set at 9 bars absolute (i.e. not ambient referenced), the pressure feed a normal critical flow orifice. The diluent gas (air) is supplied using two pressure regulators, one first reducing the pressure to 9 bar and a second one reducing the pressure down to 0.5 bar above ambient. The ambient referenced diluent pressure feeds a non-critical orifices thus resulting in a flow that slowly increases with depth as described below.
The fresh gas flow into the rebreather comprises a constant flow of oxygen, QO2, and a depth dependent flow of diluent gas, Qdil. The diluent gas oxygen fraction, Fdil, is constant; in this case it is 21%.
In the SM 1 the diluent flow, Qdil, is 2.2 L/min on the surface and increases to 4.6 L/min at 40m. The oxygen flow, QO2, is 2 L/min.
The depth dependency of the diluent gas flow can be expressed:
Where the Kdilstart is the diluent flow at the surface and Kdil is the change of diluent flow with increasing depth in L/min/bar. Depth is measured in meters.
The resulting flow of fresh gas, Qmix, thus becomes:
In the SM 1 the fresh gas flow will be as follows:
Figure 1. The diluent/air, oxygen and total fresh gas flow in the SM 1 as functions of depth.
The resulting fresh gas oxygen fraction, Fmix, will be:
The steady-state oxygen fraction in the breathing loop, FO2:
The PO2 in the circuit thus becomes:
From the equation above it is apparent that the oxygen fraction in the breathing loop has a dependency of the divers’ oxygen uptake.
Figure 2. Oxygen fractions and partial pressures in the Draeger SM 1 as functions of depth and different oxygen consumptions.
The total breathing loop volume of the SM 1 is about 9L (according to Haux – Tauchtechnik, library of congress no 79-99015) and with the relative low fresh gas flows seen the time constants will be considerably longer than in the CMF SCR.
Figure 3. Time constants in the Draeger SM 1 compared to a Dolphin with a 40% mix at 10.3 L/min. The divers oxygen uptake was assumed to be 1.3 L/min.
Under the assumption that one keeps 50 bars as a reserve, 225 L of Air and oxygen is available respectively. The duration of the unit is depth dependent and limited by the diluent/air supply at all depths, see table below:
The unit has a 2L scrubber (i.e. slightly smaller than the FGT/FGG) canister, which is not likely to limit the duration of the dive.
The benefit of the self-mixing semi-closed rebreather is that it lowers the oxygen fraction with depth and the depth range can thus be extended relative to the CMF SCR. Also, the fresh gas consumption is lower than for a CMF SCR which is especially apparent at greater depths.
The definitive drawback of the self-mixing design compared to the CMF SCR is the increased complexity as well as the new failure mode resulting from the loss of one of the supply gases.
The operating depth of the Draeger SM 1 is limited by the output pressure of the regulators, so diving the unit deeper than the specified 40 m will result in the loss of fresh gas flow with well known results.
A more modern design of the self mixing SCR RB is the Carleton (formerly the Fullerton Sherwood) SIVA+ (see www.carltech.com) which has a slightly more developed design of the diluent gas supply resulting in an almost constant partial pressure of oxygen down to its maximum operating depth of 90m. More information on that unit some other day…
Last updated 2003-03-23
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