Dräger TR60 semiclosed-rebreather

Escape Mixgas rebreather from 60 m

Dhe TR60 also called escape rebreather is a semi-closed device for escaping from depth. The gas flow and composition of the gas is set to allow the user to stay at 60 meter, e.g. to swim out of a hatch. Due to the semi-closed technique, the diver inhales a lower fraction than the oxygen fraction supplied. Partly due to the short staying times at depth, this is possible, according to the manufacturer. However, this oxygen loading applies to emergency situations and can never be applied in recreational activities. Besides the escape function, the rebreather can also be used to serve in toxic environments or as a life jacket at the surface.

Recently I was able to buy a Dräger Tauchretter Model TR 60. This life saving dive jacket was designed in the 60’s and is a surprising rebreather. One of the most interesting aspects of this rebreather is that in fact it is a semiclosed rebreather! It is well known that using pure oxygen rebreathers to escape from depth introduces a big risk of oxygen toxicity. Semiclosed rebreathers offer a improvement because the breathed gas has a much lower oxygen content. This rebreather uses EAN 50!

To understand how the rebreather works on depth I here made two calculations to show why the partial pressure of oxygen is much lower with a semiclosed rebreather compared to the standard oxygen rebreathers. The working principle is based on the semi-closed equation:

The meaning of the formula is:

The unit is equipped with a constant mass flow regulator with a gas flow of 3,0 l/min. This flow is constant to a depth of 40 meter. This means that the intermediate pressure is 10 bar on the orifice. The flow will continue to 10 bar and the stops. This will happen on a depth of 90 meters. Because the flow is constant to half the IP at 40 meter there will be exact 3,0 ltr/min. Imagine a diver who uses 0,5 ltr oxygen per minute the partial pressure he is breathing can be calculated with the mentioned equation.

FiO2 = unknown

Qs = 3,0 ltr/min

FsO2 = 0,5 (EAN 50)

VO2 = 0,5 ltr/min

FiO2 = ((3,0ltr/min x 0,5)-0,5)) / (3,0 ltr/min – 0,5 ltr/min) = 0,4 meaning the fraction of the inhaled gas is 0,4. The gas is EAN 40 breathed at 40 meter the partial pressure of oxygen at depth is 5 bar ambient pressure x 0,4 bar oxygen pressure = 2 bar ! In the 60’s military divers were supposed to be capable of breathing 2 bar oxygen! Fortunately this has been changed, although this unit would only be used in a emergency situation and therefore the 2,0 bar is not so crazy..

When the diver uses more oxygen due to swimming or stress the unit can be used deeper. Imagine a oxygen consumption of 0,8 liters per minute the inspired gas is:

FiO2 = ((3,0ltr/min x 0,5)-0,8)) / (3,0 ltr/min – 0,8 ltr/min) = 0,318. When a gas is breathed with a fraction of 0,318 the maximum depth when 2 bar oxygen pressure is allowed is 2bar / 0,318 = 6,28 bar. On a depth of 52,8 meter the rebreather feeds EAN 31,8 resulting in an oxygen pressure of 2 bar.

In the manual  you find more details about this very nice unit!