Overview of Freebreathe System

The Freebreathe system is a ‘semi demand gas reclaim system’.

1. A compressor pumps gas through an umbilical hose down to the diver at a constant rate

2. A double counterlung in the diver’s backpack accommodates the diver’s breathing

3. A hose from the supply side of the double counterlung provides gas to the diver

4. A second hose takes the exhaled gas to the return side of the double counterlung

5. The exhaled gas is then returned to the surface at a constant rate

6. The gas is reprocessed at the surface before being pumped back down to the diver after

a. Surplus moisture has been removed

b. Carbon Dioxide has been removed

c. Oxygen has been added

d. Makeup gas has been added

Because gas flows to and from the surface at a constant rate, if the diver holds his breath gas continues to circulate as it would in a freeflow helmet. When the diver breathes in, the double counterlung increases the flow to the diver whilst decreasing the flow from the diver. As the diver breathes out, the flow to the diver decreases whilst the flow from the diver increases. This is unlike a normal demand system where both the supply and return flows stop when the diver holds their breath, hence the term ‘semi demand’.

A rebreather and a surface supplied open circuit regulator are incorporated into the system as backup.

Conventional Gas Reclaim Systems

1. A conventional gas reclaim system uses pressure regulators in the supply line and backpressure regulators in the return line to control the supply and return flow to the diver. Most of the pressure boost provided to circulate the gas is to overcome the pressure drop across these regulators at peak flow rates

2. As the flow rate is cyclical the peak flow rate in both the supply and return hoses is over three times the diver’s rate of breathing so the supply and return umbilical hoses have to be sized to take these flows

3. A volume tank is needed to store the compressed gas to provide the peak supply gas flows for short periods at extreme breathing rates. Similarly, large water traps are needed to accommodate extreme return gas flows

Freebreathe Reclaim System

The Freebreathe reclaim system has no regulators in the recirculating loop. This:

1. Significantly reduces the pressure drop in the circuit allowing much smaller and lower pressure compressors to be used. Typically the power required is reduced from 35kW to 5kW

2. Removes the need for a volume tank or large water traps

3. Means that supply and return hoses can be reduced by a size: supply from 3/8 inch to 1/4 inch, and return from 5/8 inch to 1/2 inch internal diameter

4. Makes it still possible to recover all the return gas with the diver on the surface