Semi-Closed Circuit Rebreathers
Semi-closed circuit rebreathers (SCR) allow for the use of an inert gas to dilute the oxygen, which allows you to dive deeper. The base gas that is typically used is an EANx (nitrox) gas mix. An SCR is very similar to oxygen closed-circuit rebreathers except for the fact that they must have functionality to accommodate the use the inert gas.
One of these changes is the addition of an over pressure valve, which is used to vent gas from the breathing loop so that it stays at ambient pressure. Also, a metering valve is used to inject the breathing gas into the system. The metering valve’s job is to inject enough gas to make up for your metabolic consumption of oxygen.
There are three different classifications of semi-closed circuit rebreathers:
· Constant mass flow
· Respiratory minute volume keyed
· Constant ratio
A constant mass flow SCR is the simplest and most commonly used type of recreational rebreather. They are simple to set up, operate, and maintain. The system meters a constant flow of EANx into the breathing loop. The gas mix set determines the flow rate. The over pressure valve vents gas from the loop corresponding to the flow rate amount of gas. The mass flow controller is an integral part of a constant mass flow system. If must accurately regulate the flow of gas by mass regardless of your depth. Checking the mass flow controller is part of every pre-dive check.