This should really be Part One, since these features are available in the standard Sentinel rebreather. The standard system is called a Canister Duration Meter which includes a temperature sensor and an oxygen injection monitor. Together, these two devices can give a reasonable read on the state of the carbon dioxide absorbent within the rig.
The Thermal Profile Monitor (TPM) gauges the temperature within the scrubber, using 8 thermistors. The readings are displayed in a bar graph. The diver learns to interpret the shape and color of the TPM curve to predict different scenarios such as: no scrubber, leaking, expired, flooded, bypass, etc.
The full picture based on temperature only emerges about ten minutes into a dive, so the Sentinel has added a secondary feature to monitor oxygen injections.
The oxygen injection monitor works on the assumption that the carbon dioxide generated is directly proportional to the oxygen used by the diver. The oxygen that passes through the solenoid is measured and displayed in a percentage. This way, the diver’s metabolic rate is factored into the equation (note: manual injections are not included in this figure).
Additional failsafes are added to the system. If the diver has left a scrubber in the rig for more than one week, the available percentage will be reset to zero. If the scrubber has been used for five hours duration, then an alarm state will be triggered.
Using all the systems together, the diver has a reasonable picture of the state of their scrubber. If they make good choices and respect the limitations of absorbent material, these systems should offer the best safety and support available today.
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