The Optima CM uses four oxygen sensors. These sensors are threaded into the sensor disk mounted inside the electronics head. Dive Rite uses Analytical Industries, Inc. type R22D sensors.
Oxygen sensors have a finite life. They are a consumable item and must be replaced at regular intervals. They are typically replaced during the annual service, but may need to be replaced prior to this.
The sensor labeling includes a “Sell by” date of 4 months after manufacture and a “Do not use after” date of 16 months after manufacture. Sensors must not be used after this date even if they appear to still be functioning correctly.
Four sensors are used to provide redundancy and the ability to cross check their values against each other to determine if a sensor is not reading correctly. Sensors 1 and 2 are shared between the controller and the HUD. Sensor 3C is only connected to the controller, and sensor 3H is only connected to the HUD. By comparing sensor readings between the HUD and the controller it is easy to determine if a sensor is not reading correctly.
If sensor values 1, 2, and 3C do not agree, the controller automatically uses a voting logic to make an educated guess of which sensor is not reading correctly. Having an independent “4th sensor” display on the HUD allows the diver to independently verify the controller’s voting logic.
For more information on oxygen sensors, see the Oxygen Sensor Care section and also Appendix II:
Galvanic Oxygen Sensors Applied to Closed Circuit Rebreathers by Analytical Industries, Inc.