Sensors

The Electrolung used three polariographic oxygen sensors. The sensors were robust hand made ones we made ourselves. They had a central platinum cathode about 1/4" in diameter surrounded by a concentric silver anode about 3/8" Diam. In between was an annular groove for the KOH electrolyte. A .001" teflon membrane held in place by a thick silicone rubber boot retained the electrolyte. Although the sensors would run for weeks before desiccation of the electrolyte became limiting our SOP was to make them up fresh and calibrate them for each days diving. At the end of the days diving the membranes would be removed and the sensors washed with distilled water. Making up and washing off the sensors only took a few minutes and assured you always had fresh sensors.

Sensors of this type don't wear out so they were hard wired into the circuit. Unlike galvanic sensors they don't use oxygen but rather just respond to its presence. They work equally well submerged so the effect of any condensation is negligible. A drop of water fully covering the end of the sensor would only slow the response time. It practice we never had any condensation in the sensor area as this came immediately after the canister so the gas was at its warmest and driest point in the circuit. Thick plastic walls probably helped too in avoiding condensation on cold surfaces. The chief advantages of the sensors were that they were always fresh and condensation wasn't a worry. The disadvantage is that in making them up with fresh electrolyte you can screw up by contaminating the sensor via sloppy technique. Any significant change in calibration after a fresh make up would be an indicator and determining why should be mandatory before proceeding further. Still, there are the black box mentalities who will simply crank the trim pots until they get the reading they want and then assume all is well.

There were two trim pots for calibrating each sensor. One for zero. The other for gain. Zero was checked each time before the sensors were made up. Gain was calibrated initially with air then the unit was put together and a check with pure O2 was done. The permeability of teflon to O2 varies with temperature. The sensors were of potted epoxy construction with the electrodes embedded in the epoxy. A thermistor in contact with the underside of the cathode was also embedded. This thermistor had a similar response curve to the teflon and compensated for the temperature effect keeping output linear over the desired range. Our chosen set point was 0.5 Atm PPO2. The bottom line was that with proper care the sensors were very reliable. Enough so, that they could be hard wired in and I know of no case where one ever had to be replaced.