Some comments on failure modes in electronic CCRs

Hypothetical failures in hypothetical designs are somewhat hard to talk about. The possibilities are limitless. I will instead suggest some ways to avoid or minimize them as much as possible.

Electronics and sensors situated together and using short fixed Teflon insulated cables present minimal opportunity for cable failure. Electronics situated atop a back pack and operating at ambient pressure are least likely to leak and if so most likely to let gas out rather than water in. If transparent plastic is used for the absorbent canister, electronics housing and breathing bag simple visual inspection for absorbent condition, condensation and water leakage is possible. Cylindrical canisters and electronics housings can be strong, rugged, easy to make in a lathe and lend themselves to highly reliable radial o-ring seals.

The greatest danger of water entry comes via the mouthpiece. A breathing circuit whereby exhalation goes direct to the counterlung and inhalation is drawn in through the absorbent canister past the electronics provides double water/condensation removal before the sensors. Outlets from both counterlung and absorbent canister should be free standing and away from the bottom to reduce the likelihood of picking up any water which may have gotten in. Counterlungs should be provided with drains which can be used underwater.

Readouts should operate at high impedance so that any failure of them or their cables have minimal effect on the control circuit. Redundant sensor circuits should incorporate signal limiting or clipping to prevent failure in one from dragging the others past safe limits. Switchable redundant batteries should be used. Only batteries constructed with hard wired connection between cells should be used. An extra ppO2 readout which is totally independent of the control circuit is worth considering. With a well designed unit by far the greatest source of failure is operator error and failure to pay attention. Some common errors to watch out for:

1. Trying to squeeze more time out of absorbent, batteries and sensors.
2. Careless or rushed setup. Lack of thoroughness.
3. Black box syndrome. The calibration is way off so just crank up the trim pot until it reads what you want rather than finding out why it's so far out.
4. Not thinking about what you are doing. Leads to mistakes like valving in pure O2 at depth instead of inert gas.
5. The She'll be Right assumption. The alarm is sounding and the readouts are high but if I take a few more breaths it will probably drop back.
6. Leaky mouth washout. This is all too common. Experienced OC divers are pretty casual about letting some water in around the mouthpiece. Even when it's gurgling away with each breath in a CCR they may ignore it until the system is well on the way to flooded. When a mouthful of pH 13 seawater wakes them up they blame the unit, saying, "It flooded!".
7. Drunken diving. Related to 4. above. If you have N2 on the inert side whether you know it or not you are impaired at depth and more prone to not pay attention or to make simple mistakes. There is a lot of individual variation in this. Using a mix which keeps the ppN2 below 3 ata is a good idea.

Other than water leakage by far the most likely equipment failure is manifested as solenoid not operating or stuck open and readouts too high, too low or one is drifting away from the others. For solenoid problems over ride with manual control. You should be able to hear if a solenoid is working or not. If all readouts are high cut off O2 and flush with inert. If this bring them back use manual control and abort dive. If it doesn't you have major electronic problems. Go to manual SCR mode and abort dive. If all readouts are low use manual O2 control. If the readouts respond properly continue with manual control to abort dive. If they do not. Flush with inert premix and abort in manual SCR mode. If one sensor is drifting but the others are responding normally simply abort dive permitting the remaining two to effect control. If the errant one is erratic or extreme go to manual SCR mode.

This all presumes the inert gas supply is premixed with a breathable % O2. Manual SCR simply means valving in the inert premix, taking two to four breaths (depending on depth and %O2), exhaling overboard and refilling from the inert. If a decompression obligation is involved you can switch to O2 at 40 fsw. If your first stop is deeper than that then you should have a standby breathing system available at the first stop.

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