Electronics

Unlike galvanic sensors, polariographic electrodes don't generate electricity. Rather, the conductivity of the cell varies in the presence of oxygen. A bias potential from an external source is applied between anode and cathode and the resulting flow of current is a function of the molecular concentration of oxygen present. The current involved is very small so an Op Amp is used with each sensor to boost power to a level useful for control and monitoring. Hermetically sealed trim pots which incorporate an O-ring seal around the adjustment screw provide for zero and gain adjustment of each Op Amp thus enabling calibration.

The amplified signal is read out to a wrist display consisting of a stack of three edgewise panel meters. 100 microamp meters were used in conjunction with high resistance to prevent a possible short in this circuit from affecting the solenoid control. Mil Spec, so-called "shock resistant" meters were used. These resist minor bumps but still they won't stand up if you drop the display on a steel deck or concrete. In practice it wasn't a significant problem but occasionally a meter did require replacement. This was quick and easy to do.

The big advantage of this type of analog display is that you can tell at a glance everything you need to know. In use all you need to verify is that all of the readouts are in line with one another and at or a bit above the set point which was exactly mid scale. This kind of meter is also precise enough for calibration purposes. If I were doing it today I would look at bar type LCD or LED readouts for monitoring and perhaps a separate switchable numeric display for calibration. I would also seriously consider a miniature head up display in the mask instead of one on the wrist. I don't like numeric displays for monitoring as they entail reading and mentally comparing numbers which requires much more attention than just noticing if position and alignment are where they should be. Possibly some of the commercial RBs have already done all this.

The amplified signals from all three sensors were fed into a fourth Op Amp which in effect averaged them and used the resulting value to control the solenoid set point via a switching transistor. We used a fixed set point of 0.5 Atm PPO2 but it would be simple to add a trim pot to provide an adjustable set point. Clipping circuitry limited the input to the control Op Amp from each sensor to values corresponding to 0.25 and 0.75 Atm PPO2. If any one sensor began to read drastically different from the others its effect on automatic solenoid control was thus limited. Clipping came after the meter display thus they would continue to read true output even if the input to the control Op Amp was clipped. Clipping also activated an audible alarm. If the alarm sounded a glance at the meters would tell you what the situation was. If only one was off the other two would continue to exercise control. If all were high, low or different from one another you could use manual control while aborting the dive.

The Op Amps require a + and a - voltage power supply. This was supplied by a pair of 9V Manganese Alkaline transistor radio batteries. Bias to the sensors was provided from the same source via a voltage dividing resistor circuit. A second pair of the same batteries provided switchable backup power. A third pair used in parallel provided separate power for the solenoid. The solenoid did not have backup as this is non-critical because manual control of O2 is easily effected. The snap terminals used for this type of battery were securely attached to a bulkhead. A screw adjusted base plate held the batteries firmly in place and against the terminals avoiding any possibility of a loose battery connection.

All the electronics were incorporated on a single circuit board about 4x5". This was mounted on one side of a longitudinal bulkhead in the electronics housing with the batteries and audible alarm on the other. This longitudinal bulkhead was itself mounted on a transverse bulkhead which separated the electronics compartment from a plenum above the absorbent canister. The solenoid and sensors were mounted on the opposite side of this transverse bulkhead thus everything electrical other than the wrist display was immediately adjacent to each other.

In the units I made all of the electronic components were on a printed circuit board. After assembly the boards were coated with a spray-on waterproofing compound as is widely used for marine electronics. At Beckman the components were assembled into 4 micro- welded epoxy potted modules which plugged into gold plated sockets on the circuit board. In theory this is a better way to go but in practice it didn't make any noticeable difference.

With respect to reliability of electronics in this kind of application. Recently someone posed the question of when was the last time your TV failed to which Robert made the wonderful reply, "The last time I took the bastard underwater." Both comments reflect important points. Electronics in themselves can be extremely reliable. In terms of MTBF, far more reliable than most mechanical devices. Enough so that they can be trusted for things like passenger aircraft control systems where thousands of systems are in everyday use and a single failure means the loss of hundred of lives. But Robert is right too. If you flood them with water they fail.

The problem then is really a mechanical one. Can electronics be reliably enclosed so as to prevent flooding in underwater use. If it were solely a matter of constructing a watertight pressure proof housing for the electronics that alone wouldn't be too hard. Unfortunately there is also the matter of connections for sensors, displays, a solenoid, and a switch plus keeping all these external devices themselves dry. The possibilities for leaks begins to multiply. With a great deal of care in construction and use, high reliability is achievable but I think there is a much easier way to reliably keep out the water.

The key to the solution is pressure. Keeping things watertight under 100-200 psi is difficult. Doing it under 0.5-1 psi is easy. In the Electrolung everything was at ambient pressure. The electronics compartment was vented via a small canister of silica jell with the rest of the system. A standpipe for the vent orifice prevented any accumulated moisture in the canister plenum from being pushed into the electronics compartment. In anything but a head down position the electronics were above the counterlung thus any leak would normally result in gas escaping rather than water coming in. In practice with the kinds of seals involved and the very low pressure differentials leakage anywhere in the electronics section was never a problem.

Humidity and condensation were non-problems. Plastic construction probably helped in avoiding the latter and the waterproof coating seems to be quite sufficient for the former as is well attested by a wide array of complex devices and vast usage experience in the marine electronics industry.

The only practical way to get your TV underwater with this type of system is to flood the entire system. This is inherently no more likely nor any more or less disastrous than it would be with any other rebreather regardless of type.