PRISM Topaz Sensor Performance & Placement Issues
Oxygen sensors are perhaps the most critical component of any closed circuit diving system design. Oxygen sensors must be accurate under a wide range of environmental conditions including high moisture and elevated partial pressures of oxygen and various diluent gases. They must also be fast in response time to prevent unacceptable swings in breathing loop PPO2 levels. Oxygen sensors used in contemporary military constant PPO2 rebreathers were designed to work in fractional oxygen partial pressure environments. These sensors have greatly reduced life at partial pressures above 1 ATA.
Steam Machines has developed a proprietary oxygen sensor for use in PRISM Topaz. This sensor is fast response, high output and designed specifically to work at elevated partial pressures. The breathing loop pathway and sensor plenum of PRISM Topaz are designed to condense moisture and route water away from the sensor elements. Under normal operating conditions, including operation at PPO2 setpoints up to 1.5, the PRISM Topaz oxygen sensors provide useful lifespans far in excess of conventional sensors. Initial PRISM Topaz maintenance specifications require replacement of oxygen sensors on twelve month maximum intervals. Results of long term testing may allow future extension of that figure.
Oxygen Sensor Calibration Issues
Steam Machines research has found that sensors calibrated at or below one atmosphere PPO2 often fail to read accurately at elevated PPO2 levels. In fact, the most common failure mode of galvanic type oxygen sensors is falloff of response at high oxygen partial pressures. It is therefore quite common that a sensor calibrated at 1 ATA will be unable to report elevated oxygen partial pressures. For example, an actual loop PPO2 of 1.8 may well be reported by the sensors as 1.1. This causes the rebreather control system to drive the actual loop PPO2 high, risking CNS O2 toxicity.
It is important to note that this falloff in sensor response cannot be detected by traditional surface calibration methods until the falloff extends below the calibration value of 1 ATA. This is especially true if the three sensors used in a voting matrix are of the same type and age since high level falloff is typical as sensors age. It is therefore extremely likely that systems operating at elevated PPO2 setpoints but calibrated at or below 1 ATA are delivering dangerously high oxygen partial pressures even though primary and secondary displays are reporting normal operation.
PRISM Topaz PPO2 Sensor Calibration
While PRISM Topaz uses proprietary sensors to minimize this problem, it is still vitally important to calibrate sensors at actual working PPO2 levels. Steam Machines has developed a proprietary method of calibrating oxygen sensors in the PRISM Topaz.
PRISM Topaz Radial Flow Scrubber
The PRISM Topaz series platform uses a radial flow CO2 scrubber featuring an easily packed, removable cartridge-style scrubber basket. The scrubber has undergone extensive independent testing under the most extreme diving conditions. The radial flow design allows PRISM Topaz to deliver extremely low work-of-breathing. Expired gas leaves the exhalation counterlung and passes through the oxygen injection plenum and down through the permeable center tube of the scrubber canister (figure 5). The gas passes radially out through the absorbent material and through the outer permeable screen. Oxygen enriched, carbon dioxide-free gas is routed around the outside of the scrubber basket where it serves to insulate the scrubber basket from the canister shell. This results in increased scrubber bed temperature and attendant increase in CO2 absorbent performance, especially at low water temperatures.
The radial flow design also has the advantage of scalability. Longer scrubber basket assemblies, providing extended diving durations, result in increased cross-sectional area with resulting decrease in breathing resistance. This is a great advantage over an axial flow cartridge design where lengthening the cartridge for longer duration causes an increase in breathing resistance.
PRISM Topaz Functional Diagam
Counterlung Placement Issues
The counterlung system in a closed circuit diving system serves the primary purpose of providing a space of dynamic volume for the diver to exhale into during the breathing cycle. To serve this purpose it may be mounted in any of three places: 1) on the divers back 2) on the divers front or 3) over the divers shoulders.
Front or back mounting bring with them the issue of hydrostatic effects on work-of-breathing. This is especially true of back mounted counterlungs where the difference between the depth of the counterlung and the depth of the divers lungs is greatest. This difference in depth causes a diver swimming face down to have to work against higher water pressure when inhaling. If the diver swims upside down the higher counterlung pressure can be enough to blow the divers mouthpiece out. Back mounted counterlungs do have the advantage of being concealable in the main rebreather housing. This is sometimes an advantage in applications like explosive ordinance disposal where there is serious incentive to keep moving parts as far away from the ordinance as possible.
Steam Machines made the choice to design PRISM Topaz with twin over-the-shoulder counterlungs. This has the major advantage of placing the counterlung dynamic pressure axis on virtually the same plane as that of the divers lungs. The result is the lowest possible work-of-breathing in all body positions without the need to resort to complicated mechanical counterweight schemes.
A second, equally important factor in this design choice was that the PRISM Topaz counterlungs serve as an extremely effective water trap. Any water entering the system at the mouthpiece is routed first into the counterlungs. This water collects at the bottom of the front-mounted extensions where it is easily vented through purge valves provided on each side. Water cannot enter the critical sensor plenum or scrubber areas without first filling the counterlungs. The PRISM Topaz counterlungs have the additional advantage of adjustable volume through an easily operated variable overpressure valve. This makes fine tuning of system buoyancy extremely easy. The counterlungs are easily removed and disinfected which is important for safe long term rebreather use and critical in a training environment. The design also easily accommodates oversized counterlungs for divers with extreme breathing volumes.
PRISM Topaz Control System
PRISM Topaz provides proportional control of oxygen injection using a simple low power, long life solenoid-operated injection valve. This valve is pulse-width modulated allowing the PRISM Topaz control system to inject precisely the amount of oxygen necessary to compensate for diver metabolic oxygen consumption.
PRISM Topaz is powered by a long-life lead acid gelcell stored in a sealed chamber. The battery system is easily recharged without removal from the system and stores enough power on a single charge for an estimated twenty five diving hours.
The PRISM Topaz control package is sealed in a pressure vault which is totally isolated from the breathing loop. Only low power sensors are allowed in the breathing loop. Sensor and solenoid control wiring are routed internally in the sealed control/manifold section, reducing the number of watertight connectors necessary. The entire electronics package, like the rest of PRISM Topaz, is designed to reduce the potential points of failure to an absolute minimum.
[ Topaz Main - Part I - Part II - Part III - Part IV ]
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Revised: December 01, 2002.
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