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Part III - O2 Setpoints

PRISM Topaz PPO2 Setpoint Range Discussion

Higher oxygen partial pressures in the breathing loop result in lower inert gas partial pressures and consequent reduced diver inert gas loading. Prevailing medical opinion holds that continued diver exposure to PPO2 levels of greater than 1.6 ATA increase the likelihood of CNS oxygen toxicity to unacceptable levels. Accordingly, PRISM Topaz is designed to accommodate diving PPO2 levels of .21 ATA to 1.6 ATA with a typical diving setpoint of 1.3 to 1.4 ATA. PPO2 levels of up to 2.0 ATA can be maintained by the system, although this is not recommended. It is important to understand that PRISM Topaz is a constant partial pressure mixed gas breathing system. Because oxygen partial pressures are held constant, the actual gas volume percentages will vary according to depth. Delivered gas percentages of PRISM Topaz using a diving setpoint of 1.4 are shown in Figure 4. Note that diluent gas percentages and corresponding partial pressures are significantly reduced over open circuit diving at recreational diving depths.




Depth (FSW)

Pressure (ATA)

O2 %

Diluent %

Diluent PP

Surface (0') 1.0 ATA 70 30 .30*
10' 1.3 ATA 54 46 .60*
20' 1.6 ATA 87 13 .21
33' 2.0 ATA 70 30 .60
66' 3.0 ATA 47 53 1.59
99' 4.0 ATA 35 65 2.60
132' 5.0 ATA 28 72 3.60
165' 6.0 ATA 23 77 4.62
198' 7.0 ATA 20 80 5.60
218' 7.6 ATA 18 82 6.23
231' 8.0 ATA 17 83 6.64
264' 9.0 ATA 16 84 7.56
297' 10.0 ATA 14 86 8.60
330' 11.0 ATA 13 87 9.57
363' 12.0 ATA 12 88 10.56
396' 13.0 ATA 11 89 11.57
429' 14.0 ATA 10 90 12.60

*NOTE: PRISM Topaz PPO2 target is typically 0.7 above 18 FSW

Figure 4

PRISM Topaz Dual PPO2 Setpoint Operation

Maintaining elevated oxygen partial pressures while diving presents certain operational complications at the surface. At one atmosphere of surface pressure, a 100% oxygen environment is equivalent to 1.0 ATA PPO2. It is impossible to attain a higher partial pressure than 1.0 at surface pressure. In attempting to do so, conventional closed circuit diving systems using an elevated PPO2 setpoint tend to continually inject oxygen into the breathing loop. The result is tremendous waste of oxygen trying to reach an unattainable PPO2 setpoint. Experienced closed circuit divers diving at elevated PPO2 levels often turn their control systems off and operate their systems manually until reaching a depth at which it is possible to attain the setpoint PPO2. (Note that this was not an issue for most military divers since their operational PPO2 setpoints were traditionally .7 ATA.)

PRISM Topaz was designed from inception to operate at elevated PPO2 levels. Accordingly, PRISM Topaz accommodates two PPO2 setpoints: (1) "Shallow water" and (2) "Diving". Each setpoint is adjusted separately by the user prior to beginning the dive. PRISM Topaz uses a canister mounted pressure transducer to determine depth over the first atmosphere of descent. Changeover is made from "Shallow water" to "Diving" PPO2 setpoint below approximately 18 fsw (technician adjustable). This system (patents pending) allows a low "Shallow water" PPO2 setpoint to be used while shallow, preventing the system from wasting oxygen in trying to achieve unattainable PPO2 levels. A higher "Diving" PPO2 setpoint is used at depths where that setpoint is attainable to minimize inert gas loading. The 18 fsw changeover point was chosen to allow decompression at a 20 fsw stop with elevated PPO2.

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