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Rebreather Prism Topaz
A prototype Prism was the first rebreather I ever used. I took one on an expedition to the Red Sea in 1992. It required a lot of Duck Tape and faith, at that time, on the part of the user.
     It was agreeable to find that its inventor, engineer Peter Readey, is now living in California, and that the production Prism Topaz is now a polished product, honed by the requirements of the US Navy COTS (commercially off-the-shelf) purchasing policy. COTS allows the military to save millions of dollars in development costs and the private buyer gets the advantage of all that military testing and development .
     Luke Inman, an instructor at the Cortez Club in La Paz, Baja California, Mexico, is helping the American University Research shark-tagging program. That organisation uses only closed-circuit rebreather (CCR) units tested and approved by the US Navy, including the Prism. So I met Peter 1000 miles south of his home at the Cortez Club, with a view to getting to know the Prism.
     The Cortez Club is now a complete CCR facility that can support both Prism and Inspiration users with 3 litre DIN cylinders, oxygen at high pressure and Sofnolime or Sodasorb, and of course both semi-closed-circuit and CCR training.
     Most readers are by now familiar with the advantages of closed-circuit rebreathers. The user consumes only the O2 actually metabolised; the mix breathed is at a fixed partial pressure of oxygen; and the inert gas is kept to a minimum, conferring great advantages when it comes to decompression. The duration of the unit depends on that of the scrubber that removes exhaled carbon dioxide. With the Prism that is 5 hours at only 4°C.
     Until now the APD Inspiration has been the only CCR available with CE certification. Peter intends to change that. His Prism Topaz, currently sold in the USA, is second only to the Inspiration in terms of number of units in private hands.
     The Prism is modular, making it easy to transport. It will go as carry-on luggage on a plane à fully loaded it weighs around 21kg. Inevitably the Prism invites comparison with the Inspiration and it makes for a viable alternative purchase.
     The Prism uses 3 litre cylinders but has been designed to accommodate a variety of sizes. The oxygen valve is a 200 bar DIN connection and the diluent a 300 bar DIN connection, to avoid confusion. Scubapro valves are used.
     A display appears at the bottom of your mask, on a little unit mounted on the mouthpiece. It uses a series of LEDs: blue indicates dropping ppO2, red rising ppO2, and green is good.
     The warning lights flash if level changes become serious. There is also a bi-coloured LED for warnings of cell failure. The green LED pulses to indicate that it is functioning normally and this is what you mainly see during a dive. The brightness of this green light is adjusted automatically according to ambient light levels. It is not blinding or distracting.
     The other colours certainly get your attention, even if they indicate only a momentary spike in ppO2 levels.
     There is also a totally independent secondary display in the form of an analogue ppO2 meter which runs directly from the three analysing cells, no battery needed. This can also be used to check the state of the 9V battery that runs the primary LED display and controls automatic mixing of ppO2 and diluent.
     You also use it to check set-points and cell status, and even to find out if there are higher voltages than normal present anywhere in the electronics, caused by an unwanted ingress of water leaking power from the battery.
     Every time I got a red or blue light (increasing or decreasing ppO2 levels) during a dive, I was able to go to this display to see exactly what was happening. I found this very useful.
     The secondary display allows you to fly the unit manually, even if your electronics suffer a disastrous malfunction, and you can completely over-ride the automatic system at any time.
     Prism training requires trainees to use the system manually, which imparts a good understanding of what is happening to ppO2 levels during a dive. If you leave the automatic system switched on when you do this, you will have to endure the resulting colourful warning light display.
     You set up the unit to your choice of working set-point for ppO2 (usually 1.3 bar) from a range of 1.0 to 1.4 bar, but a pressure transducer changes this to 0.7 bar when you are shallower than 5.5m deep. This is entirely automatic, and there can be no diving at the low set-point by mistake.
     The electronics are plotted in re-enterable urethane (watertight but accessible). The solenoid is outside the breathing loop, so that should it fail it will not dump high levels of O2 into the mix. There is no high-voltage electricity in areas where there are elevated levels of oxygen.
     The gas is analysed after passing through the radial scrubber but is injected into the loop prior to the scrubber, thus ensuring a good mix.
     Water from the exhaled breath condenses on the face of the scrubber bucket so that the three analysing cells stay perfectly dry. The scrubber takes around 2.7kg of scrubber material.
     There is an automatic diluent-addition valve and an adjustable over-pressure valve on the counter-lung. I found that I could simply blow out into the mouthpiece during an ascent to push expanding gas in the counter-lung through this over-pressure valve. It truly is a hands-free operation. There is also a manual oxygen over-ride valve, exactly like that of an Inspiration.
     Nice touches are the drains fitted on the counter-lungs, which allow you to dump any water that might find its way into them during the dive. The absence of any T-junctions means that any flood caused by clumsy dropping of the mouthpiece while open and submerged goes directly into a counter-lung rather than any other part of the loop. The mouthpiece is negatively buoyant.
     I also liked the fact that the mouthpiece was easy to open and shut. It is operated by a large lever, which enabled me to close it during the dive and eject some water that had collected in my mouth through the drain-hole provided in that position. I didnt have to remove the mouthpiece.
     The work of breathing at depth seemed very low. I dont know if it was less than with an Inspiration. I was always very comfortable and our no-stop dives were always of several hours duration.
     A Scubapro Air II is used as open-circuit bail-out and to control buoyancy by means of a large wing-style BC. Two of my fellow-divers preferred to carry their bail-out in separate sling-tanks.
     Weight-pockets are integrated and it is possible to zip two weights on to each counter-lung to resist any tendency for them to float up.
     The harness has the usual cummerbund and straps and the works, including scrubber and cylinders, are housed inside a black-coloured shell. Peter Readey is a passionate engineer and I can understand that he was reluctant to see his work covered up in this way, but it does make the latest Prisms looked more finished than the uncovered original.
     So what did I not like about the Prism As it was originally aimed at the military, preparing the unit involves much paperwork. Most of these issues are taken care of with the Inspiration by the interrogating handset display just before diving.
     Preparation of the Prism seemed to take a lot longer. In fact, prescribed pre-dive preparation could take the best part of an hour. But it does make it very safe and, with five hours duration, you dont have to prepare so often.
     I also found to my cost that it is possible to put the radial scrubber unit into the scrubber bucket upside-down. No-one noticed me make this mistake.
     After preparing my unit with this user-error in place, I entered the water for my second Prism dive to find myself breathing open-circuit. I was pulling in diluent gas through the automatic diluent valve and exhaling it through the over-pressure valve on the counter-lung. It was like breathing through a poor-quality regulator while watching Blackpool illuminations on the display!
     I felt I was getting a dose of CO2 poisoning, such was the breathing effort. My dive was quickly aborted. I suggested to Peter that he write the word top on top of the scrubber unit.
     Five of us were diving Prisms at the Cortez Club. We were swimming along at the sea-mount El Bajo, when we saw a new video-housing lying beyond reach of any open-circuit diver.
     We retrieved it, and Luke swam to the only other boat to ask if anyone had lost anything.
     Yes, but youll never find it, came the reply. Weve searched everywhere. Itll be too deep.
     Was it a Top Dawg housing with a Sony digital camera Did it have a green lanyard asked Luke.
     Yes, but youll never find it. It was sheer joy to see the owners delighted double-take when Luke asked him if it had green Radio-Shack batteries in it!
The Prism Topaz costs around £5700 fully loaded.
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    The scrubber bucket, designed to catch condensation
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    the secondary display
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    oxygen sensors are kept safely away from moisture
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    + Very safe when properly prepared
    + Military safety specification



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    - Long prep needed