Steve Millard's PRISM Topaz review |
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The following is a review on the Topaz from the a Rebreather List posting by Steve Millard.
Rebreather list posting "Prism Topaz Review"
Aug 12, 1998
Topaz - a personal perspective
On a recent visit to California I had the opportunity to visit Peter Readey at Steam Machines Inc and to see his Prism Topaz rebreather. Peter gave me a comprehensive overview of the unit and the following report is a personal viewpoint on what I saw. It should be remembered that the 2 units I saw were pre-production units and may differ from the final units in a number of ways.
I have sent a few photographs to Bill Elliott to post onto his website and this report should be read together with these photos.
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The Prism Topaz unit looks quite similar in layout to units such as the Cis-Lunar and the Inspiration already on the market in that it has a central cylindrical scrubber unit, oxygen and diluent cylinders on each side and a split, over-the-shoulder counterlung configuration. It is about 2/3 the height of my Inspiration unit, pictured alongside, and weighs about 40lb ready to dive. |
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The top of the scrubber unit is closed with a heavy brass plate which
counter-balances the bouyancy and provides correct diver trim. This plate is also angled
at 45 degrees to improve swimmer streamlining. The scrubber unit is normally accessed from
the bottom to change the scrubber material and the brass plate would not normally be
removed in service. Also seen from the back are the integral wings that provide additional bouyancy. |
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From the front it can be seen that the counterlungs sit quite high on the chest and on the pre-production units these are made from crushed neoprene. On the final units I understand that this material is to be replaced with an inner lightweight airtight bag and tough outer cordura bags. I personally preferred the initial concept of using crushed neoprene as a bit of stretch in the counterlungs could be a good thing..but I guess the twin bag idea is well proven & easier/cheaper to fabricate. |
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A close-up of the mouthpiece shows it is attached to the rubber hosing with heavy brass couplings, which counter-balance the hosing bouyancy and the mouthpiece sits comfortably in the mouth. An on-off closure switch similar to that used on the Drager Atlantis or the Biomarine/Carlton units is used. I saw the final production version of this mouthpiece as it had just arrived at Steam Machines from Carlton & it looked very good. |
The counterlungs both have drain valves at the bottom allowing water accidentally entering to be expelled either during the dive or afterwards. Also seen on the inhalation counterlung is the automatic diluant addition valve. This looks very much like a conventional regulator 2nd stage, with a purge button for manual diluaent addition. An adjuster knob allows the sensitivity of the automatic diluent addition to be controlled. Backing the sensitivity right off turns the unit into a manual diluent addition unit. Increasing the sensitivity right up causes a continuous flow & the unit becomes a semi-closed rebreather.
In principle it should be possible to run the unit in semi-closed mode (in the case of scrubber breakthrough for example) and yet have the constant oxygen ppO2 maintained as well. In practice I didn't have time to discover how well the oxygen controller could keep up with the constant addition of diluent...but of course you could always help it along a bit by the manual addition of oxygen using the over-ride switch at the back of the unit. This switch could also be used to switch the oxygen off very quickly in the event of a solenoid 'open' failure.
Also on top of the inhalation counterlung is an adjustable exhaust/pressure relief valve.
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With a cylindrical scrubber unit it might appear that the scrubber is an
axial flow scrubber but this is not the case. Gas enters the top of the cylinders and
passes unrestricted down the centre of the scrubber basket. It then flows radially through
the scrubber material to the outside of the cylinder. Gas then passes up the cylinder over
the ppO2 sensors and out of the top of the canister. The ppO2 sensors are located at 120 degrees to each other and are situated under an inset ledge (also seen in this photo) to minimise the possibility of condensation or water collection problems on the surface of the ppO2 sensors. Sensor calibration at the working ppO2 of 1.3 can be carried out in place by filling the scrubber with oxygen and pressuring it to 0.3 bar. |
An automatic depth sensor prevents the unit from trying to achieve 1.3 at depths less than 3 metres. Thus only a single setpoint is used throughout any dive. This setpoint can be adjusted before (but not during) a dive to any chosen value.
The scrubber material itself is packed into a rigid nylon mesh basket that looked like it would be easy to pack
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Control of the ppO2 works using the same philosophy as the Biomarine/Carlton units and both of the displays are very similar to Mk.15 and 1.5 units. A manual control of the ppO2 is carried out using an analogue millivoltmeter connected directly to the ppO2 sensors that needs no additional batteries. The one shown in Photo #4 is taken from a Biomarine/Carlton unit & will be replaced by a much prettier rectangular unit that essentialy does the same thing. A manual switch allows each ppO2 sensor to be individually read. The same meter can also be used to carry out a manual battery check. |
The microproccesor voting logic control and display are similar to those used on the later Biomarine/Carlton unit, with some additional features. A green led shows that all is well. A yellow led on either side of the green led position shows that one of the sensors has deviated from the setpoint, but by no more than 10%. Low is to the left and high is to the right. A red led on the outer side of the yellow leds shows that something has gone seriously wrong & needs immediate attention. There is also a blue led showing that the oxygen solenoid is being activated. In addition the led display shows when CO2 breakthrough is occurring and when the battery is getting low.
Although a CO2 sensor wasn't fitted to the pre-production units, I saw how it will work. It is described as 'bio-optical', is very sensitive and will use a very low current to function. I am not allowed to give away all Peter's ideas, but suffice to say that you will have an electrical indication of CO2 breakthrough and will be able to confirm this by a visual inspection of the sensor unit. The sensor element is expected to last about 3 months and will be cheap to replace.
So....finally I got to dive the unit in a deepish swimming pool. Putting the unit on, it felt very light and comfortable on my back...but of course horribly unfamiliar !! In the water it felt great. Then disaster struck as I realised I had major sinus problems & couldn't get down more that 1/2 metre below the surface without inflicting a totally an unacceptable degree of pain !! After *inching* down the side of the pool I managed to reach the bottom some 10 minutes or so later with sinuses intact. I wasn't going to let a problem like this deter me having come all the way from England to see & actually try the Prism out ..!!
Once I became comfortable on the bottom I gave the unit my full attention. Breathing resistance was minimal in all orientations, as you would expect from the split over-the-shoulder counterlung configuration. In fact I could detect no difference in the breathing resistance to that of my own unit. I swam hard around the tiny pool, bouncing off the corners (!) for several minutes until I was puffing like a train but was still getting all the gas I wanted.
I then vented all the air through my nose until the counterlungs were empty and tried to breath in further. The automatic diluent addition kicked in and gave me some air as it should. The resistance was a bit 'tight' at first...but after adjusting the sensitivity knob this action became much easier.
The led display was very simple to read and easy to understand. I found the analogue meter a little small to read & Peter said that the simple addition of a lens cover plate would fix this. It wasn't that I couldn't see where the neelde was, but to read an actual ppO2 number from the display was tricky for me. There again...all you Biomarine owners who already know what I'm talking about I will probably say I'm as 'blind as a bat' :-)
I played around adding diluent & O2 manually to see how easy it was and what the effect on the displays was & everything worked as I expected. The yellow leds came on as they should and the analogue meter went up and down as it should.
A fun unit to try out and a serious contender to the rebreather market...once it reaches the market. Peter says they will be in production by Jan'99. I'd like to think so, but as they say 'the proof of the pudding is in the eating'. A price of under $7000 is being talked about..and it *has* to be kept down to compete with the Inspiration, which in my view has really set the standard for a low cost rebreather (but then I am biased as I already own one).
Taking my Inspiration to the USA generated a lot of recent interest and questions and the inevitable comparison with the Prism. Which I would buy if I were to start again is a difficult question to answer. They are different units, function quite differently, both have different advantages & drawbacks. One is available *now*...the other should be available *soon*. The prices are different, but I honestly feel that you get what you pay for. The cost, availability and location of training and the availability of support however must be a serious factor & one is made in the UK & the other in the USA. I expect that the markets will always reflect this geographical difference.
Anyway..thanks for letting me see & try it Peter. All opinions are mine alone.
Regards, Steve M.
************************************************************************** * Dr. S. G. Millard, * E-Mail : ec96@liv.ac.uk * * Senior Lecturer, * * * Dept of Civil Engineering * Tel : 0151 794 5224 (UK) * * University of Liverpool, * 44 151 794 5224 * * PO Box 147, * (International) * * Liverpool L69 3BX, * * * UK. * Fax : 0151 794 5218 (UK) * * * 44 151 794 5218 * * * (International) * **************************************************************************
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