Rebreather Evaluation – Part 3: The PRISM Topaz

I had seen some photos of the PRISM coming back from DMEA, and it wasn’t too long before I started to hear some very positive things about it. In fact, one of the divers I admire most decided that the PRISM was the rebreather for him. Ergo, I decided that my rebreather evaluation would be incomplete without a good long look at the PRISM. Here are the PRISM specs, as well as a nice teardown of the unit.

PRISM stands for Peter Ready’s Incredible Steam Machine. Peter is, of course, the father of the PRISM, and head of Steam Machines Incorporated (SMI). Not only is this system incredibly well designed, it has two very nice features not found on either the Inspiration or the Meg. The first, of these, and the one that has everyone buzzing is the analog secondary display.

Sometimes called “the brick”, PRISM secondary gives a readout of battery voltage, as well as a PO2 reading from each sensor. “Big deal” you say. Every CCR on the market can do this, right? Wrong! The thing about the PRISM’s analog secondary that makes it so special is that it draws its power directly from the O2 sensors, and can continue to function even in the event of a total electronics failure. It is for this reason that the PRISM takes special, high output oxygen cells. Very very cool!

The second unique feature that the PRISM has to its credit is a radial scrubber. Unlike the more typical axial scrubbers in which the breathing gases move from top to bottom (or vise-versa) through the scrubber, the radial design moves the breathing gases from the middle to the outside (or vise-versa) through the absorbent. is quick to tell us that “Only very complex tests show that the quantity of scrubber material, flow, form of the housing and many other factors determine the quality of a scrubber. Axial or Radial is not a general recipe for a good or bad scrubber.” Radial scrubbers do, however, have the added advantage of allowing moisture to condense on the much cooler inside of the scrubber bucket, thus, arguably keeping the O2 cells dry. Or at least that’s how the theory goes… It’s always hard to tell how well these theories translate to real-world experience.

The PRISM has some pretty nice other features as well. It is very light, which is nice if you have to hike to your dive sites. The solenoid exists outside of the breathing loop, meaning that it would not affect the PO2 of the breathing loop should it leak. The heads up display is by far the most well developed of any on the market, and there is an even cooler one as vaporware that will give a digital readout. The PRISM also has an optional shell if that is your kind of thing.

So what are the downsides? Well, in my mind, the fact that the analog secondary display relies on jeweled movement is a pretty serious problem. In the end, this is a millevolt meter that has been waterproofed and calibrated against the high-output O2 sensors to read PO2. As an electronics geek, I can testify that these millevolt meters break pretty easily when subjected to a shock. Even though I love the idea of being able to read my PO2 independently of the electronics, I worry that “the brick” just won’t stand up to the inevitable bumps and thumps on a dive boat. I also don’t like the little wheel it uses to select the individual sensors. I do a lot of diving in cold water, and I worry that this might be difficult to operate with thick gloves on.

The PRISM is made of plastic. Is this a bad thing? Most likely not, but I’m just not much of a plastic sort of guy. There is just something about nicely machined alluminium that makes me happy, and this rebreather doesn’t really have much of that going for it. It’s nicely machined, but it’s nicely machined plastic and to me, that’s a downside. Granted, there are some very good arguments for using plastic as a material. It makes the PRISM the lightest rebreather I evaluated, and plastic is wonderful about not corroding in salt water. Will the fact that the unit is plastic be cause for concern about the durability of the unit? Who knows?

The tanks are attached via velcro straps. This is really not a problem, but it’s certainly not as cool as the Tiger Gear hard mounts found on the Meg. From everything I’ve heard, the unit is very solid when put together, but I don’t think I would be able to strap aluminum 80’s into the thing.

Finally, and most importantly, there is something that bothers me about having only a heads up display and an analog secondary as instrumentation. Sure I can tell what the PO2 is by watching the HUD, and double check it against each individual sensor by toggling “the brick”, but I really want to be able to easily see each sensor’s readout on redundant digital displays. In my opinion, SMI has taken away from the usability of the unit in every day diving by making sure you can stay fully closed circuit even with a total electronics failure. It’s always a balance, and I certainly appreciate the fault tolerance, but I think they’ve planned a little too much for the worst possible case. This is, of course, all just conjecture, as I have never even dived the thing. It’s just how I imagine it would be.

So that’s my feelings on the PRISM Topaz. Remember, I have never even seen the unit, let alone dived it, so please comment if I have made any glaring errors.

Rebreather Evaluation – Part 2: The Inspiration >
Rebreather Evaluation – Part 4: The Megalodon >

7 thoughts on “Rebreather Evaluation – Part 3: The PRISM Topaz

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  2. Hello Cliff, thanks for the informative comparisons. As a point of interest, the lack of digital displays on the Prism does not interfere with my “everyday” diving. The Prism HUD will tell you everything you need to know and is supplimented by the 2dry for certain things, like figuring out which cell has been voted out. Remember that every digi display has batteries, amps and the display itself and is thus more complex and must also employ some sort of averaging to make the digi gages stable enough to be read. None of this is needed to know your PO2 on the Prism 2dry.
    Also, while you may be able to use the Prism radial in the Meg, the gas flow on it is opposite to the Prism-out to in. While still a better design from a duration/WOB stand point than an axial, this is not the optimal way to run a radial. It should flow inside to out to for the best WOB and duration. The Prism scrubber is the most tested scrubber out there and a lot of resources have been put into realizing the optimal proprtions, flow, insulation, etc… -Andy

  3. Thanks for your comments Andy! It’s interesting to hear about your experience using the Prism HUD. I only got my Meg back in March, so I’m still a little green as a CCR diver, but I have to admit that as I get more experience, I rely more and more on the Meg HUD. Granted, I still double check it against the primary handset quite a lot, but it does seem that your are right about the HUD telling you just about eveything you need to know. I had not imagined myself liking the HUD as much as I do.

    Also, tanks for your comments about the scrubber! I know Peter has gone to great leingth to model that gas path through his scrubber, and I have no doubt that the in to out path is optimal not only for WOB, but for “drying” the gas as well. The only argument agains it that I have heard is that when the absorbant is exhausted, it is SERIOUSLY exhausted, and the Carbon Dioxide will hit you harder than it would using an axial scrubbber. Again, I think this kind of argument is a lot of theory and hair splitting. If I could have gotten my Meg with an in to our radial scrubbber I would have.

    Thanks again for your comments! It’s great to hear from those who are diving these rigs!

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  7. Prism is all I’ve used. Works OK however backup service is non existant. I have 1500 hours on unit and consider 3 to 9 months delay for parts unacceptable.

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