The Prism Topaz

Steam Machines PRISM Topaz
By Stefan Besier


Over the past year I’ve been repeatedly asked why I choose the PRISM, what I like about it, what makes it a good rebreather. So I ended up writing this little article and e-mailing it upon request. Now that Stuart has created a place on the web to make this and other rebreather information publicly available I have found the right place to publish it.
applaud his effort.


Steam Machines PRISM Topaz


The reason I chose the PRISM Topaz over other CCRs available today is that it is a very well engineered rig, which makes it very user friendly to dive and maintain.

While it lacks some of the bells and whistles that came with the MK-5p and are available on some other units, it excels in the most basic engineering parameters. Size and weight in relation to its capabilities. Excellent WOB and hydrostatic properties. Transparency of operation. And safety considerations. Peter Readey’s many years of experience with rebreathers are obvious all over the PRISM.


To explain what I like and prefer about the unit, let me take you “on a trip around the loop” to point out the thoughts that went into its design.


The Dive/Surface Valve has a pair of brass rings that make it neutrally buoyant, so if you let the bite piece go it won’t automatically raise above your head. The weights in this location also make the DSV comfortable during longer dives, and the lack of weights on the hoses means you won’t get hit in the side of the head by them in a strong current.

The breathing hoses (all four of them) have annular corrugations, allowing the diver to cut them to proper length. They connect to the counterlungs with “elbows” rather than T-pieces. So each hose as its own connector, angled 90 degrees away to prevent any water from bypassing the bag and heading straight for the loop. The elbow is lower on the counterlung than a T-piece usually is and pointing slightly outward. This has the added benefit of the front hoses being out of way.


The exhalation bag (diver’s left) has a OPV in the upper half of the front panel, and the manual oxygen addition valve on the lower right side panel. That position allows easy reach with either hand and protects the valve from unwanted activation when squeezing through a restriction.

Like the inhalation counterlung, the one on the exhalation side has a water drain on the bottom. This is a very important safety feature in my opinion as they allow a partially flooded loop to be recovered, and are a must have for overhead environs or deco diving.

Next, the exhaled gas enters the head through the exhaust plenum that routes the gas to the center tube. It is is into the plenum that the solenoid adds the O2. The solenoid itself is located outside the loop for two reasons: First, it’s a battery powered device, electricity and high O2 don’t go well together. In case of solenoid damage no sparks will be possible inside the loop. Second, if the solenoid should develop a leak, O2 gets dumped into the water rather than the loop.


The gas then enters the radial scrubber, from the center tube moving outward. This is the main reason for the PRISM’s high efficiency. The gas enters the scrubber in the center, where temperatures are the highest. It is well heated when it moves across the scrubber bed. The Navy tested the PRISM’s scrubber to 300 minutes (the data on the web site is for the the smaller 5.4lbs. scrubber that was originally used. Current units have a 6lbs. scrubber) at 4.5 degrees C and a WOB of 1.35 ltr/min.
When the gas has passed through the basket it hits the clear scrubber bucket which has a considerably lower temperature. It is here that most of the condensation occurs, the generated moisture is caught by sponge water traps in the bottom. The gas then flows back into the head where the three sensors measure the O2 contend of the loop.


The O2 sensors are the only “electrical devices” inside the loop, Steam Machines uses proprietary sensors (their design, specifically for use in an UBA). The SMS202 are high output sensors generating between 16mV and 22mV during their useful life span (1 year according to manufacturer’s specs). While some people insist that high output sensors are no good, I’ve never seen any data on SMS202 supporting such claims. However Steam Machines has data from the cell testing where they passed with flying colors, even for possible loop contamination (off gassing and materials). The high output is needed to drive the analog secondary, which runs directly off the current generated by the O2 cells.


From here the gas flows into the inhalation bag (diver’s right), again connected to hoses by separate fittings. The head connector for the hose is located below the sensors when in a normal diving position. Small amounts of water that may have found its way into the scrubber basket can be moved into the inhalation bag and drained from the loop with the drain valve. That procedure of recovering the loop is part of advanced PRISM training. The inhalation bag also houses the valve for diluent addition. It’s placed just below and to the left of the elbow, again well protected from unwanted activation yet easily reachable by either hand. The valve operates both as an ADV (so that is a standard feature of the Prism) and for manual addition. It’s a plunger type valve that hits a striker plate sonically welded into the counterlung. It can be rotated so the supply hose can be routed form either the shoulder or armpit (the latter eases switching to off board dil supply).

Fitting the addition valves in different places on their respective counterlungs prevents accidentally activating the wrong one during a stressful emergency situation.

From there it’s back to the DSV.


The main item left are the electronics. The computer controller is potted into the before mentioned compartment on the top of the head. The potting completely seals the electronics from any water, both from the loop underneath and the battery compartment above. There are two separate and sealed trim pots in the battery compartment: One is the calibration pod for the secondary (three small screws that adjust the analog secondary to read 1.0 during calibration), one for the primary. The latter contains a small button for calibration (once the secondary reads 1.0, you “tell” the electronics that the current output of the cells is 1.0 ata pO2 by gently pressing the cal button. That’s it.) as well as another small screw that adjusts the setpoint. 0.7, 1.0, 1.2, 1.3 and 1.4 are available, and are chosen before the dive. As the Prism has a depth sensor, the setpoint switches automatically from a low 0.7 to the chosen setpoint on descent and back on ascent. Forgetting to switch setoints during either is impossible and the danger of incurring the related deco obligation by accident eliminated.

The standard 9V battery is located next to the trim pods. It lasts for about 40 hrs., is inexpensive and easily available at just about any supermarket.

Three cables exit the head towards the diver. On is for the power switch. The switch is conviniently located on the inflator hose. It is a potted magnetic reed switch that is waterproof. It’s a physical switch that disconnect both positive and negative battery leads, thereby completely separating the battery from the loop. This is a very important safety feature frequently underestimated as it prevents stray current from flowing across the sensors in case of a short or like malfunction.


The second cable is for the primary display. The standard display is a three LED heads down display mounted to the DSV. The cable runs out of the way along the inhalation hose, reducing clutter and snag points. (A wrist mounted 5 LED display as seen in the pool pictures is an option, though both display the same info during calibration and diving.)

The left LED is bi-colored and gives sensor warnings as well as low battery warning.

When the battery drops below 7.6 V the solenoid gets voted out, the remaining battery power is used for the HDD display. The diver manually controls the pO2 while retaining both primary and secondary readouts.
The former is remarkable as the PRISM has had cell verification for several years now (even the old SM1600s had this feature). Not only will the diver know when a sensor reading is off and voted out. The electronics record the sensor output and response to O2 addition and compare it both against previous response as well as the other cells. This means when two cells go bad the PRISM “knows” and notifies the diver. In that case the solenoid gets voted out and the diver can fly the PRISM either SC or CC on the remaining sensor and finish the dive on the loop. Rather than voting out the good sensor and injecting O2 as most all current controllers do. Another major safety plus for the PRISM Topaz.

The blue center LED signals low pO2

The right bicolored LED signals that the system is running on setpoint and the computer is working as well as high levels of pO2.
Having the HDD (or any HUD) constantly in your field-of-vision is in and by itself a major safety feature. During training I used both HDD and wrist mounted display, the HDD is much more convenient.

When the electronics are switched on all LED flash twice (and the solenoid fires twice if shallower than 18 ft.) to verify operation.
When calibrating the green setpoint LED flashes twice if calibration is accepted, or all LED flash continuously when calibration is rejected (sensor(s) disconnected or out of range).

The LED displays are completely potted, and like the power switch and secondary use water stop cables that don’t let water reach the contacts on either end in case of damage. There is no lens over the LEDs, it’s clear, optical potting.


The final cable connects the analog secondary to the head.

The secondary is basically a voltmeter with a pO2 scale and is completely independent and separate form the primary display. It runs directly off the sensor’s currents and doesn’t need a battery. The lack of a battery also means that no current can flow back to the sensors and influence the cell output. This complete separation ties in with the before mentioned physical disconnection of the power switch and is the single most important engineering feature in regards to safety.

The secondary is also completely sealed off by potting, with a reed switch in form of a small thumb wheel to choose settings. These are 1-3 for each of the sensors as well as setpoint display and battery volt meter. The display will also alert the diver when there is a short anywhere in the system.

Last but not least, an optional cowling can be fitted over the head/scubber assembly and the standard 3 alu liter tanks to protect them form damage. As the PRISM was designed without it, it is quite sturdy and can be used without the cowling. All of the hoses are routed out of the way. In that case visual checks of the clear scrubber canister are easily performed by a buddy during the dive.

Since both the rebreather and tanks are mounted on a board, the PRISM can use different tank sizes from 2 liters to 7 liters. A convenient ability when using tanks at a destination as traveling with your own gets increasingly harder.


The PRISM is a fairly small and light (47lbs./21kg) package, even more so considering the excellent scrubber duration. It comes complete and, safe for the gas fills and absorbent, ready to dive. The only options are the cowling and the alternate wrist mounted primary. The BCD is comfortable and easy to adjust, don and doff. The clear scrubber and low condensation in the sensor area means that there is no fiddling and disassembling the rig between dives.

Finally, the PRISM is likely the most thoroughly tested rebreather available today as it was tested over several years by the US Navy to their standards. Even the current Navy issue MK16 units are not as thoroughly tested as new test parameters (such as loop gas analysis) was added after their issue.


And unlike any other manufacturer Steam Machines makes the actual testing data available to the public (some on their web site, more in their office, as much as the Navy allows them). Most of the data important to recreational divers is disclosed, such as setpoint control accuracy, scrubber duration and WOB/hydrostatic differences.

All of which adds up to the PRISM Topaz being my choice.


The few things I don’t like ...

The O2 and diluent tanks on the PRISM, compared to other units, are on the "wrong"side. SMI has broken with the old tradition of carrying the rich gas (O2) right and the lean gas (diluent) left. Unfortunate, as this is taught by most agencies and adhered to by most divers, open or closed circuit. The diver must be conciously aware of this unusual gas arrangement, especially when switching between rebreathers.

The secondary shows one sensor readout at a time. It’s easier to compare the cells reaction to O2 addition when all three readings are side by side. You can still see the sensors respond when you switch between settings, just not all three at a time. As the secondary has a jeweled movement, it can also be damaged fairly easily. Attention must be given to the handling of it to avoid damage from impact.

But truly independent and separate sensor readout is more valuable to me. Being able to monitor the sensors without having to rely on any electronics whatsoever outweighs the convenience of a simultaneous cell display or inconvenience of having to pay some attention not to damage it.

There will be a console or DSV mounted deco computer in the near future that has all three cell readouts on screen, so that problem won’t be an issue anymore. The computer will have full RGBM (read real, not folded into a Bühlman or Haldane) and be located in a replacement battery cap (sensor data will be transferred by infrared), again with its own independent and separate battery supply.

I would prefer a buckle type connection between the head and the scrubber, rather than the groove and pin lock currently used. When you drop the bucket at the wrong angle the groove can be damaged. This is, however, usually fixable in the field.

I would like to see an OC-integrated DSV for bailout, rather than an Air II. They are a convenient and safe feature for any rebreather.

That’s it for disadvantages.

Safe diving
Stefan


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Note: The PRISM I photographed inside belongs to Robert Landreth, a former SMI emplyoee who added some custom features to his rig. Aside from being polished he added custom primary and secondary displays that are smaller than standard ones. It's the last one build in California.
Thanks to Robert for giving me access to his rigs.

The pictures by Vicky Newmann on the other hand show me diving one of the early PRISMs during my training. It had originally been submitted to the NAVY for testing. They entered it into the rental fleet where it has remained ever since, still going strong.

Thanks to my classmate for the pictures.