Keep It Simple....Stupid !

The KISS Rebreather, from Vancouver

A review by Steve Millard

The end of the last decade and the beginning of this one saw an increasing variety of rebreathers becoming available to the sport/technical diving world. Drager led the way by introducing the Atlantis semi-closed rebreather. This was quickly followed by a launch of the Inspiration, the Cis-Lunar Mk.5p and more recently the Prism Topaz closed circuit rebreathers. However, one rebreather that has escaped the attention of most divers is the KISS closed-circuit rebreather. This has been quietly developed by Vancouver diver Gordon Smith and has a small following of devotees, who like to dive the cold waters of the Pacific Northwest. On a recent diving expedition to explore dive sites north of Vancouver Island, I met up with Gordon and had the opportunity to see his KISS rebreather and to test dive it.

The philosophy behind the design of the KISS is, as its name suggests, to keep everything as simple as possible and to leave the operation and maintenance of the rebreather firmly in the hands of the owner-diver. In the picture below Gordon is seen with his rebreather which can be seen to superficially resemble other units, with two small inverted cylinders on each side of a central scrubber. However an obvious difference is the location of the inhalation & exhalation counterlungs. Instead of wrapping these over the shoulder, they are located within hard protective cases right behind the shoulder blades of the diver. This gives the diver a clean and uncluttered chest area. From a top view of the KISS, the blue counterlung cases are clearly seen, as is the location of the three oxygen cells in the top of the scrubber. These cells are used to monitor the partial pressure of the oxygen in the breathing mixture. The counterlung volume is selected to be only marginally larger than the diver's lung capacity. The means that buoyancy changes with depth are very small indeed. An automatic diluent addition valve and a counterlung dump valve ensures that the diver always has an adequate volume of gas to breathe regardless of changes in depth. The scrubber is linear and gives between 4 to 5 hours duration in cold waters, using coarse grade Sofnolime. Using the finer grade that is more popular in the UK would increase the scrubber duration by 50%.

{short description of image}

Gordon Smith with his KISS rebreather

{short description of image}

Top view of KISS rebreather

The KISS rebreather is a fully closed circuit unit which dispenses with all the fuss of solenoids, injector valves, computer voting logic and other complexities seen on other closed-circuit models. Instead the oxygen is continuously fed into the breathing loop at a measured rate that is roughly expected to match the metabolic rate of the diver during normal diving activity. This rate can be preset during preparation of the rebreather but is not altered at all during the dive itself. It is the responsibility of the diver to monitor the actual oxygen partial pressure at regular intervals during the dive and to make any adjustments needed manually. The partial pressure is displayed on three independent LCD units mounted together and worn on the left forearm. Each LCD unit can be manually calibrated by flushing the breathing loop with oxygen and making trimming adjustments using a potentiometer screw sealed in the back of the LCD display. Each LCD display unit is sealed completely separately from the other two units. If one were to fail or flood the others would still function independently & enable a safe termination of the dive. During the dive, oxygen can be added simply by pressing the injector button for a short while if the partial pressure displayed drops below the desired setpoint. If for any reason the oxygen partial pressure rises above the desired value then diluent can be added by a short exhalation through the nose to vent gas from the breathing loop. The subsequent inhalation causes diluent to be added automatically when the counterlungs bottom out.

{short description of image}

Oxygen partial pressure display

{short description of image}

Oxygen injection control

From the front the KISS looks very uncluttered, with just the breathing hoses visible in front of the standard back-plate, harness and buoyancy wings. A picture below of Vancouver diver Dave Dillabough diving the KISS, shows clearly just how unencumbered the diver's chest area really is. The oxygen feed control is visible on the diver's right. Finally, another valuable feature that comes as standard with the KISS rebreather is an automatic bailout mouthpiece that enables an immediate switch to open-circuit access of the diluent gas simply by twisting a large handle on the front of the mouthpiece. Like all other parts of the KISS design, this is designed for cold Canadian waters and can be easily operated whilst wearing thick neoprene mittens.

{short description of image}

Front view of KISS rebreather

{short description of image}

The KISS in action

{short description of image}

Automatic bailout mouthpiece

When I was given the opportunity to dive with the KISS I found out just how simple it all really was. At the start of the dive the oxygen is turned on and can be barely heard with a gentle continuous hiss from the top of the scrubber. A few judicious presses of the oxygen control valve quickly brought the setpoint up to my chosen value of 0.7. Once a breathing rhythm was established the setpoint barely seemed to change as the continuous oxygen feed replaced the oxygen metabolized by the body.

During the descent at the start of the dive I expected to have to make large inputs to prevent the oxygen partial pressure from rising excessively as I descended. However in practice as the water pressure increased this caused a collapse of the counterlungs and diluent was automatically added as I inhaled. With a small adjustment at the bottom (20m) the oxygen partial pressure was easily raised to the chosen setpoint of 1.2. For the following one hour of diving I expected to have to make changes continually to keep the setpoint at 1.2. In reality this really wasn't necessary. About every 10 to 15 minutes a small oxygen injection was all that was needed. However I was aware that if I suddenly started working hard & metabolized more oxygen then a more frequent adjustment would be needed.

A good indication as to when a serious check of the oxygen partial pressure display is required is either whenever diluent is felt being added to the breathing loop or whenever gas is heard to be venting. For example, if you are working hard and using up more oxygen than is being continuously added then this will cause a reduction in the volume of gas in the counterlungs. This in turn will cause an automatic addition of diluent to make up the lost volume and the diver should ask himself why diluent is being added and should check the LCD displays to find out what is happening. A sobering thought was that there are no alarm buzzers or flashing lights to warn the diver that the oxygen partial pressure is not within safe limits. It is the responsibility of the diver to know his oxygen partial pressure at all times and not to trust the unit to keep him alive automatically!

During the ascent part of the dive I was aware that expansion of the breathing gas was causing a venting of the counterlungs and also that the oxygen partial pressure was falling. It was necessary to inject additional oxygen manually to maintain the set-point and for my first KISS dive I found the ascent very busy as I was injecting oxygen into an expanding system where gas was being vented off. In practice the setpoint can be more easily maintained by exhaling gas from the breathing loop through the nose and replacing the lost gas with an oxygen injection during the inhalation.

At the end of the dive I asked myself what did I like & what didn't I like about the KISS rebreather. On the negative side I found the inhalation resistance needed to force fresh diluent into the breathing loop uncomfortably high. However this can be adjusted to be much easier and on the unit I was diving it was set up just the way Gordon Smith likes it. I also found that the LCD displays, which gave the oxygen partial pressure to 3 decimal places rather distracting, as the last 2 digits danced around continuously. It was pointed out that this was a useful indication of the responsiveness of the oxygen cells and anyway it could be easily fixed by the judicious application of black insulating tape to block out the last two figures (!) If I owned a KISS I would probably learn to live with this feature.

On the positive side, I found the KISS suprisingly simple to dive with and it needed very little proactive control to keep the setpoint just where I wanted it throughout the dive. Its simplicity will make it very easy for any diver to maintain. All parts of the design have had diver-friendliness in mind. The construction of the KISS is rugged to the extent that all seals are done using double o-ring seals rather than one. However the pleasantest surprise about the KISS is its selling price. At around 5000 Canadian dollars (approx. £2000) it must be one of the rebreather bargains of the century and is almost half the price of my Inspiration. A word of caution though...KISS Engineering is not a mass production facility. Units are made in a small volume throughput, typically 5 units at a time. You may need to place your order early to avoid disappointment. Standard diving components such as the backplate and harness and the cylinders are not provided with the unit. You will need to provide these yourself, which is not really such a bad thing. Most technical divers end up customizing the standard equipment to suit their requirements and so you may as well provide what you really want rather than what the manufacture thinks you might need.

(Gordon Smith can be contacted directly by mail at Jetsam Technologies Ltd., 2817 Murray St., Port Moody, BC, V3H-1X3, Canada or by email at


Where to buy Sofnolime...

Copyright Northwood Designs, Inc. All rights reserved.
Revised: January 09, 2003.
All trademarks or product names mentioned herein are the property of their respective owners.