The Cochran Lifeguard is a constant PPO2 computer using a Haldanean 12-tissue group model. The design of the unit is based on the popular Nemesis IIa Nitrox computer utilising a wrist unit and tank sender.

The wrist unit is for display of dive data and can be substituted for another wrist unit or even the new Cochran head up display unit should the desire arise! During my tests I managed to match the wrist unit from my Nemesis IIa computer to the tank sender.

The tank unit is where all the work is done. It is a small box about 6"x2"x2" with a short yellow cable coming from one end and a row of white clips for attaching to an LP hose. Holding these three clips in are three metal screws which are also used as connectors for the data probe and field programmer. The yellow cable is where the unit differs physically from the Nemesis, instead of a high-pressure sensor the cable attaches to what appears to be a normal Vandengraph (Teledyne) O2 analyser cell but is in fact a Cochran proprietary cell developed specifically for use with rebreather systems. Four AA type batteries power the main unit and careful selection of batteries is crucial. Duracell batteries with the tester strip cause considerable problems. When the airspace in the unit compresses under pressure the test strip is activated causing the battery voltage to drop, which then causes the unit to switch off! A good choice of batteries is the Eveready Energiser series. It is advisable to replace the batteries before each dive trip and carry plenty of spares. Fortunately they are easily replaced in the field using the supplied special 'tool' - an American Quarter! Using the data received from the O2 sensor on a second by second basis the unit makes the relevant decompression calculations. This provides the diver with a very accurate decompression schedule.

Calibrating the Lifeguard is a fairly simple procedure. Using the data probe provided you put the unit into programming mode after first activating the sender unit and the wrist unit. You then step through the programme settings until you reach the calibration section. At this point you put the Inspiration into calibration mode and allow the O2 to flow into the loop. As the Lifeguard sensor is so close the Inspiration's own sensor and the O2 injection valve pure O2 flows up the hose and past the Lifeguards sensor to give a good calibration. Once a steady reading has been achieved you store the calibration and exit the programming mode. It is only necessary to calibrate in a single O2 source and not dual calibrate in air.

Comparisons of the cells with the Inspiration's own show a difference of less the 0.03 bar. This is above the calibrated Inspiration settings and the Lifeguard at the surface but as depth increases the difference drops to around 0.02 bar.

Using the Lifeguard version of the Analyst (tm) software you are able to fine-tune the unit to your own requirements, including the most important setting which is converting the unit to metric. This conversion also produces an interesting quirk on the wrist unit display by moving the decimal point in the PPO2 reading so that a reading of 1.30 bar would appear as 13.0 bar. Not a major problem once you learn to interpret the display.

The display in the wrist unit displays plenty of relevant information including depth, time, NDC or Deco time, water temperature and PPO2 reading. On the alternate display, which is accessed by tapping the unit or a quick flick of the wrist the current FO2 is also displayed along with dive time. Having used Cochran products extensively the display was familiar and easy to understand, however new users to the unit may find the display a little busy at first but should adapt quickly. The unit also incorporates Cochran's superb taclite technology, which makes the display on the unit easily readable in any lighting conditions. Tapping the display of the unit activates the taclite for about 10 seconds. This is in contrast to the Commander unit that is switched on in programming mode before the dive and stays on for the duration.

After finally getting the sensor installed into the breathing loop the time came to actually run some test dives. These dives were to be run in both a cold water environment to validate the cold water conservatism algorithm and test the reliability of the sensor in cold conditions and then a second series of dives were planned for a warm water environment. The test dives were planned using a Cochran Commander computer as a back up to the Lifeguard unit. We ran each dive profile through both Proplanner and ZPLAN, the two most commonly used Closed Circuit Rebreather planning applications. The depth changes and durations were then transferred onto a slate and used for in water reference. A constant check was made on the PPO2 readings referencing against the inbuilt sensors of the Inspiration.

The unit dealt well with the changes in PO2 and rapidly adapted the remaining no stop time based on this new information without being overly reactive toward minor changes. Installing the sensor further away from the main canister seems to have helped ensure that the gas was mixed properly before hitting the sensor and therefore giving a steady reading. Being able to view the FO2 on the alternate display was also very helpful.

In my view the Lifeguard is a well thought out robust piece of diving equipment capable of providing accurate information in a comprehensive manner to the diver. Despite the earlier problems we had with the algorithm's conservatism the unit performed well in our tests. As an existing Cochran products user I found the display easy to understand with plenty of useful information. The unit gave very accurate PPO2 readings that were generally within .03 bar of the Inspirations own on board sensors and I found it very useful to have a completely redundant source of PPO2 data at hand. It was also very comforting to finally have an accurate source of decompression data that reflects the real benefits of using a rebreather. During all of the trials I had normal levels of condensation in the loop and passing the cell face yet the unit still continued to provide accurate PO2 data. To verify the accuracy of the data during use I regularly flushed the loop to check that the PO2 reading dropped and rose again as the Inspiration corrected the drop in PO2.

My main reservations were with the fragility of the plastic clips that hold the CTU onto an LP hose and the frightening rate at which the unit consumes batteries. I also still find Cochran's desire to make the water temperature the most prominent item of data on the display very bizarre. The only other problem we are likely to encounter in the future is Cochran being unwilling to provide a t-piece with a smaller bore as the new AP hoses will not stretch to fit.

Review by Steven Copeland