Introduction

We have been testing a Lifeguard PO2 computer kindly supplied by Brian Bickell of UWI Circle Ltd to test its compatibility for us with the AP Valves Buddy Inspiration constant PPO2 Closed Circuit Re-breather. UWI are the sole distributors and service agent in the UK of Cochran products.

The Cochran Lifeguard is 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 test’s 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 on this unit, 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 3 clips in are 3 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 a 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. 4 "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! The battery of choice is the Eveready Energiser series of batteries. 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! The unit using the data received from the O2 sensor on a second by second basis makes the relevant decompression calculations. This provides the diver with a very accurate decompression schedule.

Fitting the Lifeguard to work with the Inspiration

We were supplied with an O2 cell which was mounted in side a white PVC T-Piece. The cell had been mounted tightly into one of the sides of the T-piece and encapsulated with a gel layer. The top of the cell had also been roughly cut away to expose the face of the cell flush with the inner bore of the tube. The outside of the moulding had then been sealed with cellulose putty type paint. The outside bore of the T-piece is approx. 38mm whilst the inner bore of the hoses supplied with the Inspiration are 35mm. This is where the problems for me started! Getting the hoses to fit securely over the outside of the adapter bore was extremely difficult. The original new hose we had obtained from AP Valves proved impossible to stretch in a secure manner and all of the stretching and pulling succeeded in splitting the hose along one side and losing the airtight seal. Even very hot water and several different lubricants (including the old fallback fairy liquid) failed to get the hose over the adapter. After much searching around for alternative ideas I spoke to Terry Fisher at AP Valves who informed me they had managed to get hoses to fit around a similar adapter some time ago and they still had them sat around in Martin Parker’s office. We were saved! They kindly agreed to let me have these and they were dispatched overnight. These older hoses were stretchier than the new ones and had given a little during the time they were attached to the other T-piece and so fitted a lot easier. I know what the garage inventors feel like now! Also supplied with the computer was an additional O2 sensor with a connecting cable and bulkhead connector. This connector is designed for use with rebreathers that have a hard counterlung. One option we are considering is mounting the spare sensor inside the canister lid and drilling a hole in the top of the lid to locate the bulkhead connector. This would then allow the computer to be attached straight to the canister and located inside the case for protection during the dive. A further option would be to fit a junction box to the Inspirations onboard sensors and take a feed from there into the Lifeguard. The possibility of doing this is still under investigation and if successful will be the subject of a further report.

We located the adapter in the hose on the inhalation side of the loop coming from the top of the canister, this would allow us to calibrate the lifeguard at the same time as we calibrated the Inspirations onboard O2 sensors. Positioning of the adapter in the hose is crucial, fitting it to far away from the stack could result in incorrect calibration and fitting at the wrong angle would allow moisture in the loop to run onto the sensor face affecting the accuracy of the PPO2 reading. We finally settled on locating the adapter about 8" up the hose from the stack on the outside of the case. The original idea of fitting the adapter inside the case caused the hose to kink badly as we fed it out of the case. Using 2 jubilee clips on each end of the T-piece we made sure the hose was totally airtight and in no danger of coming loose during the test dives!

When attaching the hose to the unit we wanted to make sure the sensor would be pointing down at the divers back when in a normal swimming position. This proved more difficult than predicted, as with the sensor in the best position the cable connector was pointing at an angle that could easily be caught if we were foraging around in a wreck or cave. So we settled for a more off set angle but still in a position to prevent moisture hitting the cell face as much as possible. My major concern at this time was with accuracy of the PPO2 reading due to moisture. The inspiration has additional water traps installed in the end of each sensor to ensure that as little moisture as possible actually reaches the cell face. Cochran however assured me that the modifications to the sensor would prevent any problems. Attaching the computer sender unit, which does all of the calculations, was a simple matter of connecting the cable from the sender to the back of the t-piece. The sender unit is then easily clipped to an LP hose. As you can see from the photograph we managed to achieve a streamlined fit. One problem we did encounter with attaching the sender to the hose was the fragility of the small plastic clips, the centre clip snapped off when we attached it to the hose. This has unfortunately been a common experience I have had with the Nemesis series of computers. UWI carry the little plastic clips as stock items so replacements are not a problem! Another minor addition we have made is having a custom cover made for the hose, which encloses the t-piece inside the normal heavy-duty nylon fabric, which covers the standard hose. This helps protect the t-piece from accidental knocks when in overhead environments and improves the cosmetic appearance of the unit.

Calibrating the Lifeguard

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 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 Inspirations 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.

Lifeguard Display & Analyst ™ Software

Using the Lifeguard version of the Analyst ™ 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 as 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 make 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.

The Analyst ™ software available for the unit allows downloading of dive date and uploading of new parameters to the unit. One of the first things we did using the software was convert the unit from imperial to metric measurement. Other relevant settings include a maximum (1.6bar) and minimum (0.20) PO2 alarm, a maximum depth alarm and automatic activation settings that are all user definable. Another additional setting is the ability to change the conservatism factor of the unit, during the test’s we opted to run the unit with a conservatism factor ranging between 0% and 20% although the maximum is 50%. Connecting the PC to the dive computer requires a serial cable with a data probe similar to the field programming probe and running the Analyst ™ software the probe is available as part of the Analyst ™ software package. Before any changes to the dive computers setting can be made you must first download the computer, after that changes to the profile are a simple matter of choosing the relevant menu option and highlighting the parameter you wish to change. The interface is quite easy to understand but would benefit from the greater clarity of a Windows ™ interface. A Windows ™ version is promised soon.

 

Diving With The Lifeguard PO2

After finally getting the sensor installed into the breathing loop came the time to actually run some test dives. These dives were to be run in both a cold water enthronement 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 dives 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.

If a discrepancy occurred between the Inspiration and the Lifeguard displays that was greater than 0.05 bar then the time and depth along with the values displayed was transferred to the slate. All decompression was carried out using the stops provided by the Cochran Commander computer as we felt this would provide a greater margin of safety and cater for the open circuit support divers watching us for any problems. As can be seen from the graphs the Lifeguard maintained a very accurate level of PPO2 readings. The spikes on the end of the graph are test’s I performed to check the Lifeguards ability to deal with rapid changes in the PO2 level.

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. As can be seen from the graph and as expected from a constant PPO2 re-breather the FO2 percentage matched perfectly the dive profile.

As you can also see from the graph I lowered the setpoint from 1.3 bar to 0.7 bar at 41 minutes and the FO2 dropped off as the O2 in the loop was metabolised. The spikes at the end of the profile at 54 minutes are the result of manual injection of O2 into the loop to test the unit’s response to manual control. The temperature column has been included in the table to indicate the amount of condensation in the loop that would be likely to occur during the dive.

Dive Profile’s

No	Max Depth	Time	PO2	Temp	Deco	Comments
1	24m	61 min	1.3	14C	N	None – Unit performed within parameters
2	37m	72 min	1.3	10C	Y	Deco Error – Unit stuck at 1min 3mtrs unable to clear deco so ascended and computer recorded violated stop & entered gauge mode for 24hrs
3	37m	71min	1.3	10C	Y	Deco Error – Unit clocked deco up correctly and then failed to clear the 1min 3mtr stop as previous dive. Possible hardware fault!
4	37m	91min	1.3	10C	Y	Deco Error – Unit clocked deco up correctly and then failed to clear a 2min 3mtr stop even after 23Min 100% O2 decompression. Unit then defaulted to gauge mode on exit from water.
5	36m	81 min	1.3	10C	N	None – Unit performed within defined parameters. This was a no decompression dive.
6	25m	30 min	1.3	12C	N	None – Unit performed within parameters. Conservatism mode set to zero. No decompression dive.
7	25m	32 min	1.3	12C	N	None – Unit performed within parameters. Conservatism set to zero. No decompression dive.
8	36m	40 min	1.3	11C	Y	None – Unit performed well with operational parameters. The new decompression algorithm appears to work correctly.
9	36m	42 min	1.3	11C	Y	None – Unit performed exactly as required. PPO2 tracking by sensor matched the Inspirations own perfectly.
10	36m	71 min	1.3	11C	Y	None – Unit performed exactly as required.
11	25m	62 min	1.3	16C	N	None – Unit performed exactly as required.
12	25m	50 min	1.3	16C	N	None – Unit performed exactly as required.
13	20m	30 min	1.3	16C	N	None - Unit performed exactly as required.
14	32m	66 min	1.3	16C	N	None - Unit performed exactly as required with full deco information.

The problems experienced during dives number 2, 3 and 4 were tracked down to what we believed at the time to be a faulty CTU, the down-load files were duly forwarded to Cochran and a new unit was despatched to try the profiles again. The second unit displayed exactly the same problems as the first and this was also duly reported to Cochran. After further on investigation Cochran identified that the algorithm being used in the unit was far more conservative than any other unit in the range and as such did not deal well with the deep repetitive dives I was doing. The 20% conservatism factor I introduced to the program on top of the algorithm’s own built in conservatism helped push the unit over the edge! A new unit with amended algorithm more suitable for the more "aggressive" diving being performed by rebreather divers was made available to me to enable continue the testing. This unit was used from dive 8 onwards and performed without problems. As a result of the problems we experienced with the algorithm Cochran have released a general upgrade free of charge to existing lifeguard users. A statement to this effect is available from Cochran and UWI circle.

Conclusion

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 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.

From a warranty and support perspective UWI circle responded very quickly to the problems we had with the early units and provided us with constant support and the replacement units as they arrived from Cochran in Texas.

 

Acknowledgements:

I would like to thank the following people without whose help my research would not have been possible.

Brian Bickell – UWI Circle
Mike Cochran – Cochran Undersea Technology
AP Valves – For endless advice & Spare parts
And of course all of the other poor mugs I roped into spending
long hours in freezing conditions testing decompression profiles!
All trademarks are acknowledged.