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Test Dive 60m Trimix - Poseidon Discovery MK VI

In April 2013, the College and Center for Rebreather carried out Tec Fundamental Instructor and Tec Advanced User and Instructor courses on the Italian island of Elba. Markus Schempp & his team from the iac dive Omnisub welcomed the members of the College and the Center for Rebreather.

Due to the good conditions in the Mediterranean Sea the College and Center for Rebreather took this opportunity for a dive to 60 meters with the Poseidon Discovery MK VI. Two units were equipped with the 48m Trimix battery and one unit with the new 60m Trimix battery (trial version). One of the divers using the 48m Trimix had a simple bottom timer for backup; the other two divers used a HeinrichsWeikamp OSTC 2

The dive was planned and carried out under the guidance of George Mueck and Sven Böckelmann.
Basic information:
60m max. depth, bottom time 5 min, gases 100% O², 18/45 Trimix as diluent, bailout gas 80cf TX 18/45 and EAN 50 80cf

The divers using the Poseidon Discovery MK VI Rebreather with 48m Trimix battery switched to open-circuit at 45 meters and started breathing from the Trimix Stage (TX 18/45). On the way back to the surface to 48 meters depth, the diver switched back to the rebreather into closed-circuit. One of the two divers with 48m Trimix version switched back into open-circuit using the EAN 50 stage after setting the surface marker buoy. Each gas switch has been entered into the OSTC 2. The diver using 60m Trimix version conducted the entire dive in closed-ciruit, he did not switch to bailout or stage at any time.

The rebreather equipped with the 60m Trimix version displayed the shortest total time to surface when arriving at 60m. The built-in Poseidon Discovery MK VI computer showed a total time to surface of 21 minutes. The HeinrichsWeikamp OSTC running closed-circuit mode suggested a total time to surface of 24 minutes.

At 60m the integrated computer of the units in open-ciruit equipped with the 48m Trimix batteries showed a total time to surface of 32 minutes and 28 minutes. (We will take closer look into that later on.) While the HeinrichsWeikamp OSTC backup computer displayed a total time to surface of 26 minutes. After switching to EAN 50 at 15m, the total time to surface on this computer barely changed. However, the integrated computer of the Discovery MK VI was assuming Trimix 18/45 as breathing gas in open-circuit, resulting in an increase of 20 minutes. It was showing 15 minutes before switching the BOV to bailout, now it showed 35 minutes. The other Poseidon Discovery MK VI rebreather completed the dive in closed-cicruit after switching back at 48 meters depth. There is no HeinrichsWeikamp OSTC data available for comparison because this diver only had a bottom timer.

After switching to open-circuit the integrated computer of one of the Poseidon Discovery MK VI rebreathers with 48m Trimix battery kept displaying a depth 53 meters even though the diver kept descending further down to 60m. A reason for this might be technical design. The Poseidon Discovery MK VI determines the depth by use of an air mass meter instead of a simple pressure sensor, which is commonly used by other dive computers. It can be assumed that the diver had the counter lungs filled up with less gas when he switched to open-circuit than the other divers whose computer displayed the depth properly. This phenomenon is most probably the cause the differing deco information mentioned earlier. When remaining in closed-circuit, this phenomenon will not occur because the rebreather can then continue to work with an appropriate amount of gas and the air mass sensor will deliver meaningful measurements. One lesson we can learn from this phenomenon is that it should be mandatory to carry a backup computer when performing deep and demanding dives. Anything else is would be gross negligence.


We want to point out that highly experienced technical divers carried out this test dive. We have to advise you not to perform any deeper dives than you and your unit is qualified for! Especially in stressful situations, a distinction between depth alarm and sensor/software failure is almost impossible.