mixing technologies technical dives

Mixing Technologies on Technical Dives

In the past few years I have been exploring the coast off Durban, South Africa for some of the many wrecks that have sunk off our coastline. Many of these wrecks are to be found between 50 meters and 135 meters below the surface. These dives are done in conditions that range from truly amazing conditions with over 100 meters of visibility to conditions with screaming currents and absolutely zero visibility. I am often asked why I sometimes do these dives on Open Circuit and sometimes on a Rebreather.

For me my decision on which technology to use is based on the conditions and the type of dive that I am expecting to do. I always use Open Circuit on the first exploration dives to an unknown wreck. For me I know that the Open Circuit is going to work. When I arrive at the dive centre in the morning before the exploration dive I am not going to find myself with dead cells, faulty batteries or machine errors. I put the Open Circuit regulator in my mouth, It gives me gas and as long as there is sufficient supply in the tanks, I am going to be breathing what I want to be breathing and don’t have to give it a second thought. In strong currents I can pull myself down the anchor line to the bottom and not have to still worry about over breathing the rebreather, what I am breathing and monitoring a machine. I am not having to think or worry about what the rebreather is doing and can focus on the mission at hand.

Once I have done the initial exploration dives and have a good idea what the wreck is about, what the conditions are expected to be and what the best times of the year are to dive the newly found site, then I am happy to switch over to my rebreather. I now can focus on the rebreather and knowing what to expect from the dive, I can plan and execute the dive with knowledge I have gained from the initial dives.

Yes, I know that you are thinking about the gas bills on Open Circuit versus Closed Circuit and yes, we could go on forever on the advantages of the rebreather, but this is what works for me based on the conditions off Durban and the types of dives we are doing. The ocean is not a static environment and for me, removing the machine from the equation on the initial dives ensures that I will get down and be able to explore what I had intended to achieve

Then comes the next argument. Do we mix Rebreathers and Open Circuit on the same Technical dive? My answer is Yes. Why not?

A GROUP OF OPEN CIRCUIT AND REBREATHER DIVERS DISCOVERING A WRECK OFF DURBAN

Lets look at a typical 60 meter dive off Durban. Here is the Open Circuit plan.

The best gas for the bottom with a 35m Equivalent Narcotic Depth and 1.4 PPO2 is a 20/35. Deco gasses are a 50/10 and a 100%, (Although I do like a 72% I must be honest). With a deep stop this will give me the following plan with a total runtime of 88 minutes:

V-Planner 3,91 by Ross Hemingway,
VPM code by Erik C. Baker.

Decompression model: VPM – B

DIVE PLAN
Surface interval = 1 day 0 hr 0 min.
Elevation = 0m
Conservatism = + 3

Dec to 60m (2) Trimix 20/35 22m/min descent.
Level 60m 27:16 (30) Trimix 20/35 1,39 ppO2, 30m ead, 35m end
Asc to 41m (32) Trimix 20/35 -9m/min ascent.
Level 41m 2:00 (34) Trimix 20/35 1,01 ppO2, 19m ead, 23m end
Asc to 36m (34) Trimix 20/35 -9m/min ascent.
Stop at 36m 0:20 (35) Trimix 20/35 0,91 ppO2, 16m ead, 20m end
Stop at 33m 1:00 (36) Trimix 20/35 0,85 ppO2, 14m ead, 18m end
Stop at 30m 1:00 (37) Trimix 20/35 0,80 ppO2, 13m ead, 16m end
Stop at 27m 2:00 (39) Trimix 20/35 0,74 ppO2, 11m ead, 14m end
Stop at 24m 3:00 (42) Trimix 20/35 0,68 ppO2, 9m ead, 12m end
Stop at 21m 2:00 (44) Triox 50/10 1,54 ppO2, 6m ead, 18m end
Stop at 18m 3:00 (47) Triox 50/10 1,39 ppO2, 4m ead, 15m end
Stop at 15m 4:00 (51) Triox 50/10 1,24 ppO2, 3m ead, 12m end
Stop at 12m 5:00 (56) Triox 50/10 1,10 ppO2, 1m ead, 10m end
Stop at 9m 8:00 (64) Triox 50/10 0,95 ppO2, 0m ead, 7m end
Stop at 6m 24:00 (88) Oxygen 1,60 ppO2, 0m ead
Surface (88) Oxygen -9m/min ascent.

OTU’s this dive: 130
CNS Total: 75,3%

Lets have a look at the Closed Circuit plan. The best gas for the bottom with a PO2 of 1.3 and the same END would be an 18/35. The bailout gas would be a 20/35 and the bailout deco gasses would be a 50/10 and 100%.If I were doing deeper dives or not sure of the work I was going to do on the dive I would drop the PO2 for the Rebreather and OC to 1.2 or 1.1. But with the same PO2 differences the plans would run the same. Based on this plan knowing the conditions and depth the run times would look like this:

V-Planner 3,91 by Ross Hemingway,
VPM code by Erik C. Baker.

Decompression model: VPM – B

DIVE PLAN
Surface interval = 1 day 0 hr 0 min.
Elevation = 0m
Conservatism = + 3

Dec to 60m (2) Diluent 18/35 0,70 SetPoint, 22m/min descent.
Level 60m 27:16 (30) Diluent 18/35 1,30 SetPoint, 31m ead, 36m end
Asc to 41m (32) Diluent 18/35 1,30 SetPoint, -9m/min ascent.
Level 41m 2:00 (34) Diluent 18/35 1,30 SetPoint, 18m ead, 25m end
Asc to 33m (35) Diluent 18/35 1,30 SetPoint, -9m/min ascent.
Stop at 33m 1:00 (36) Diluent 18/35 1,30 SetPoint, 12m ead, 20m end
Stop at 30m 1:00 (37) Diluent 18/35 1,30 SetPoint, 10m ead, 18m end
Stop at 27m 1:00 (38) Diluent 18/35 1,30 SetPoint, 7m ead, 17m end
Stop at 24m 2:00 (40) Diluent 18/35 1,30 SetPoint, 5m ead, 15m end
Stop at 21m 3:00 (43) Diluent 18/35 1,30 SetPoint, 3m ead, 13m end
Stop at 18m 3:00 (46) Diluent 18/35 1,30 SetPoint, 1m ead, 12m end
Stop at 15m 4:00 (50) Diluent 18/35 1,30 SetPoint, 0m ead, 10m end
Stop at 12m 5:00 (55) Diluent 18/35 1,30 SetPoint, 0m ead, 8m end
Stop at 9m 7:00 (62) Diluent 18/35 1,30 SetPoint, 0m ead, 6m end
Stop at 6m 27:00 (89) Diluent 18/35 1,30 SetPoint, 0m ead, 5m end
Surface (89) Diluent 18/35 -9m/min ascent.

OTU’s this dive: 130
CNS Total: 49,1%

On this plan the runtime would be 89 minutes.

So what have we got here now? In a mixed group of rebreather and Open Circuit divers on a 60m dive we have run times that match to within the minute. We have the same bottom gas if the rebreather has a problem and the diver has to bail out. If you have planned the reserves right there will be plenty of gas for the divers at the bottom should there be an emergency. The deco gasses are the same. So should either the Rebreather or Open Circuit diver have a problem then gasses are compatible with each other and in a team of two to six divers there would be plenty of  gas available should there be a problem on either the machine or Open Circuit during Deco. Even if the rebreather diver has had to bail out the bailout plan then matches the run time of the Open Circuit divers. If the Open Circuit Diver has a lost gas scenario then within the team of rebreather divers there would be a compatible gas to use.

The times at the different stops are within a minute of each other and in practice we will stay at the stop for the longer of the two required times and merge the plans. In my experience when diving on multi gas computers and diving the computer plan with the same settings within the team then these times are actually almost identical.

You can work on these plans for any depth and you will see that for the plans you will end with very similar run times and stops. The team needs to be in agreement on the gasses and plan and fill the gasses accordingly.If the Open Circuit divers have not been exposed to Rebreather divers before then you need to get the Rebreather divers to explain their setup, the procedures and the differences to the Open Circuit guys prior to doing the dives. Its helpful to do some build up dives to get the teams working together and sorting out any procedural differences if need be.

I have had many a rebreather diver turn up at the dive centre for the dive with less than ideal bail out gasses in his cylinders and wanting to use these as he will make them work and doesnt want to spend anything on changing his bailout gasses. I would rather then give him  the correct gas as he is more than likely not going to need it and I will use it in any case on another dive and so it doesn’t cost him any extra.

In the past few years I have been fortunate to have done many dives with a mix of Open Circuit and rebreather divers on the same dive. We have had the pleasure of finding a WW2 submarine at 105m, a WW1 Submarine chaser at 75m, a fishing trawler at 60m, an as yet unidentified wreck at 75m and searching for Coelacanths at 130m to list but a few as well as many hours of exploration dives.

Plan the dives, dive the plans and you will have many years of exploration diving in mixed groups of divers ahead of you.

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