Diving a Rebreather with a 3 Mix Nitrox Computer.

Dave1w

While commonly done, I don't think I've seen it explained, so I thought I would take a few minutes and describe how I use a Suunto VyTec with my Sport Kiss to get pretty decent dive times with minimum deco obligations.

The Vytec is a 3 mix nitrox computer, designed for Advanced Nitrox divers that allows the user to switch mixes underwater.

On our courses, we were all ( I guess) told that we can simply work out the mix we will be diving on the bottom on our chosen set point, and set a simple nitrox computer to that mix. All we are doing here, is taking this a little further. We should be able to maintain our set point (right?), so all we need to do is remember to switch mixes at the right time.

If we look at a chart of setpoint (PPO2) Against Depth, we can choose 3 mixes to put into the computer that match various depth ranges.




Taking for example a Setpoint of 1.2, when I am below 27 metres, I will select a 21% mix, between 27 and 12 metres, I will change my computer to 32% nitrox, and above 12 metres, I will set it to 50% nitrox.

If I feel at any point I need to add conservatism, then I can simply change back to the weaker mix.

You can play with the numbers and perhaps use 21%, 36% and 60% with the respective depths, and if I decide that will raise my set point for the shallower stuff, I can do the mathematics in my head and switch deeper (although a little slate table would make sense.)

With a 2 mix computer, I can simply have a bottom setting and a 9 or 6 metres up setting.

( On the chart, XL rounded up to the mix, rather than down, but I'm sure you get the gist)

Is that straight-forward enough?

Dave

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cramerdn

I used to do the same with my Nitek HE. With 7 gases and a little forethought I could have my HE reading within .1 PO2 of my set point through 90% of my dive.

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Odin

I use a Vytec in computer mode as a back-up to my plumbed in VR£.
If using AIR dil:
I set the first gas to match the bottom PPO2/Eanx
I set the second gas to match the PPO2/Eanx at a 6m stop
I set the third gas to match my bail out gas.

Doesn't match the deco of the VR£, but not too far off, and if I have an O/C buddy I have a better idea of their deco requirements.
But, have bent it a few times!

If using a Mix in as the dil:
I run the Vytec in gauge mode and use the bail out tables.

Paul

Guilhem

This means the third mix you program into the computer has a greater fO2 than the second. I have found out that *some* old versions of the Vytec then wouldn't let you change mix when they're not stored in increasing fO2 order. This is the case of the unit I own, an other people have found the same as well. More recent units don't seem to be affected by such a bug. For the record, my unit is about 4 years old.