BSAC Nitrox Deco Tables - CNS uptake

[Scubi]

Looking for some clarification please...

When using the Oxygen Toxicity table to calculate CNS%

do I calculate the TOTAL time in the water including stops
or
the total dive time up to the first deco stop

My understanding is that Oxygen uptake is measured, by the minute, at the maximum pO2 the diver experiences, therefore, it would be total time in the water including stop?

Look forward to hearing from you...

[Gareth]

Looking for some clarification please...

When using the Oxygen Toxicity table to calculate CNS%

do I calculate the TOTAL time in the water including stops
or
the total dive time up to the first deco stop

My understanding is that Oxygen uptake is measured, by the minute, at the maximum pO2 the diver experiences, therefore, it would be total time in the water including stop?

Look forward to hearing from you...

Scubi

You should calculate CNS ( & UPTD) for each phase of the dive.

So 32% for 40 minutes at 30 meters, deco on 50%

Dive 30m (PO2 = 1.28)[1] 40 minutes = CNS 22.4
Deco 6m (PO2=0.8) 3 minutes = CNS 0.66

Total CNS = 23.06

[1] use PO2 of 1.30, 1.28 not in CNS table.

Similarly, UPTD should be calculated for each phase of the dive. For modest recreational dives there is little risk of hitting the limits unless a lot of repetative dives are preformed on optimum PO2's. So a trip to Scapa doing two dives a day is not going to trouble you, four dives a day in the Red Sea is another matter.

The problem generally start to occur for series of dives that involve large amounts of decompression on richer mixes, where you are not only doing a dive at a PO2 of 1.2-1.4, but decompression on a PO2 of between 1.4 - 1.6, then the CNS clock ramps up rapidly, as does the UPTD dose.

Gareth

[Scubi]

Brilliant, thanks - all came flooding back as I was reading it.

Many thanks