Chronicle of an older diver

CCR, O2 Consumption & Exercise October 31, 2016

With a little less than 12 hours diving a CCR, I’m now venturing to comment on a technical aspect of the experience which dawned on me recently. Surely I’m not the first to think about a CCR in this way, but I’ve not come across this analysis previously and wanted to record my thoughts about it here.

The more I dive the more I concern myself with staying fit, as I’m not getting any younger, and that gets me thinking about both capacity to expend energy and calorie consumption. Rebreather training tells me that when things are going right, bubbles are only released upon ascent, which is just the excess volume from the loop as the gas expands with the decreased ambient pressure. Other than this, all the Oxygen consumed during the dive is metabolized by the diver.

Fitness evaluations are frequently done by measuring inspired Oxygen, and except for what is released on ascent the CCR will measure that based the change in O2 tank pressure. I found this article that directly relates calories burned to O2 consumption and also relates O2 consumption to METS (metabolic units, a measurement of energy used in exercise) and body weight.

I’ll give an example, which for simplicity I’ll use metric units. My CCR has a 2 litre tank, which can be filled to 230 bar. So its capacity is 460 litres. Say I do a 60 minute dive and the pressure in the tank goes from 230 bar to 180 bar, a consumption of 100 litres of Oxygen. Ignoring the bubbles on ascent, I’ve burned 500 calories (100 litres at 5 calories/litre).

I weigh 72.3 Kg right now. 1 MET energy output uses 3.5 ml of O2 per kg of body weight per minute of activity, so if we know our O2 consumption in litres the average number of METs used in the dive will be (litres x 1000) / (body weight x minutes x 3.5). So if it were me doing this dive my average energy expended would have been about 6.6 METs, which is a little less than this source gives for slow cross-country skiing.

Coincidentally, according to the calculator on this site, my current treadmill regime of 3.5 MPH at 6% slope also requires 6.6 METs (BTW tomorrow I increase to 7% slope or 7.1 METs).

Conclusions

So I’m inspired (pardon the pun) to think that this yet another great feature CCR diving. I can now, with reasonable accuracy, determine the calorie expenditure on a dive, and replenish accordingly. I also know how hard I’ve been working on the dive and train to that level.

Regarding the bubbles on ascent, there’s probably reasonable way to estimate how much O2 is going overboard. Fodder for a future post, no doubt.