Are you diving with Nitrox? The increased O2 partial pressure might explain why you consume less volume of gas.
Just my $0.02.
Another important factor which influences air consumption is body size/weight. Or what do you think?
I am male, 189 cm, and currently weigh around 110 kg. I attached my SAC calculations for your convenience.
Cheers,
LarsB
Way back in the day when nitrox first started being accepted as mainstream I heard this argument alot, particularly among the PSDs we worked with who had just enough EMT traiing on O2 use to be dangerous in terms of not understanding the CO2 side of the equation.
The problem is that regardless of the FO2 or PO2, your body still has the same metabolic requirement and for every liter of O2 it uses, it produces about .8L of CO2. That is significant as it is the CO2 levels that drive the urge to breathe. If you supress that urge and artificially reduce your respiration rate to reduce your gas consumption, you will succeed only at the expense of elevated CO2 levels. Elevated CO2 levels tend to increase the effects of narcosis (as CO2 is quite narcotic at depth) and in addition CO2 levels are associated with increased risk of ox tox.
In the 1950's when the US navy first experimented with enriched air mixes, they used a sample composed of divers with extensive hard hat diving experience who for the most part had developed a high tolerance to high CO2 levels in the sometimes poorly ventilated Mk 5 helmet. The end result of that, combined with higher PO2s than we consider prudent now, was an unacceptably high rate of somewhat unpredictable incidences of oxygen toxicity. The navy concluded that enriched air was not worth the trouble in terms of US Navy operations and dropped it - delaying the adoption of nitrox in civilian diving for about 35 more years.
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In short, even apart from nitrox diving, artificially lowering your respiration rate to improve your SAC will increase your CO2 levels. In addition to the narocis effects noted above and any anxiety that may come with a "dark" narc, high CO2 levels can significantly increase anxiety since you are in effect trying to short circuit an automatic response in your brain, and that anxiety creates muscle tension and increases in metabolism that can increase O2 use and CO2 production. It's all bad news with no upside, so don't do it.
You are far better off reducing SAC by reducing your metabolic requirements and that can be accomplished by reducing all uneccesary muscle movement or tension and by just relaxing on the dive.