A couple more things worth mentioning about SAC and gas planning.
1. Too many divers focus on SAC. Once you get to the point where you are a) relaxed underwater, b) eliminate any extraneous movement, c) minimize drag, d) get your trim to the point where you do not need to use any trunk muscles to maintain trim and e) establish a slow deep inhale-short pause-slow deep exhale breathing pattern, SAC pretty much is what it is.
2. Too many divers try to improve SAC by modifying their breathing pattern with a longer than normal or prudent pause between breaths (skip breathing) or otherwise try to gt more O2 out of a lungful of gas by reducing their respiration rate.
When nitrox first started to be used, many divers felt it greatly improved their SAC. The logic was with more O2 in the mix, they could breathe slower and extract more O2, therefore greatly improving SAC. The problem is that for each liter of O2 actually used by the body, you produce about a liter of CO2. You can use a higher percentage of the O2 in a breath of air by breathing slower/holding your breathe, but the the CO2 produced is the same whether you are taking 6 breaths per minute or 4 - but with fewer exhalations per minute to eliminate that same amount of CO2, you end up retaining more CO2 with higher CO2 levels that increase the effects of both narcosis and oxygen toxicity. I saw a lot of divers do exactly that with nitrox and in the process experience more narcosis.
It also happens as divers begin to venture deeper. SAC is no big deal when you have lots of gas and come back aboard with lots of excess gas, but at 4 or 5 atmospheres or deeper, many divers get very conscious of how much gas they use with each breath and again try to reduce the breathing rate to improve the SAC and keep more gas for more bottom time or a greater reserve. The most immediate result is greatly increased narcosis feeding into the belief that END's below 100' or even shallower than 100' mean debilitating narcosis in cases where END alone is not the problem.
So if you have improved your SAC to the limit you can with good technique and relaxation and still need more gas or a better SAC to get where you need to go in terms of depth or penetration, just take more gas.
I cringe when I hear cave divers talk about being 100-130 ft deep in a cave on Nitrox and skip breathing the last few/several hundred feet to make a goal for their turn on thirds. They may make it, but they have elevated CO2 levels, increased narcosis and an oxygen debt that gets paid on the way out - putting them in a situation where a failure at or near the pentration point may not allow them to get out on the remaining gas. If you just gotta get there, take a stage rather than try to stretch the gas by artificially limiting your respiration rate to stretch a smaller amount of gas.
I also cringe when I hear cave and other techncial divers bragging about SAC rates that are ridiulously low. A full size guy with a real world working dive SAC much below about .6 is probably retaining excess CO2, and is an idiot, not a hero.
1. Too many divers focus on SAC. Once you get to the point where you are a) relaxed underwater, b) eliminate any extraneous movement, c) minimize drag, d) get your trim to the point where you do not need to use any trunk muscles to maintain trim and e) establish a slow deep inhale-short pause-slow deep exhale breathing pattern, SAC pretty much is what it is.
2. Too many divers try to improve SAC by modifying their breathing pattern with a longer than normal or prudent pause between breaths (skip breathing) or otherwise try to gt more O2 out of a lungful of gas by reducing their respiration rate.
When nitrox first started to be used, many divers felt it greatly improved their SAC. The logic was with more O2 in the mix, they could breathe slower and extract more O2, therefore greatly improving SAC. The problem is that for each liter of O2 actually used by the body, you produce about a liter of CO2. You can use a higher percentage of the O2 in a breath of air by breathing slower/holding your breathe, but the the CO2 produced is the same whether you are taking 6 breaths per minute or 4 - but with fewer exhalations per minute to eliminate that same amount of CO2, you end up retaining more CO2 with higher CO2 levels that increase the effects of both narcosis and oxygen toxicity. I saw a lot of divers do exactly that with nitrox and in the process experience more narcosis.
It also happens as divers begin to venture deeper. SAC is no big deal when you have lots of gas and come back aboard with lots of excess gas, but at 4 or 5 atmospheres or deeper, many divers get very conscious of how much gas they use with each breath and again try to reduce the breathing rate to improve the SAC and keep more gas for more bottom time or a greater reserve. The most immediate result is greatly increased narcosis feeding into the belief that END's below 100' or even shallower than 100' mean debilitating narcosis in cases where END alone is not the problem.
So if you have improved your SAC to the limit you can with good technique and relaxation and still need more gas or a better SAC to get where you need to go in terms of depth or penetration, just take more gas.
I cringe when I hear cave divers talk about being 100-130 ft deep in a cave on Nitrox and skip breathing the last few/several hundred feet to make a goal for their turn on thirds. They may make it, but they have elevated CO2 levels, increased narcosis and an oxygen debt that gets paid on the way out - putting them in a situation where a failure at or near the pentration point may not allow them to get out on the remaining gas. If you just gotta get there, take a stage rather than try to stretch the gas by artificially limiting your respiration rate to stretch a smaller amount of gas.
I also cringe when I hear cave and other techncial divers bragging about SAC rates that are ridiulously low. A full size guy with a real world working dive SAC much below about .6 is probably retaining excess CO2, and is an idiot, not a hero.
