I got my Computer Nitrox certification last week. During the class, the instructor talked about figuring out your SAC.
He said to go down to some depth, note your SPG reading, stay at that depth for 10 minutes, then note your SPG again. Using those numbers, you can calculate your SAC. And I understand that (the physics/math).
But, it seems like the number you come to would never match the real world of actually breathing while swimming around on a dive. Even if you're mostly just floating or drifting.
I mean, if you use that number, and you assume a square dive profile, and you calculate your BT so that you should arrive back at the surface with, say for example, 500 psi, and then you dive that profile, as close as you can, I would expect you to always hit the surface again with less than 500psi. Maybe even by a significant amount.
It SEEMS obvious to me that if you use an air-integrated computer and download the data afterwards, you'd be able to have software tell you what your real SAC was. I.e. give you an SAC that is accurate enough to plan a similar dive in the future and get out with pretty close to the predicted amount of air (assuming similar profile and conditions).
But, since I know you more experienced folks, and especially the tech diver folks, mostly eschew air integration, how do you really figure out an accurate SAC for yourselves?
As best I can figure, it seems like you'd start with a baseline number, as my instructor described, then use that to predict results. And when your results were off, you'd adjust your baseline number accordingly, so that, as you repeat the process, you gradually work out a SAC number for yourself that works to give you somewhat reasonably accurate predictions of actual consumption.
Example: You follow the instructions and calculate that you have a .9 l/min SAC. You plan a dive based on that that should give you 45 minutes of BT and put you back on the surface with 500psi left. You dive your plan and end up having to ascend after only 38 minutes, arriving back at the surface with 500psi.
Based on that, you revise your SAC from .9 to (for discussion purposes, 0.9 * 45/38 =) 1.1 l/min. So, you plan your next dive based on an expected SAC of 1.1, instead of .9.
Is that more or less how it goes? Or is there a better or easier way?
Or do you just take the number from sitting at a fixed depth for 10 minutes and go with that because it's close enough?
Thanks!
He said to go down to some depth, note your SPG reading, stay at that depth for 10 minutes, then note your SPG again. Using those numbers, you can calculate your SAC. And I understand that (the physics/math).
But, it seems like the number you come to would never match the real world of actually breathing while swimming around on a dive. Even if you're mostly just floating or drifting.
I mean, if you use that number, and you assume a square dive profile, and you calculate your BT so that you should arrive back at the surface with, say for example, 500 psi, and then you dive that profile, as close as you can, I would expect you to always hit the surface again with less than 500psi. Maybe even by a significant amount.
It SEEMS obvious to me that if you use an air-integrated computer and download the data afterwards, you'd be able to have software tell you what your real SAC was. I.e. give you an SAC that is accurate enough to plan a similar dive in the future and get out with pretty close to the predicted amount of air (assuming similar profile and conditions).
But, since I know you more experienced folks, and especially the tech diver folks, mostly eschew air integration, how do you really figure out an accurate SAC for yourselves?
As best I can figure, it seems like you'd start with a baseline number, as my instructor described, then use that to predict results. And when your results were off, you'd adjust your baseline number accordingly, so that, as you repeat the process, you gradually work out a SAC number for yourself that works to give you somewhat reasonably accurate predictions of actual consumption.
Example: You follow the instructions and calculate that you have a .9 l/min SAC. You plan a dive based on that that should give you 45 minutes of BT and put you back on the surface with 500psi left. You dive your plan and end up having to ascend after only 38 minutes, arriving back at the surface with 500psi.
Based on that, you revise your SAC from .9 to (for discussion purposes, 0.9 * 45/38 =) 1.1 l/min. So, you plan your next dive based on an expected SAC of 1.1, instead of .9.
Is that more or less how it goes? Or is there a better or easier way?
Or do you just take the number from sitting at a fixed depth for 10 minutes and go with that because it's close enough?
Thanks!