SAC Rate Test

[Rhone Man]

Couple of things, freshly plagiarised from one of the TDI manuals:

- Make sure you have been in the water for a few minutes for your tank to cool before you start measuring. Otherwise the drop in pressure due to cooling will distort your calculations.

- Much easier to type your numbers into this link rather than caculate them from scratch.

- I find going slightly deeper enables more accurate calculations because you amplify the amount of gas you consume (at the surface breathing for 5 minutes, you will consume a couple of hundred PSI - hard to get a read on a convential pressure game). At 100 feet (4 ATAs) you are going to rattle through four times as much - reduces the scope for rounding errors.

- In the TDI manual, they suggest calculating 3 SAC rates: completely at rest, normal swimming, and swimming like hell (ie. pushing against something swimming as hard as you can). You should expect the last one to be about 10-12 times greater than the first one (can't say whether that is true - I have never done it - not a massochist).

- Remember to conduct the exercise afresh every so often. People change, esp. when conditions or environment changes, but even just generally as we get either older or more experienced, or physical conditioning changes.

- Much easier than all of these for calculating SAC rates with pinpoint accuracy - get an AI computer which will calculate your SAC rates for you. Or borrow one.

 

[Dr Neil]

For those of us who tend to lean towards the geeky side, just tracking diving data in an Excel spreadsheet is exciting. Although one cannot expect dead-on accuracy and draw exact conclusions from the calculations and data, it's a far cry better than ignorant bliss.

Knowledge is Power!

 

[SailNaked]

I know I am a nerd, but my idea was to sit on the couch watching TV and breath through a tank. Surface Air Consumption, cant get any more surface than that. since I will have dry paper, and a pen to keep exact notes. The number is only to do calcuguessamations with any way. obviously it is better to do so with real data as suggested and then back calculate the surface guess. As has been posted, water temp, excitement, blood alcholol content exertion, activity, libido all will effect the actual consumption rate. So basically you have a meaning less number that will not give you and exact answer to any question.

By the way the only use I have seen for this number is to calculate about how much gas you need for your stop(s). may be others I have no clue about.

I have been trying to capture the info for this number for the last 20 dives and so far not one usable number, I never have had a dry peice of paper to write on and my tank pressure when I get in the water after 5 minutes is never a nice round number for me to remember. plus I have been diving for 5 minutes. I bought a slate for this reason and will try over the next 20 dives to get a usable gesstimation.


so why not sit on the couch with a tank. nice base number, then round down. watch Star wars so you can breath along with Darth Vader.

 

[Cajuncru Diver]

I just download my dives onto the computer from my Oceanic Pro Plus 2 and look in the little box titled sac rate and read the number it computes for me. Saves me a lot of math equations.

 

[TSandM]

The biggest use for SAC rate is to do dive planning. If your buddy says, "Hey, I hear there's a wreck down at 100 feet we should go check out!" If you know your gas consumption and your tank capacity, you can calculate how long you can stay and look at that wreck and maintain adequate safety reserves -- with a small tank, it may be a pretty short time. If you KNOW you're going to run low on gas after ten minutes, it will help you make sure you check your gauge and are headed back within that time.

I think each year I have been diving, we have lost someone in Puget Sound who did a deep dive and ran short on gas when they didn't expect to, and panicked. Doing some planning BEFORE you get in the water will avoid that, but you have to have some idea of how much gas you use to know what kind of depths and times are reasonable for you.

 

[MXGratefulDiver]

I think each year I have been diving, we have lost someone in Puget Sound who did a deep dive and ran short on gas when they didn't expect to, and panicked.

Well, they didn't all panic, but I can think of 11 divers who have died in Puget Sound since 2001 due to OOA. At least one of them was a member of ScubaBoard.

... Bob (Grateful Diver)

 

[DA Aquamaster]

In my opinion a resting SAC is totally useless. At best you take it times some arbitrary constant to get normal working SAC and hard working SAC rates.

So in essence, how accurate it is is meaningless as it all gets washed through an arbitrary constant of very questionable accuracy and validity.

In my opinion, there are two methods that give useful data.

1. Use a dive computer with an average depth function and use the formula provided previously to figure your SAC or the entire dive based on the actual average depth (not your estimate) and the psi actually used. That will give you a real world idea of SAC in the conditions for that dive and over time you will get a very good grasp of the increases that occur with anxiety, water temp and workload.

2. Swim for several minutes, then note the time, your SPG reading and your depth. Then continue swimming at your normal medium/fast rate while maintaining a constant depth. 60 feet works well as it is deep enough to actually use enough gas to make an accurate estimate of SAC. Swim constantly at constant depth for 5 to 10 minutes, then note the end pressure.

You can then figure the volume used and the atm of the depth you were at to figure out how many cubic feet per minute you used and convert it to a SAC rate.

This method also gives you a useful real world estimate that does not require any artificial manipulation.

 

[Blackwood]

By the way the only use I have seen for this number is to calculate about how much gas you need for your stop(s). may be others I have no clue about.

How about: calculate how much gas you need for... everything else.

The stops are just one part of the dive and, in fact, how much stopping you need to do is predicated on how long you dove and to what depth. Your consumption rate and gas supply constrain how long you can remain at a given depth.

Say you know the reef you want to hit is at oh... 120 feet. If you have a good idea about your consumption rate, you can figure out pretty closely how much gas you'll need to stay on the reef as long as you want to. In a single 72, you may not be able to stay more than a few minutes. If you want to stay longer, you can figure out how much gas you need to bring for the bottom portion. Only then can you know how much gas you need for your stops.

Planning is about more than the end of the dive.

so why not sit on the couch with a tank. nice base number, then round down. watch Star wars so you can breath along with Darth Vader.

Because sitting on the couch in no way relates to dive exertion (unless I suppose you plan to sit on the sea floor). And why would you round down?

 

[DA Aquamaster]

How about: calculate how much gas you need for... everything else.

The stops are just one part of the dive and, in fact, how much stopping you need to do is predicated on how long you dove and to what depth. Your consumption rate and gas supply constrain how long you can remain at a given depth.

Say you know the reef you want to hit is at oh... 120 feet. If you have a good idea about your consumption rate, you can figure out pretty closely how much gas you'll need to stay on the reef as long as you want to. In a single 72, you may not be able to stay more than a few minutes. If you want to stay longer, you can figure out how much gas you need to bring for the bottom portion. Only then can you know how much gas you need for your stops.

Planning is about more than the end of the dive.

Exactly. Knowing how much gas you will use/can use on the bottom and how long you can expect to stay is the heart of gas planning, especially once you get into deco.

Also you should have a good idea of what your gas consumption looks like, what you shoudl use and what the SPG should look like at various points in the dive. If things are not progressing per the plan, you need to take action sooner than the end of the dive or it really will be the END.

 

[AzAtty]

Also you should have a good idea of what your gas consumption looks like, what you shoudl use and what the SPG should look like at various points in the dive. If things are not progressing per the plan, you need to take action sooner than the end of the dive or it really will be the END.

What does Equivalent Nitrogen Depth have to do with this discussion?

Sound advice from all.