Do you use your SAC for anything? POLL

[Peter Guy]

Lake County -- IF you dive with a computer or bottom timer, you should be able to get a readout from it that gives you the "avg depth" of the dive. This "avg depth" is usually a "time weighted" avg and so is useful for determining the avg SAC rate for the dive. If you don't have that readout, then one way to get a "Time Weighted Avg" is to just mark down your depth every five minutes, totally it up and divide by the number of markings to get a time weighted avg depth. (Or you can just try to keep a running total in your head -- 85'=9; 71'=7 -- 7+9=16/2 -- avg 80' which is good enough.)

Charlie -- you responded while I was writing -- the "avg depth" works really well ONLY IF it is time weighted as opposed to saying -- "I was at 90 and then 70 and then 45" and expecting a useful number by just averging those three numbers since the time at each depth is what is really important.

 

[SCUBA482]

I use an RMV of .8 or .9 for planning, which is what I can expect if swimming into a VERY light current or surge, or if I'm working harder than normal or slightly stressed for some reason.

So, for me planning is the answer. It helps me decide if I can do the dive with the tank(s) I have available.

During the dive, I compare what the SPG actually reads to what I anticipated it would read, and respond accordingly.

Best wishes.

What he said

 

[BoltSnap]

SAC rate is "equivalent" SAC on the surface and thus "multi-level" is not a factor as much as different circumstances that would change breathing rate such as exertion level due to anxiety, etc. Average SAC is the most practical and useful way to measuring SAC rate for dive planning.

 

[diver_doug]

Since I did not want to hijack a thread I will start this one and it was brought on by the "Check my math" thread.

What do you guys actually use your SAC rate for? I know it is used in formulas to find a few different things. Its just how accurate do you really think it is since there is so much that can affect your SAC. Does anyone actually use their SAC in planning dives???

I know how to find mine thats not what I am asking. I am just taking a poll if you will to see who actually uses there SAC for anything. I just see so many divers (new mainly) that get so triped up on their SAC rate and it is just something that is a part of teaching and diving that is irrelevant to me.

Everything effects it temp/viz/current/surge/day/night. WTF!!!!

I'm with you...I remember at one point I was inquiring with an LDS about a Solo Cert and the divemaster asked me what my SAC rate was; I remember thinking "are you kidding me, that's gotta be like the most meaningless thing anyone could know". Like you said, SO MANY things are going to affect your breathing rate: temp, vis, surge, how you're feeling, if you run into trouble, if you see a lot of cool stuff, get a cramp, remember that you left the iron on, etc. Plus if I'm measuring my SAC (I'll refrain from making a joke here) my SAC rate will be different than it is when I'm simply breathing on the surface and not testing myself. I dunno, I suppose in some instances (or at least for some people) there are meaningful applications, but it's not something I find useful in the slightest.

 

[pollywogg]

I mainly use it as an excuse to reference my scrotum in an oblique way.......low sac, high sac, etc.

 

[tbone1004]

I know what my "average SAC" is and you can see slight changes across your SAC in your log. Mine stays between .4 and .6 unless I'm kicking hard in the current or really really uncomfortable. For normal diving with constant kicking it's about a .6
Once you get truly comfortable in the water, the only thing that noticeably changes it is your exertion rate. Kick harder, you breathe more.
"Everything effects it temp/viz/current/surge/day/night. WTF!!!!"
I.e. did OWT this summer at Looe Key. Babysitting students, warm water, clear vis, slight current, near constant kicking and a lot of depth fluctuations hovering above them. .58
Two weeks ago, 41* water, 15 feet vis, got stuck diving a buddies gear since he was using my nomad. Using tanks I'm not familiar with and his harness wasn't long enough to buckle so I was holding the tanks down. Yes I know this was stupid but it was a 30 minute dive in a quarry, 30 ft max. SAC was .57.
Jill posted last week that when she had that regulator issue and was feathering in a panic situation her sac stayed what it usually does.

I put .8 in as my planning SAC for gas consumption. I've only had 2 dives with a .82 sac, one was a nav course in my OWT class years ago, and the other was babysitting the same nav course last year. Both times I was kicking like a madman.
It's also good to know your buddies SAC, especially in a situation when you may lose visibility because you can estimate his pressure fairly accurately *within 100psi or so*.

 

[Peter Guy]

DiverDoug wrote

I dunno, I suppose in some instances (or at least for some people) there are meaningful applications, but it's not something I find useful in the slightest.

Then how do you plan for how much time/depth you'll do for a dive -- or do you just not plan your dive and just figure things (like NDLs) after the fact?

No one has ever claimed that knowing an average consumption rate (or more precisely, knowing one's average consumption rates (plural) which take into account different types of dives) guarantees anything -- but having some idea of whether you have the proper amount of gas on any dive certainly sounds like a good idea to me.

IF I know I'm going to dive on a wreck at 80 feet and I know I want to spend 40 minutes exploring the wreck, then knowing that I need "Y" amount of gas sounds like a much better planning method than just jumping in and diving. (BTW, I'd need about 85 ft3 for the bottom portion of that dive so my 130 would give me plenty of cushion -- my 95, not so much!)

YMMV

 

[LakeCountyDiver]

DiverDoug wrote
Then how do you plan for how much time/depth you'll do for a dive -- or do you just not plan your dive and just figure things (like NDLs) after the fact?

Thats is how most dives are executed. You do have to do a few dives with out knowing your SAC before you ever get an average SAC to use to plan dives right???

No one has ever claimed that knowing an average consumption rate (or more precisely, knowing one's average consumption rates (plural) which take into account different types of dives) guarantees anything -- but having some idea of whether you have the proper amount of gas on any dive certainly sounds like a good idea to me.

IF I know I'm going to dive on a wreck at 80 feet and I know I want to spend 40 minutes exploring the wreck, then knowing that I need "Y" amount of gas sounds like a much better planning method than just jumping in and diving. (BTW, I'd need about 85 ft3 for the bottom portion of that dive so my 130 would give me plenty of cushion -- my 95, not so much!)

YMMV

This is the exact reason you want to know your sac rate but most divers dont know thers or dont care.

I have not calculated mine because all my dives are multilevel. I just now found a good way to figure it out on a multilevel so who knows maybe I will start using it.

 

[BoltSnap]

I have not calculated mine because all my dives are multilevel. I just now found a good way to figure it out on a multilevel so who knows maybe I will start using it.

I am not sure that I understand what difference "multilevel" dives make on the SAC rate and having to calculate the gas consumption rates for different depths. Once you find out your gas consumption rate for one depth, you find the equivalent SAC rate (Surface Air Consumption Rate) and from that you can calculate the gas consumption rate for any depth and ultimately your gas needs for the time and depth. What is more important here that one has to be concerned about is the possible gas consumption rate changing due to any condition that would increase your gas consumption rate beyond the anticipated changes that are due to depth changes alone.

For example,

Let's assume that your equivalent SAC rate (surface) is 1 ft3 (I am making the number to be easy for calculations).

Your gas consumption rate at 33fsw will be 2 ft3 and at 99fsw will be 4 ft3. All of these numbers assume that you are at the same exertion level as you were on the surface.

You can find the equivalent gas consumption rate at ANY depth using the same appropriate equation and adjusting for depth (In ATA's).

You SAC rate would be effected by any change in your exertion due to many causes such as increased activity level, cold, anxiety, etc. If you were fighting a very heavy current at 30fsw, your gas consumption might be higher than your gas consumption at 60fsw.

Now, a question I have is, how relevant is the SAC rate at the surface in determining an "exact" gas consumption time if there would be substantial factors that would alter exertion rates? For dive planning purposes, one would use the absolute worst case scenario and use a very high SAC rate that would cover changes due to unpredictable environmental conditions that would increase the exertion levels.

At the end, a dive computer that shows your consumption rate and air time remaining is the only practical and realistic method of knowing for sure you gas consumption and how much air time you have left at any specific depth and at a specific exertion level. Pen and paper method is a good exercise but would only be a best effort "guesstimate" but not a precise and accurate tool to predict air time.

Does this cover what you are looking to find out?

 

[Eric Sedletzky]

I use my SAC rate for doing vintage dives when I use no SPG. The SAC is the only way to get an idea of how much air I will need to do a dive for a certain time at a certain depth.
I also use a J valve in combination but usually I'm already back close to shore about ready to finish the dive when I need to pull the J.
Instead of looking at my pressure gauge I look at my watch and depth gauge.