SAC decreasing with depth

[petmal]

Possibly bad news. I looked at my profiles and the blue line is the actual pressure profile. I can even "damage" it by manually editing the recorded pressures. It has a pressure record for every 10s. It is just the scale of the plot that masks the changes and makes it look like a straight line.

 

[petmal]

OK. I may have the answer. Suunto DM can somehow simulate (whatever it means) the air consumption for any given profile. Simulated consumption leads to our "mysterious effect". SAC profile is nearly flat for "real data" (?). I searched the help but didn't find anything helpful... My guess would be that the simulation is there for computers that do not record pressure every time and it is just a linear fit (pretty useless I have to say).

 

[Majickyl]

The "depth" read out on the computer is, or should be interpreted as, pressure---measured in feet sea water (fsw), for example. Thus, if the computer is set for salt water, it will read 33 feet when the diver is 33 feet deep in salt water and will read 33 feet when the diver is 34 feet deep in fresh water. Since the readout is pressure (not depth) air consumption, SAC calculations, etc., are not affected, it's just that you may be a bit deeper (in the example above) than the computer indicates.

Ahhh... duh.

I think the thread is veering closer to what's actually happening than what I was posting here, which on reflection makes no sense.

Of course the computer is measuring pressure and would use those actual values in the calculations.

 

[eponym]

Suunto DM can somehow simulate (whatever it means) the air consumption for any given profile. Simulated consumption leads to our "mysterious effect". SAC profile is nearly flat for "real data" (?)

Exactly right.

SDM can display a graph of consumption during the dive but on my installation it very intelligently chooses not to do so because my downloads from dive computers do not include real-time pressure info.

Here's a very quick and rough sketch of what you could expect if you have a dive computer that records pressure at regular intervals during the dive and downloads that to a divelog program.

You'll notice that the deeper you go, the faster the pressure (blue line) drops. It's always dropping, of course, but the line's downward slope is steepest when you're deep because you're going through gas faster.

Consumption (red line) may vary some during a dive depending on your comfort, exertion, etc. It would tend to go up if you get cold, down as you get comfortable.

-Bryan

 

[Majickyl]

I imagine that the Suunto DM program would calculate SAC rate like this. I may be utterly wrong, but this is how I would do it if I were the computer.

Take the average depth/pressure from the last 10 s.

Take your gas pressure you were at 10 seconds ago, and the gas pressure you are at now, and subtract.

With that depth/pressure, that change in gas pressure, and a time of 10s, calculate the rate.

Because your depth is changing not necessarily in a linear fashion in that 10 sec, taking an average would be less than ideal, and the since the rate of gas pressure change is also dependent on the depth, which is changing, the model fluctuates a bit.

I think the artifact we're actually seeing is SAC rate is decreasing on a descent and increasing on an ascent, and that's due to the way Suunto DM is calculating the instantaneous SAC rate.

I think I'm confusing myself now...

But it should always fluctuate right around the important number, the average SAC rate.

 

[Majickyl]

Exactly right.

SDM can display a graph of consumption during the dive but on my installation it very intelligently chooses not to do so because my downloads from dive computers do not include real-time pressure info.

Here's a very quick and rough sketch of what you could expect if you have a dive computer that records pressure at regular intervals during the dive and downloads that to a divelog program.

You'll notice that the deeper you go, the faster the pressure (blue line) drops. It's always dropping, of course, but the line's downward slope is steepest when you're deep because you're going through gas faster.

Consumption (red line) may vary some during a dive depending on your comfort, exertion, etc. It would tend to go up if you get cold, down as you get comfortable.

-Bryan

Nice figure!

Though I'm under the impression now that the OP's computer is air integrated and is recording pressure at regular intervals. Is that the case?

If so, we're still not at why the red line isn't flatter and is so responsive to depth in his actual profile?

 

[petmal]

Nice figure!

Though I'm under the impression now that the OP's computer is air integrated and is recording pressure at regular intervals. Is that the case?

If so, we're still not at why the red line isn't flatter and is so responsive to depth in his actual profile?

The problem appears when I choose to "simulate air consumption" (that's how they call it) and my guess would be that it uses a linear approximation (constant pressure decrease over time from the initial pressure to the final value) that does not reflect the actual tank pressure changes. That would be why the SAC profile follows depth changes (pressure decrease over time is constant while depth varies every 10s).
When I choose not to simulate the consumption it probably uses the real recorded data (if and when available) and I get a nice nearly flat SAC profile.

 

[CuracaoJ]

petmal,

What version of the Dive Manager are you using? If you use the latest, (3.1.0) and click on the "Air Consumption" tag at the bottom, you should get a window to put in all your cylinder info. If you fill that in correctly, and if your transmitter was hooked up, working, and recording during your dive, then all you have to do is press "re-calculate," and you will get your SAC rate. It shouldn't be a solid line like in your pic, more like a fine dotted line with each dot representing (I assume) a sample pulled off the transmitter. I have about 1,000 dives with SAC info on my Suunto, and none of them look like the one you posted, they are all very irregular and dotted. I think what you have there is more of an "averaging" line that has been smoothed as some people are speculating.

Hope that helps.

 

[Puffer Fish]

I was fine, swimming at very much the same rate all the time. I understand it is often more work on ascends and descends, but I don't think this can explain everything.

There are a couple of things that may distort your numbers.

First, SAC tends to decrease as the dive goes on...that intial adrenaline rush will slowly go away.

Second, there are other aspects of the dive that can effect you besides just depth.

We dive a lot here, where you cannot see the bottom when on the surface.. most people are a lot more relaxed when they know there actually is a bottom. And the surface is not always your friend...rush to get down, waves, current tend to be a lot worse on the surface.

My second dive is always a lower sac rate...not sure why... I can dive 10 days in a row, and the first dive is always higher...the rest the same... go to sleep, get up and it starts all over again......

Check your sac rate on the safety stop, it should be the lowest when you are just relaxing.

 

[petmal]

petmal,

What version of the Dive Manager are you using? If you use the latest, (3.1.0) and click on the "Air Consumption" tag at the bottom, you should get a window to put in all your cylinder info. If you fill that in correctly, and if your transmitter was hooked up, working, and recording during your dive, then all you have to do is press "re-calculate," and you will get your SAC rate. It shouldn't be a solid line like in your pic, more like a fine dotted line with each dot representing (I assume) a sample pulled off the transmitter. I have about 1,000 dives with SAC info on my Suunto, and none of them look like the one you posted, they are all very irregular and dotted. I think what you have there is more of an "averaging" line that has been smoothed as some people are speculating.

Hope that helps.

I have 3.1.0. When I go to "Air consumption" and hit "re-calculate" it asks "Do you want to simulate air consumption for dive profile?". And yes, when I choose "No" I get something like what you described. Unfortunatelly, it does not seem to recalculate dives for which I have already chosen "Yes". Dunno why... :depressed: