EON Steel SAC Calculation

[RandomA]

I'm a very new diver, so maybe I'm just calculating incorrectly, but I'm confused by my EON Steel's SAC calculation. Basically, at the beginning of a dive, the SAC rate is very large - here's a couple charts from DM5:

In this shallow dive, the SAC rate is so high it doesn't even show on the chart - it starts at 12cf...

In this, deeper dive, the SAC starts 2.3cf - at the surface before dropping to .6ish...

Shouldn't the SAC be much higher at the 8 minute mark, at 120ish fsw (physically active) than at the 24 minute mark at roughly 20fsw? (hanging on a line, not physically active)

 

[RainPilot]

Silly question but did you enter the correct tank sizes?

@Diving Dubai is our resident Eon man...

 

[RandomA]

Silly question but did you enter the correct tank sizes?

Yes. That was my first thought.

 

[KenGordon]

I see stuff like the second dive. I put it down to filling the wing and dry suit on the way down. It may also be down to cooling. Were the cylinders in the sun and then directly into cool water?

I cannot explain the first one.

 

[Diving Dubai]

Okay. This is my understanding

You have a 10 Sec sample rate.

What the computer does, is measure pressure drop over time and extrapolates it over time. This is a measurement of consumption per minute

At the beginning of a dive the computer sees significant pressure changes. Your tank might cool slightly as you hit the water, your breathing might be slightly elevated before you settle down, and of course you are adding gas to your BC and perhaps Drysuit during descent

Thus a "large" pressure drop but over a very small time gives a high average SAC. During the dive this all starts to average out and the SAC levels out flattening out to the true level.

Taking one of my logs, the Sac starts off at 52l/m - (you'd have to work some to breath that) But it takes until the 15 min point for the averaging to have settled to the actual average of between 10.5 and 13l/m

On a lot of my logs I get a spike at the end of the dive as I inflate my wing


Your car fuel consumption works in a similar fashion. If you reset it and start to drive you'll get a wildly different average MPG for the first few miles until it settles down.

Personally I only use the "flat" area of the SAC line to gauge my rate. You only need to know your SAC as an average over a dive for gas calcs. If you are on a dive where things get "a little interesting" then it's worth seeing the SAC for that portion. Mine under stress is 35l/m and while that's an instantaneous peak rather than a constant rate, I use that value for Emergency gas calculations.


I hope that helps. As I say this is my understanding rather than being the definitive answer

Regards

 

[RandomA]

No gas used for the wing, no dry suit... Cylinders were in the shade for the second dive and the first dive was at 9AM, so not much sun...

However, I've noticed this behavior for basically all dives in pretty much all conditions, where the first part of the dive has a really high, if not off the chart, SAC rate. (I got the computer used and it has almost 200 dives on it, so I have a lot of samples).

I'm wondering if there is a disjunct between when the air consumption is calculated and where the depth is calculated. So, for example, if the depth used for the calculation is taken at time A and the air consumption is taken between A and A+10seconds (or some other period), and you are descending, then the SAC would appear to be higher than it actually is. Conversely when surfacing the opposite would be true, however, descents are usually faster than ascents, so they would show more deviation.

For the shallow dive, based on time at depth, my calculation for SAC is approximately 1.2cf, whereas the Suunto is calculating 1.4-6cf, which I'm pretty sure is not correct. That's based on 9 minutes, at an average of 17', using 646psi of gas in an AL80. The DM5 doesn't show exact resolution of time, so the 9 minutes is probably +/- 30 seconds. That calculation is using the data that is recorded by the Suunto, so DM5 should come up with similar results but doesn't.

 

[scubadada]

Hi @RandomA

I know nothing about Suunto or your Eon Steel. However, I have a lot of experience with my Oceanic VT3 and Oceanlog software.

I see little to no reason you would want or need to follow your instantaneous RMV. I follow my RMV for complete dives, they actually vary very little. I assume your Eon Steel and download software will calculate your RMV for the dive.

Here is an example of one of my dives. I use 30 sec sampling rate. My gas consumption is quite regular during the dive, it does flatten out a little at the safety stop. I'm not at all sure what your profiles reveal.

Good diving, Craig