High SAC reported at the beginning of dive

[wetb4igetinthewater]

Maybe you were nervous about the transmitter exploding? Then said "screw it, I'm going to enjoy my dive" and your SAC came down.

 

[aviator8]

My Eon Steel does the same. I always figured it was that it was averaging and there is some base assumption at the start that throws the average off. As more sampling points are taken the initial assumption becomes insignificant in the average. I just ignore my sac for the first few mins as it is always way off. If you come up to less than 5 ft it resets and does the same thing all over. I could be in a pool and have a sac of .35 then get above 5 ft and re-descend and nothing has really changed in my breathing but it jumps up to 1.2 or something.

 

[Andrei Astra]

My gut feeling is that the computer SAC calculation is skewed during the descent. SAC = dV/dt / D(t). If the depth D (expressed in atm) changes with time t rapidly, comparing to the pressure sampling rate, the SAC calculation will be off.

 

[drainaps]

My 2 cents:

1) Algorithm to calculate SAC will not deliver accurate results until things average themselves out. Beginning of any dive is a steep descent (high pressure gradient for the mathematically inclined) and that's going to affect how SAC is calculated and probably the accuracy of the algorithm in that transient part of the dive.

You also see in your graph a spike in SAC during your ascent (gradients again) , 20 something minutes into the dive, delayed vs the beginning of the ascent, that is probably algorithm-induced as well.

Depends also on what the pressure sensor is delivering to the watch at the very beginning, including the tank cooling.

Probably Shearwater did the right thing by displaying nothing instead of cr@p. Don't think too much about it. To me SAC is an after dive metric, not a live dive metric as others have already said.

2) These algorithms are funny. My Teric will continue to calculate SAC on my twins once I've switched to deco gas, which it clearly shouldn't. It should stop there and then, so obviously the results are not what they should be.

Safe dives all.

 

[Diving Dubai]

Algorithm to calculate SAC will not deliver accurate results until things average themselves out.

Absolutely. Average SAC can be affected disproportionately by mid dive events. I can read my final SAC and see that I've had a spike mid dive throwing it off.

I use the results as a guide only. And then for planning and mid dive maths, I round up to values which are generally conservative but also are easy to calculate on the fly.

In my case I use 15l/m SAC - which because I dive 15L steels means 1bar per minute at the surface, and thus easy to extrapolate with ATA allowing me to make decisions on the fly as appropriate

 

[Diving Dubai]

You also see in your graph a spike in SAC during your ascent (gradients again) , 20 something minutes into the dive, delayed vs the beginning of the ascent, that is probably algorithm-induced as well

A classic example of this (and why you need to understand teh data and not take it at face value)

My wife who has a seriously low SAC rate, once recently was "disturbed" when she saw her computer turn Red with a TTS (or GTR) of Zero

She was just inflating her dsmb at depth from her Alt reg. Of course the computer saw a large pressure drop in a short period of time and extrapolate that with her depth and current gas pressure and and gave her a warning. At the next sample everything was back to normal and the computer adjusted accordingly.

Post dive after thinking about it, she realised what was happening, but at the time it caused a little concern for a moment.