Shearwater Perdix AI SAC calculation

[sabbe]

You can click on the graph and view SAC for each stage of your dive. Went back through my history and not sure why some dives have it and others don't, Shearwater's display of this seems to change over time.

Quite a useful feature, you can look at your usage for example SAC for descent, SAC for high work during transit, SAC during low work / drifting phase etc and use that to plan future dives rather just getting a rounded total dive number.

Sam

 

[lukeb]

You can click on the graph and view SAC for each stage of your dive. Went back through my history and not sure why some dives have it and others don't, Shearwater's display of this seems to change over time.

Quite a useful feature, you can look at your usage for example SAC for descent, SAC for high work during transit, SAC during low work / drifting phase etc and use that to plan future dives rather just getting a rounded total dive number.

That is a really good idea. That way you can get a 'true' average for the main bottom portion of the dive. I know looking at the graph my SAC rate is very high on the descent as I add air to my wing (Shearwater's SAC rate ignores the first 30 secs of a dive for just this reason, but most descents take at least a minute or two).

 

[lukeb]

Any gas use during unmeasured deco should give you a lower SAC than actual. But light deco -- even 15 mins -- doesn't use much gas, so should not lower the SAC too much. For example, if you use 70 cu ft for the total dive, of which there is 7 cu ft for the deco (assuming actual RMV=0.5 cuft/min) at 15 ft for 10 minutes), your Perdix SAC would be calculated on the basis of 63 cuft, since it would not know about the 7 cuft used for deco. A actual SAC of 20 psi/min would be reduced by the factor 63/70 so would appear as 18 psi/min. You might not even see this, given the flucation in SAC due to work load, temp, equipment config, etc.

That makes a lot of sense. Thanks for explaining it so clearly.

 

[Markos Valsamis]

I asked shearwater tech support who said that if you use a gas that doesn't have AI then the overall calculation won't be correct....

I was hoping for some more insight into that but didn't get any (despite giving the theoretical scenario I posted on this thread).

 

[tursiops]

will it display average SAC as 0.6 or 0.3?

0.3
All it knows is time, depth, and the amount of gas used that it measures. If you double the time for the same amount of gas used, that halves the apparent SAC. How could it do otherwise?

 

[Markos Valsamis]

0.3
All it knows is time, depth, and the amount of gas used that it measures. If you double the time for the same amount of gas used, that halves the apparent SAC. How could it do otherwise?

Just wondered if it can identify that zero means like you are breathing from something else so as to remove those readings from it's calculation of the average.

 

[inquis]

It could, but that would require a change to the software. Maybe someday...

 

[tursiops]

Just wondered if it can identify that zero means like you are breathing from something else so as to remove those readings from it's calculation of the average.

How would it know that you are breathing from something else, vs not breathing?

 

[tursiops]

Just wondered if it can identify that zero means like you are breathing from something else so as to remove those readings from it's calculation of the average.

When you download the data to Shearwater Cloud, it shows the SAC during the dive. So you can see what your SAC is up the moment that you stop using the tank with the transmitter on it.

 

[AJ]

How would it know that you are breathing from something else, vs not breathing?

There's two options when it still records depth: a) you're breathing from another tank or b) you're dead. If you're dead you won't be interested much in SAC I guess