Shearwater Perdix AI SAC calculation

[Markos Valsamis]

Hello all,
I have a transmitter for my back gas but not for my deco cylinder. During a gas-switching dive, does the Perdix account for the fact I have switched to a non-AI gas when calculating the average SAC for the dive?

For instance, if you breathe at a steady 0.6 bar/minute, and do half the dive with your back gas (with a transmitter), and switch to your deco gas (with no transmitter) for the second half of the dive, will it display average SAC as 0.6 or 0.3?

 

[NorCalDM]

This is probably a question for the Shearwater tech guys and they are pretty responsive. I would expect it to be about 1/2 the actual number as it only has data from 1/2 the dive and it's just a math problem. I wouldn't bank on it being exact as your SAC rate is more than likely going to change depending on workload.

 

[Aaron Harmon]

No, how could it?

 

[tursiops]

No, how could it?

Exactly. Shearwaters are good but not THAT good. If they don't measure it they can't display it.

 

[inquis]

If they don't measure it they can't display it.

True enough, but I'd rather not have that non-measured portion corrupt the measured portion. If it does display a SAC that is 1/2 that displayed at the gas switch, that's effectively "measuring" (i.e., assuming) zero -- which is undesirable. I would rather that it update the SAC only when the pressure changes. Of course, the user would have to understand it's not the the SAC over the entire dive. Although, it would an upper bound estimate on the entire dive SAC since that would typically decrease if the deco phase were to be measured.

 

[lukeb]

I was just wondering this same thing about my SAC rate when it came to switching to a deco gas without a transmitter. Comparing my deco dives to my non deco dives the SAC rate is very similar, making me think that somehow the Perdix does compensate for when you change gas, but I'm not sure how?

 

[tursiops]

I was just wondering this same thing about my SAC rate when it came to switching to a deco gas without a transmitter. Comparing my deco dives to my non deco dives the SAC rate is very similar, making me think that somehow the Perdix does compensate for when you change gas, but I'm not sure how?

Is the displayed SAC on a deco dive usually less than on a non deco dive?

 

[lukeb]

Is the displayed SAC on a deco dive usually less than on a non deco dive?

Right now I can't see a noticeable difference, some are slightly higher, some are slightly lower, but I haven't had my Perdix that long, so probably need to do some more dives to get a true comparison. Also my deco is usually 15 minutes or less. I imagine on much longer hangs any difference will stand out more.

I have a longer deco dive to the USS Rankin planned for two weeks time, so I will compare again after that and let you know.

 

[tursiops]

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 fluctuation in SAC due to work load, temp, equipment config, etc.

 

[scubadada]

I was thinking that you could not get the SAC for an entire dive using back gas and deco gas, as the SAC measurement is just one tank. However, you could probably get the entire dive SAC if you used sidemount mode and your back gas and deco cylinders were the same size and both had transmitters. SAC is cylinder size dependent and you would have to multiply by a single tank factor to get RMV. This is certainly not the intended use of the sidemount mode and would interfere with tank switch notification and RTR. Perhaps it wouldn't work