Solo Diver Redundant Air Supply vs SAC Calculation

[miraclemax]

Hi Nick,

I don't know if this falls into your plans for multiple tanks but I thought I'd post my request anyway.

I'm currently doing the SDI Solo Diver course (and I truly think everyone should do this course!) and as part of the equipment you need for this course, is to plan to have a redundant air supply. For this I use a Luxfer 40 tank I just purchased. I want to tell MACDIVE that its a tank since I could use it for stage deco or some other use. The problem is: if I enter it as a tank and add working pressure and capacity, then add this tank to my dive gear, it'll ADD the Catalina 80 I have and calculate SAC on the total amount of available air.

Now the Solo diver training tells us that the redundant air source is NEVER to be counted in our planning so essentially, this tank "does not exist" during the dive or the planning phase and so should not be part of SAC calculations afterwards.

My feature request would be to have a per-dive way of excluding a tank's capacity from the air consumption calculations of this dive. This would ensure that a tank is added correctly as a gear item and added correctly as "something I took on my dive" but not added to the calculations for SAC since this gas was ONLY there for an emergency and not available to extend dive times. Does that make sense?

I don't know if this would serve other purposes but it would sure help me Right now I've had to identify this tank as being a "Reserve Tank" so it would not count it in the SAC.

Thanks!
Max

[nick]

Only one tank should be used in calculations, though it would be random as to which one was picked.

If you want it to explicitly be ignored, change its type to something other than "Tank".

This will all be better when I add multi tank/gas support and there will be a way to choose what tanks/gases were used and which weren't.

[TnT]

... and calculate SAC on the total amount of available air.

SAC stands for Surface Air Consumption. Therefore it does not matter how much air you have available, because only the amount you actually consumed is taken into account. If your spare tank isn't used (e.g. p_end = p_begin), it shouldn't affect the SAC rate at all.

SAC = (p_end - p_begin) * V / (p_avgdepth * time)

[miraclemax]

Hey TnT,

Yeah I totally agree about the definition of the air consumption, thats how my training (SDI) defined it as well.

However, when I add 2 tanks to a dive, the number calculated changes. So I'm forced to assume that MACDIVE might be adding the pressures or tank baselines (I'm in imperial) to do the calculations. For example, I add a Luxfer 40 liter (3000 PSI working pressure) tank to a dive with a Catalina 80 liter (3000 PSI working pressure) tank at average depth of 23.78 ft having consumed 1450 PSI over 46 minutes, the SAC goes from 0.47 cf/min (just the 80) to 0.24 cf/min (with the 80+40). At first go, my calculations are showing me that MACDIVE is using 6000 PSI (3000 for the 80 and 3000 for the 40) to calculate the SAC. I haven't tried figuring out if its adding the tank baselines but I could do that if you want.

Anyways, I'll wait until the multi-tank feature arrives so I can identify the tank as a tank but not count it in the dive calculation.

Thanks!

PS I can post calculations and screenshots if you need, I'm not making this up hahah. also, I don't want to start a math war, just pointing something out about software behavior and a request about adapting it.

[TnT]

If the SAC changes, there is indeed an error in the calculation. I'm sure Nick is already aware of this problem.

[nick]

Yes, but multiple tanks are not currently supported. If you see weirdness because of it, then, well - it will be an issue once multiple tanks ARE supported.

For now, my previous suggestion would be the workaround - change its type to "Pony Tank" or "Secondary Tank" or something. Crude, but until multiple gases are properly supported, you will potentially get odd behaviour.

[kevincasey]

... and calculate SAC on the total amount of available air.

SAC stands for Surface Air Consumption. Therefore it does not matter how much air you have available, because only the amount you actually consumed is taken into account. If your spare tank isn't used (e.g. p_end = p_begin), it shouldn't affect the SAC rate at all.

SAC = (p_end - p_begin) * V / (p_avgdepth * time)

I know this is an old thread, but I'd like to +1 the multi-tank / multi-gas recording.

[D-Zyl]

Yes, it would be awesome !
+1

[Jean-Francois]

Indeed it would be

+1