Back-and-forth gas switches?

Suggestions? Comments? Features you'd like to see?
Post Reply
mikemike
Posts: 10
Joined: Tue Jan 05, 2016 3:26 am
Dive Computer: TDC-3

Back-and-forth gas switches?

Post by mikemike »

Hi,

I ended up with an (admittedly unusual) situation that I'm not sure how to handle in Mac Dive.

Basically I often do dives that involve a gas switch... back to a previously used gas. (E.g., travel gas gets reused in deco). In Mac Dive the two separate times this gas shows up in my "Gases" detail, it seems to be treated as two separate tanks... throwing off my SAC/RMV calculation. Is there an easy way to tell Mac Dave that "those two same-blend gasses in my list are actually the same tank"?

Thanks!
User avatar
nick
Site Admin
Posts: 4360
Joined: Sat Apr 12, 2008 8:33 am
Dive Computer: Shearwater Teric
Contact:

Re: Back-and-forth gas switches?

Post by nick »

Currently the list of tanks is not a list of tanks you had with you. It's what you were breathing at a given point in time. To think of it as "two separate tanks" is incorrect, in terms of what it is trying to describe.

You breathe from mix A. Then switch to mix B. Then switch back to mix A, so there are three entries reflected in the gases table. This is not three tanks, this is three "states" as it were. Does that make sense? They are not 1:1 in terms of what tanks you had with you. So your setup is correct, there should indeed be three if you switched twice.

It also shouldn't throw the SAC off. You should already have a separate SAC calculated for each of those setups as it were being used, and then the overall dive SAC is a weighted average. I suppose what you want is a weighted average SAC for a subset of tanks? Which you can't do right now. But I'm not sure if that makes sense, really. You switched to a different tank, then back. What does combining the first and third SAC rate mean?

Sorry if I'm missing something here, I'm just not sure I understand how treating those two SAC rates as combined makes sense. You were only breathing from one of them at a time, so that SAC rate is correct for the time period.
mikemike
Posts: 10
Joined: Tue Jan 05, 2016 3:26 am
Dive Computer: TDC-3

Re: Back-and-forth gas switches?

Post by mikemike »

Thanks!

Sorry, to clarify: let's take an example:

Suppose I breath:
  • - Mix A for the first 10 minutes of the dive
  • - Mix B for the next 60 minutes of the dive
  • - Mix A for the last 30 minutes of the dive
I dove with two tanks. For the tank containing Mix A, I know its starting pressure at the beginning of the dive and its ending pressure at the end of the *second* time I breathed it. (I don't always remember the intermediate pressure at the end of the first 10min.) I would like to know my sac rate across the 40 min total I breathed it... or it's ok to know my SAC rate total across both tanks, i.e., (total cuft of mix A + total cuft of mix B) * depth correction factor / time.

Thanks!
Post Reply