SAC and RMV?

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mstevens
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SAC and RMV?

Post by mstevens » Sun Feb 17, 2019 4:16 am

There are convincing reasons that some prefer to use SAC while others prefer RMV.

There are different use cases for these computed values, so the same diver might want to have both values available.

Given that it's mildly tedious to convert from SAC to RMV or vice-versa and that computers tend to do such things quickly and easily, could we please have RMV calculation added to the Gases screen?
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nick
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Re: SAC and RMV?

Post by nick » Sun Feb 17, 2019 4:54 am

The sac on the gases screen is actually rmv.

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Re: SAC and RMV?

Post by mstevens » Sun Feb 17, 2019 9:29 am

nick wrote:The sac on the gases screen is actually rmv.
I'm aware that there's conflicting terminology when it comes to RMV and SAC.

Divers commonly think of RMV as being expressed in units of volume/minute and SAC as being expressed in units of pressure/minute. Since "volume" is right there in the name of RMV, I think units of volume/minute make sense for RMV.

They're useful for different purposes. SAC can be computed independent of tank size, which can be at least somewhat useful when one isn't sure who manufactured that tank used on vacation a couple of years ago.

My Shearwater software gives me SAC in PSI/min or bar/min irrespective of whether I specify a tank size.

MacDive gives me what I'd call RMV, labeled as SAC, for the same dives from the same computers in CF/min or L/min but contingent on specifying a tank size.

DiverLog (the manufacturer software for my Aeris computers) gives me both SAC in units of pressure per minute and RMV in volume per minute. DiverLog used to show only SAC, with the same units (and the same values) as MacDive. In an update, they added a new label "RMV" for that value and changed "SAC" to show pressure/minute. In that software, RMV varies dependent on tank size entered, while SAC does not.

Currently I copy-and-paste from Shearwater Cloud to get SAC in PSI/min into MacDive because it's a bit quicker than using a spreadsheet (and because I copy-and-paste RMV in CF/min from MacDive to Shearwater Cloud).

I'm not concerned with what terminology MacDive uses so much as with having the results of both calculations.

I'm hoping for a feature, like that in DiverLog, that shows me both SAC and RMV.
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Re: SAC and RMV?

Post by nick » Sun Feb 17, 2019 6:00 pm

I have renamed SAC to RMV, because it was technically wrong, and has been for years, and added SAC as an extra in a couple of places (like the gas tab).

This will be in the next update. I haven't done iOS yet, but I will look in to that.

Thanks.

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Re: SAC and RMV?

Post by mstevens » Tue Feb 19, 2019 1:01 am

nick wrote:I have renamed SAC to RMV, because it was technically wrong, and has been for years, and added SAC as an extra in a couple of places (like the gas tab).

This will be in the next update. I haven't done iOS yet, but I will look in to that.
And that, folks, is an example of why MacDive is such a great program: unparalleled responsiveness in terms of support and updates.

I totally understand that some people only use the iOS app, but I started on the Mac and use the Mac for most of my analysis. We use the iOS app primarily for downloading Bluetooth computers, so it's less important to me personally to look at RMV or SAC on the boat.
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Re: SAC and RMV?

Post by nick » Tue Feb 19, 2019 3:18 am

I'm not really fussed whether you use one or both, but I do like to keep them consistent.

Further to what I did earlier, I have now renamed SAC to RMV on iOS as well. Also back on macOS, I've added added SAC as an option for smart logs (the existing option has been renamed RMV). This is the only major problem here, any existing smart logs for SAC will now be incorrect in that they're looking at the new SAC, not RMV. I've no idea if anyone even has a smart log with this, but, we'll find out soon when I get an angry email I suppose. Extra bonus here, I fixed the text box when editing certain smart log criteria (SAC, RMV, depths) so the text box is a little wider, as it was too short for most values.

As another fun tidy up, I've also changed the main dives table to calculate RMV and SAC the same way the gases tab does. For performance reasons it used to do a quicker calculation where possible, but this was always slightly off. Now it uses the same calculation and I did some extra work to cache the results to keep the performance up. So that should be more consistent also, resulting in one less confusing thing.

I spent far more time on this than I intended to, but I think it's nice to finally fix all this up. It's all just been a little wrong for far too long. There are betas available (email if you want access).

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Re: SAC and RMV?

Post by reefduffer » Fri Jun 14, 2019 1:51 am

nick wrote: ...
I spent far more time on this than I intended to, but I think it's nice to finally fix all this up. ....
Sorry, another loose end that needs cleanup - Export to CSV.

I am happy with the change to use SAC/RMV correctly. What I care about for my logs is RMV. I've been content to live with it being mislabeled SAC, this is better. But...

For many years I have used a workflow that exports selected fields to csv and then postprocesses that to create a 1-dive-per-line index file. SAC has been one of the fields I export, knowing that it's actually RMV. Now I discover that exporting SAC gives me ... SAC. Which is not the value I care about. And the "Export to CSV" popup doesn't have a checkbox for RMV. Could you please add RMV as one of the fields that can be exported to CSV? Thank you.

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Re: SAC and RMV?

Post by reefduffer » Mon Jun 17, 2019 12:44 am

Ooof! Just noticed a more serious related issue. "File > Print ..." (Print Dives) reports SAC Rate (psi/min) where the older version reported SAC Rate (CF/Min), which is of course RMV, the more interesting value. The format seems hardwired, not configurable.

Could you either replace SAC Rate with RMV in this print format, or add RMV? Thanks.

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Re: SAC and RMV?

Post by Craig » Fri Oct 04, 2019 12:35 am

My wife and I use G2's. She recently used 63CF tanks on a trip, and she had Macdive using 80CF as the default. As I understand it, all SAC/RMV calculations are done on data from the G2 which just shows time, depth, pressure, temp, and ppO2. Fall in PSI over time if everything else is equal depends on tank size, correct? If I go into the gas tab and change her tank size to 63, I get an appropriate reduction in RMV in the gas tab, but not in the profile. SAC does not change in either the tab or profile. She wants to see how she is doing as she gets more experience. I guess RMV would be the thing to moniter if we make sure tank size is correct. Is this right?

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Re: SAC and RMV?

Post by nick » Fri Oct 04, 2019 1:57 am

SAC doesn’t depend on the tank size, RMV does.

If the RMV doesn’t change on the profile after changing the tank, try changing dives and seeing if that triggers it. If that still doesn’t work, try restarting MacDive, and seeing if it updates then?

A bunch of this stuff is cached internally for performance reasons, it’s possible it just isn’t refreshing correctly in this case. I’ll look in to it.

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