Tracking SAC / RMV without air integration

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d^2b

Worse diving through photography
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Recently I've been tracking my SAC with subsurface. I download dives from my Shearwater Peregrine, so all I need to to record is starting and ending PSI (and what kind of tank). I record the starting pressure on the boat, since checking the pressure is part of my pre-dive routine. In warm locations with a compressor on the boat, that typically means a drop of around 300-500 psi when I hit the water. I guess it doesn't matter since it makes the SAC more conservative for planning purposes (if I actually did that...), but it bugs me in some small way. Do people bother trying to correct for that e.g. by recording in-water pressure, or does it just not matter that much?
 
I don't worry about starting and end pressure for the whole dive. If I am going to check my rate, I will do so at depth on about a 10 min swim.
 
Your SAC is going to vary a little on each dive due to conditions. Over time you will get a good sense of a starting point for calculations. In planning, you will want to round to the more conservative anyway.

I disagree with the 10 minute swim concept, because that is an artificial construct that may or may not reflect the full dive with normal diving practices. You will be consciously thinking about your SAC throughout those 10 minutes, and that will alter the way you dive--your attempt to measure it changes it. I think looking at the full dive (as you are doing) is more accurate, especially if you follow the trend over multiple dives.
 
I track entire dive though if the bottle hasn't stabilized then I will typically wait until I hit the bottom then write time current time and pressure.
I don't believe in SAC swims and am with @boulderjohn that you need to average the entire dive. All of my students calculate their sac on every dive based on start time/pressure vs end time/pressure and are taught how to use it.
 
Giving things more thought....

I regularly dive in a huge sinkhole with a constant, year-round temperature of about 60° F. In summer, the air temperature can reach 100° F. In winter (as in last weekend), it can be in the 30s. That means we get temperature changes in our cylinders in both directions. I have not noticed much of a difference seasonally, although, as I indicated above, there is no point in getting too anal about your SAC since it varies by dive conditions.
 
Keep in mind, your RMV will be equipment dependent, too, probably. For example, diving a "baggy" tri-lam drysuit and a single 8" O.D. cylinder likely will net you a higher RMV than diving a 5 mil wetsuit and a single 6.9" O.D. cylinder--all other things being equal.

Bottom line: You probably will not have only the one RMV.

rx7diver
 
I have a AI pod on my rec set, so I do get those numbers as well, but I like to track what my actual breathing rate is. And yes I will do some tests where I am focusing on my breathing and seeing what I can do with that. I will also run tests kicking much harder than I reasonably would need to to get those numbers as well. If you understand the purpose of those tests you can use them. But if you are just taking them to get a score, then yeah you can game them and the results aren't really applicable to anything.
I definitely wouldn't use my lowest SAC rate to determine gas requirements for a dive.
 
Hi @d^2b

I'm with @boulderjohn and @tbone1004, measure your RMV for entire dives. After a good number of dives, you will be able to calculate an average RMV and the variation around that average. It is interesting to go back and look at dives or dive series that vary significantly from your average Average Gas Consumption

I have been diving AI since 2010 and my RMVs have been calculated with computer data since then. Before that, I used to write my start and end pressures on a small slate or wet notes. I would write down the start pressure soon after hitting the water and the end pressure upon surfacing.

I have never attempted to make any correction for a change in cylinder pressure due to temperature. My wife and I were recently in Bonaire and dived cylinders available on the pier right after they had been filled and were hot. There was a noticeable pressure drop right after hitting the mid-80 degree water. These cylinders would generally start at around 3200 psi and then drop, perhaps 100 psi, did not pay much attention. You would think this would adversely affect my RMV, the series of dives on this trip were the lowest average RMV I have had
 
I use the pre-dive pressure, no alteration for temperature. Knocking off 200 psi only changes my SAC by 0.04 cuft/min on the one dive I just checked (single AL80 tank). It's lost in the noise due to other variables (how much kicking, who my buddy is, how close the shark comes, and so forth).
 
Do people bother trying to correct for that e.g. by recording in-water pressure, or does it just not matter that much?
Doesn’t really matter that much. Unless you are using AI, you are probably dealing with rounded numbers as well. While my AI transmitter can tell me that I have 647 pa at the end of the dive, if I were using an SPG, I’d record that as either 600 or 700.

As long as you are consistent with what you record, it will work for you. Similar dives should give you roughly similar numbers. Higher than expected could be something to look into if there isn’t an obvious difference on that dive.
 
https://www.shearwater.com/products/teric/

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