Dive computer-dumb question

[dianna912]

My husband and I are arguing over the way the dive log works on our dive computers. (Oceanic Atom 3.1 if that matters.) I think that the log shows the air left on that dive when the diver gets back on the boat. The true air left in that tank. He thinks that the actual log (not what displays on the computer, but what goes into the log) shows the air consumed only during the dive itself, irrespective of any surface swim. Who is right?

 

[Seaweed Doc]

Does your software have a way to display air pressure remaining throughout the dive? (My Shearwater software does.) Looking at that graph should tell you. Then compare the number to "what goes in the log."

Or do you mean what you write in a paper log book? In that case, I'd record the air left after you no longer needed it. If you chose breathing off the tank to get back on the boat, you needed the air. If you decided to do a weight check after the dive and burned your tank down to 500 psi by purging, I'd write what you surfaced with.

On the other hand, why not record BOTH?

 

[Belzelbub]

My husband and I are arguing over the way the dive log works on our dive computers. (Oceanic Atom 3.1 if that matters.) I think that the log shows the air left on that dive when the diver gets back on the boat. The true air left in that tank. He thinks that the actual log (not what displays on the computer, but what goes into the log) shows the air consumed only during the dive itself, irrespective of any surface swim. Who is right?

It’s possible that it may display both, in a way. If the log displays RMV, or SAC, that is a measure of consumption. Not true consumption, as it is normalized to surface pressure.

I never actually looked at the log on my Oceanic computer, so I’m not positive what it shows. My Perdix AI shows start and end pressure along with SAC.

 

[scubadada]

Hi @dianna912

I believe your husband is correct. I have many, many dives logged on an Oceanic VT3. All the log entries look just like this, the gas use ends upon surfacing. It is that gas use that is used in the calculation of your RMV, as is appropriate. The small red area on the dive profile is a short period of decompression.

 

[Belzelbub]

Hi @dianna912

I believe your husband is correct. I have many, many dives logged on an Oceanic VT3. All the log entries look just like this, the gas use ends upon surfacing. It is that gas use that is used in the calculation of your RMV, as is appropriate. The small red area on the dive profile is a short period of decompression.

View attachment 660949 View attachment 660950

Apart from SAC and RMV, where do you see gas use on the graph? I interpreted her question as she was saying the 1180 PSI is what was left in the tank, and her husband was saying that was what was used on the dive. In your example, actual use during the dive would have been 1920 PSI.

Just re-read her post. I say neither are correct the way it’s worded. Air left in the tank when the diver reaches the surface is what is displayed in the log. A surface swim would only be included if the diver descended enough after the surface swim provided the end dive time wasn’t exceeded during the swim. Maybe that’s what the OP meant in what her husband thought, but I read it to be PSI consumed, not left.

 

[scubadada]

@Belzelbub

Gas pressure is the blue line, superimposed on the dive profile. Gas use ends on surfacing, yes that is what is used in the SAC and RMV calculations, 1920 psi in my example (start gas pressure - end gas pressure).

The dive itself does not end until 10 min after surfacing. If you descend again, within that time, it will be counted as a singe dive, rather than 2 dives.

Edit: @EFX was posting at the same time I was. My Shearwater Teric captures the same start and end pressure that my Oceanic VT3 does. Calculation of the SAC is identical. Shearwater does not automatically calculate RMV, as cylinder characteristics are not captured. RMV is calculated from SAC by multiplying by the tank factor (vol/pressure, cu ft/psi for imperial)

My VT3 has a default end dive time of 10 min. My Teric is adjustable, between 10 seconds and 10 min, I have it set at 10 min to match the Oceanic. You can end the dive earlier with a button push when a prompt comes up on the screen. I don't think I've ever found this in the owner's manual, but it is intuitively obvious.

 

[EFX]

He thinks that the actual log (not what displays on the computer, but what goes into the log) shows the air consumed only during the dive itself, irrespective of any surface swim. Who is right?

the gas use ends upon surfacing.

I believe the computer logs only the data recorded during the dive and it keeps recording data until the dive ends. So, the question is: when does the dive end? I haven't read the Oceanic computer manual but I can tell you how the Shearwater Perdix behaves. There are two conditions for the end of the dive: (1) a return to the surface which is confirmed by matching the surface pressure with a stored surface pressure taken before the dive began. Once these pressures are matched (within a narrow window) then (2) a timer is started whose elapsed time is compared with a preset value set by the user. When the values match the dive "officially" ends and data recording stops. I believe that most computers set that time to 1 minute as the default. This means the dive will end 1 minute after surfacing and logging will stop. On my Perdix I've set the dive end time to be 2 minutes. This is because sometimes my buddy and I need to surface, before the dive ends, to briefly exchange information and I don't want that very short SI to end the dive. Read your manual to see how this works for your computer.

 

[dmaziuk]

What @EFX said: normally dive "starts" when you descend some 5-10 feet and "ends" a couple of minutes after surfacing. So if you get back on the boat within a couple of minutes of surfacing, you'll both be right, if your surface swim is longer than that: only your husband is. If you breathe off your reg during a surface swim before the "dive starts", that won't be logged either.

 

[dianna912]

@Belzelbub

Gas pressure is the blue line, superimposed on the dive profile. Gas use ends on surfacing, yes that is what is used in the SAC and RMV calculations, 1920 psi in my example (start gas pressure - end gas pressure).

The dive itself does not end until 10 min after surfacing. If you descend again, within that time, it will be counted as a singe dive, rather than 2 dives.

Edit: @EFX was posting at the same time I was. My Shearwater Teric captures the same start and end pressure that my Oceanic VT3 does. Calculation of the SAC is identical. Shearwater does not automatically calculate RMV, as cylinder characteristics are not captured. RMV is calculated from SAC by multiplying by the tank factor (vol/pressure, cu ft/psi for imperial)

My VT3 has a default end dive time of 10 min. My Teric is adjustable, between 10 seconds and 10 min, I have it set at 10 min to match the Oceanic. You can end the dive earlier with a button push when a prompt comes up on the screen. I don't think I've ever found this in the owner's manual, but it is intuitively obvious.

Okay, so with the Oceanic, we have the same 10 minute setting. Am I understanding you correctly, though, that if we dive a normal dive, the surface with for example, a 5 minute surface swim, and never descend again, the end pressure will be based solely on that moment we surfaced, regardless of the surface swim? But if we descended a second time within the 10 minutes, the end pressure would reflect the second time we surface?

That’s how your log looks to me. (I haven’t looked at ours on a computer in a very long time.) Sounds like he’ll be glad to hear he is right. I guess it’s a good idea to let it happen every once in a while.

 

[Belzelbub]

Okay, so with the Oceanic, we have the same 10 minute setting. Am I understanding you correctly, though, that if we dive a normal dive, the surface with for example, a 5 minute surface swim, and never descend again, the end pressure will be based solely on that moment we surfaced, regardless of the surface swim? But if we descended a second time within the 10 minutes, the end pressure would reflect the second time we surface?

That’s how your log looks to me. (I haven’t looked at ours on a computer in a very long time.) Sounds like he’ll be glad to hear he is right. I guess it’s a good idea to let it happen every once in a while.

That’s correct. Once on the surface, any gas consumed on the swim won’t be counted when the dive is logged, provided you don’t go deep enough to be still diving. If you do, then it’s all counted and the end pressure from the next time you surface is logged.