Tanks sizes and pressures

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Insta-Gator
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Tanks sizes and pressures

Post by Insta-Gator »

I tried to find this via search but didn't, so if this is a duplicate question I appologize.

Under equipment I've added different tank profiles (material/size/working pressure)

When I go to the log, the new tank descriptions are not added to the drop-down menu selections. If I enter a tank description not available in the drop-down (i.e. steel ; 117 ; 3442) will the SAC be calculated properly?

It would be helpful to be able to edit the tank selection data to reflect the tanks we might actually use. But if the manual entry will work properly with other fields/calculations, I suppose manual entry will suffice.

Insta-Gator
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Re: Tanks sizes and pressures

Post by Insta-Gator »

Also, when I calculate my SAC cu/ft (RMV) for a dive it doesn't match the SAC cu/ft reported in DivingLog. Can you share your calculation for SAC that is presented diving log? Perhaps the cylinder constant figure used is different or doesnt' carry as many decimal points as I use?

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Re: Tanks sizes and pressures

Post by Insta-Gator »

While I have determined the discrepancy between your SAC calculation and mine, i would still like to see the equation you use.

What I've discovered is than in your drop-down window for tank size, if 80 is selected, the calcualtion is based on 80 cu/ft. However, an aluminum 80 is actually 77.4 cu/ft (not a menu option). Also, it appears your calculation is based on average depth, not max depth. Is the average depth based on the mean of minutes at any specific depth? How is the average derived?

I've been using these formula: http://www.ndc.noaa.gov/pdfs/AirRqmtFormulas.pdf

divinglog
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Re: Tanks sizes and pressures

Post by divinglog »

Hello

The equipment section is completely unrelated to tanks and SAC calculation. The tank size field is a free field, so you can enter any value you want, even when it is not in the list. For working pressure you have to enter it into the working pressure field. If you don't see it, use the "Extended Layout 2" from the layout tab. The SAC calculation is explained here:

https://www.divinglog.de/blog/?p=765

Dive time and avg. depth are calculated from the profile data (ignoring zero depth values).

Kind regards,
Sven

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Re: Tanks sizes and pressures

Post by Insta-Gator »

Thanks for the reply. Please not on the blog link, the calculation for imperial also shows meters/10+1 rather than feet/33+1

divinglog
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Re: Tanks sizes and pressures

Post by divinglog »

It doesn't matter for the end result and internally meters are used, because all data is stored in metric units.

whodunit68
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Re: Tanks sizes and pressures

Post by whodunit68 »

I know this is really old (almost a decade) but it was top on the list of 2 searches.

I'm wondering about SAC calculations. I read the linked blog post by Sven
divinglog wrote:...
https://www.divinglog.de/blog/?p=765
Dive time and avg. depth are calculated from the profile data (ignoring zero depth values)...
and each of the criteria are met such as:
  1. dive profile data available via Shearwater import
  • all individual tanks entered manually
  • data on tanks maintained properly.
However, the SAC values represented are bizarre and it *seems* far from accurate (I'm only providing that they are apparently way off but I'm not experience, hence my question.)
one example:
dive 498 had 2 individual sidemount tanks for a cave dive, both Steel 100 @ WP of 3442. I used one AL 80 stage bottle which was filled to 3300 (WP of 3000), 800 psi used. Dive time was 84 minutes.
DL says my SAC was 0.17 cu. ft/min.
Manually here is what I have (by the way, there is one parenthesis missing from the formula on the link):
ImperialSAC = ((TankPressureDiff / WorkingPressure) * TanksizeCuFt) / (Divetime * ((AvgDepthMeter / 10) + 1))
ImperialSAC = ((TankPressureDiff / WorkingPressure) * TanksizeCuFt) / (Divetime * ((AvgDepthMeter / 10) + 1))

3800 / 2600 = 1.215
3900 / 2700 = 1.215
3300 / 2500 (80) = 1.5

HP100s: ((1200/3442)*100) / ((84 * (49.07/10)+1))=34.86/(84*5.91) = 2.45/2 (for 2 bottles) = 1.215
AL80: ((800/3000) * 80) / ((84 * (49.07/10)+1))=21.33/(84*5.91) = 1.50 (assuming 80 cu ft though really only ~77.)

The unfortunate reality is that the SAC for this dive was awful but that's not the point of the question. I'm not sure where to take it from here though with some concentrated though I could probably figure it out. Any ideas what the differential is?
By the way, this was just an example. I notice on almost every dive it appears that the "calculated" SAC by DL is significantly different than what I believe it should be (and of course only where I maintain the tanks, dive profile data, etc.)

Any ideas/help/suggestions are appreciated.

divinglog
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Re: Tanks sizes and pressures

Post by divinglog »

Have you entered the tank switches into the profile? If not, Diving Log cannot calculate the dive time for each tank. Not the overall dive time is used for multi tank calculations, it is the dive time per tank. Right click into the profile and select "Add gas switch" and enter your tank switches at the correct location in the profile, if you've not done yet.

Also in your manual calculation, you cannot use 84 minutes for the whole dive, just use the dive time of each tank (you hadn't used the HP100s for 84 min?)

whodunit68
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Re: Tanks sizes and pressures

Post by whodunit68 »

you are correct, the tank switches weren't, and generally aren't, entered and could explain the consistent difference I see. Right about the manual calculation....thanks for the tips. Probably explains and answers the question I had.

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