Quick Reference: SAC & RMV Chart

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I brute-forced my way into learning/reinforcing the math for cuft>>>psi conversions over the course of about 5 hours yesterday evening. I initially figured it out by cross-multiplication, but good friend and good buddy @Cr.padlo helped me refactor it into an equation that morphs into an Excel-friendly function.

For people (like me!) who have to see the math in action to understand it, here's how to solve for the cell at the intersection of 20 ft (1.6 ATA) and 0.4 cfm SAC for LP85 doubles.


Cross-multiplication way:
eunp0iY.jpg


Variable way:
zILCjht.jpg

  • Cc is cubic feet consumed (known value)
  • Pr is the working pressure of the tanks
  • Tr is the tank's rated capacity, in this case x2 for doubles;
  • Pc is psi consumed

To replicate the variable way in Excel, use =ROUNDUP(SUM(B2*(2640/170)),0) -- replace the B2 reference with whatever cell holds your Cc value. The inner =SUM formula does the conversion math and the outer =ROUNDUP(..., 0) makes the number pretty.

After a lot of find & replace batch formula editing, I derived PSI charts for a handful of different tanks that I or buddies are likely to be using.

Click here to see the current version of the chart.
 
@dewdropsonrosa one thing to keep in mind is the Z-factor for gas compressibility. At 2640 it's fine, but your psi/min consumption is higher in the first third by about 5% than it is in the second third and that's where using tenths instead of hundredths is important. You also only have so precise of resolution in the gauge itself.
 
Out of curiosity why have it in freshwater mode? Advantage you know your "true" depth disadvantage you have a harder calculation to work out ambient pressure which is useful for many things. (like sac/rmv calculations)
 
If you leave your computer set to saltwater it shouldn't be a problem no?

Out of curiosity why have it in freshwater mode? Advantage you know your "true" depth disadvantage you have a harder calculation to work out ambient pressure which is useful for many things. (like sac/rmv calculations)

It's really not that critical since everything is based on pressure and the depth gauges aren't that precise in recreational depths. I leave mine set for freshwater when I end up diving in the ocean, it's just not that critical
 
Brought up under British Imperial system but when volume is involved!!!! Hate that. Log table, slide rule until the appearance of calculator.
A lot easier and simpler in metric.

In our food market 3 different weighting systems are mingling with each other!!!! Chinese(catty), Imperial and Metric!

Catty - Wikipedia


I am still telling people that I weight 9 stones 4!
 
Extra rows and digits are a nice distraction from the howling black void of being done with grad school but not having heard back about my most recent job interview yet!

Non-scuba update:
I just accepted a job offer yesterday! Very excited to have a meaningful dive budget again.
 
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