Tank calculations

[Kevrumbo]

Did a little reading and this is all pretty straight forward for me now.

With imperial measurements you have the total volume at the service pressure. With the metric measurements you have the volume of water and this happens to be the volume of air at 1 bar.

So when I try to calculate my SAC rate with imperial measurements I do the following:

- Go to 33 feet
- Note my PSI
- Dive, swim, etc. while staying at 33 feet for 10 minutes
- Note my PSI
- Take the difference in PSI and divide by 10 to get my PSI/minute
- Convert to 1 ATA (the dive was at 2 ATA) by dividing by 2.
- Convert PSI to cubic feet

The last step requires me to know the cubic feet at service pressure plus the service pressure (e.g. HP100 is 99.5 cubic feet at 3442 PSI). This is where the real math comes in. For example, if I calculated that I use 25 PSI/minute then 99.5 cubic feet / 3442 PSI = 0.03 cubic feet/PSI then 25 PSI = 0.75 cubic feet / minute.

Now if I did the same thing in metric it would be:

- Go to 10 metres
- Note my bar
- Dive, swim, etc. while staying at 10 metres for 10 minutes
- Note my bar
- Take the difference in Bar and divide by 10 to get my bar/minute
- Convert to 1 ATA (the dive was at 2 ATA) by dividing by 2.
- Convert bar to litres

The last step requires me to know the size of the cylinder. For example, my HP100 is 12.2 litres. So if I used 1.7 bar I would take the bar used times the size of the cylinder or 1.7 * 12.2 or 20.74 litres/minute. The math seems to be a little easier because there is not converting PSI to cubic feet.

Hey, I think I got it. If I have 20.74 litres / minute and 1 litre = 0.03666 cubic feet then I have 0.76 cubic feet / minute. With the occasional rounding error, I think that is pretty close.

To take it a little further...

On my HP100 at 99 feet I would breath 0.7 * 4 cubic feet / minute or 2.8 cubic feet / minute. If the cylinder is 99.5 cubic feet it would last approximately 35 minutes.

Using metric, my HP100 at 30 metres I would breath 20.74 * 4 litres / minute or 83 litres / minute. If the cylinder is 12.2 litres and I started with 230 bar I would have 2806 litres. At 83 litres / minute it would last ~34 minutes.

I now see what V&G was doing. I just remember my RMV at the surface and the volume of air in the cylinder. I then calculate the usage at depth by multiplying by the atmospheres at that depth.

It is interesting to see when dealing with litres and bar the numbers are so small that rounding errors can built significantly. With an analogue gauge it would be really hard to figure out what your SAC rate was in 10 minutes.

Make it more applicable . . .remember your "gas gauge" --the SPG-- is in bar units using the metric system.

Your Surface Consumption Rate in bar units: 1.7 bar/min*ATA
(That comes from dividing your RMV of 20.74 litres/min*ATA by your tank rating of 12.2 litres/bar. 20.74-divided-by-12.2 equals 1.7bar.)

1.7 bar/min*ATA multiplied by 10 minutes is 17 bar/ATA (consumed on the surface, an arbitrary interval of nominally non-stressed breathing for 10 minutes for example).

But 17bar/ATA at 30 meters depth for 10 minutes: you've now nominally consumed 68 bar of gas. (That result comes from multiplying 17bar/ATA by 4 ATA [30 meters depth is 4 ATA]; 17 times 4 equals 68)

So for 20 minutes at 30 meters: you've consumed 136 bar of gas.
(That's 68 times 2 equals 136)

And for 30 minutes at 30 meters: you will expect to consume 204 bar of gas.
(That's 68 times 3 equals 204).

You had a fill of 230 bar in your tank, so 30 minutes bottom time at 30 meters is really pushing it with regards to gas planning for decompression requirements, and an emergency contingency reserve amount to get you and a buddy back to the surface while gas sharing.

 

[USMCNJ]

i remember all this from my physics portion of divemaster...... ouch my head still hurts!!!

 

[scubadiver888]

Make it more applicable . . .remember your "gas gauge" --the SPG-- is in bar units using the metric system.

Your Surface Consumption Rate in bar units: 1.7 bar/min*ATA
(That comes from dividing your RMV of 20.74 litres/min*ATA by your tank rating of 12.2 litres/bar. 20.74-divided-by-12.2 equals 1.7bar.)

1.7 bar/min*ATA multiplied by 10 minutes is 17 bar/ATA (consumed on the surface, an arbitrary interval of nominally non-stressed breathing for 10 minutes for example).

But 17bar/ATA at 30 meters depth for 10 minutes: you've now nominally consumed 68 bar of gas. (That result comes from multiplying 17bar/ATA by 4 ATA [30 meters depth is 4 ATA]; 17 times 4 equals 68)

So for 20 minutes at 30 meters: you've consumed 136 bar of gas.
(That's 68 times 2 equals 136)

And for 30 minutes at 30 meters: you will expect to consume 204 bar of gas.
(That's 68 times 3 equals 204).

You had a fill of 230 bar in your tank, so 30 minutes bottom time at 30 meters is really pushing it with regards to gas planning for decompression requirements, and an emergency contingency reserve amount to get you and a buddy back to the surface while gas sharing.

Yes, absolutely. My calculation were solely based on how long the air would last at depth. There was no consideration for staying within NDL or having additional gas to do deco, ascend to the surface or handle emergency situations. If I used all my gas on the bottom I'd have to do a CESA or rely on my buddy. Not a good plan. Proper plan would be calculating how much air you use for:

- Ascending at a rate of 1 foot/second (do metric divers use 0.3m/second?) from depth to safety stop
- Time spent at safety stop
- Ascending the final 15 feet
- Double it in case your buddy is on your octo (OOA situation)

Or if using rule of thirds, calculate your personal gas consumption and multiple by 1.5 (keeps 1/3 in reserve for emergency).

i remember all this from my physics portion of divemaster...... ouch my head still hurts!!!

I like this sort of stuff but then I'm a professional teacher who learned to scuba dive.

 

[knowone]

It may be great to have 500psi in a 120cf tank.

But NOT so great to have 500psi in a 63cf tank.

 

[Jax]

I'm looking for the Gas Management 101 .pdf, but here's a couple to further confuse the issue.

NWGratefulDiver.com

http://www.ndc.noaa.gov/pdfs/AirRqmtFormulas.pdf

EDIT: Found it! This is the very best explanation of the metric and English calculations:

http://decodoppler.wordpress.com/2008/11/25/gas-planning-101-sacrmv-and-ways-to-make-sure-you-have-enough-gas-to-complete-your-dive/

 

[scubadiver888]

I'm looking for the Gas Management 101 .pdf, but here's a couple to further confuse the issue.

NWGratefulDiver.com

http://www.ndc.noaa.gov/pdfs/AirRqmtFormulas.pdf

EDIT: Found it! This is the very best explanation of the metric and English calculations:

Gas Planning 101: SAC/RMV and ways to make sure you have enough gas to complete your dive Doppler's Tech Diving Blog

Thanks for finding that last quote. I just started reading Steve Lewis' blog last month. Haven't gotten back to 2008 yet.

He has a part 2 at Gas Planning 101: Continued Doppler's Tech Diving Blog.

 

[Doppler]

EDIT: Found it! This is the very best explanation of the metric and English calculations:

Gas Planning 101: SAC/RMV and ways to make sure you have enough gas to complete your dive Doppler's Tech Diving Blog

Thank you Jax.... have a happy holiday and yet another eye-catching avatar!