Spreadsheet for helping pick a pony bottle

[scubadada]

I'm with @Diving Dubai, I make a very conservative calculation, knowing that there are many things I can do to decrease the amount of gas I use. However, I would rather have more gas than I need rather than too little. Having a buffer does take some of the emergency out of the emergency. I do the same calculation as used by @mi000ke

1) Emergency at 130 ft, the limit of my recreational diving
2) 1 minute at depth
3) An ascent rate of 30 ft/min to the safety stop and to the surface
4) A 3 minute safety stop
5) Gas consumption at twice my average RMV (I have my average RMV from my last 1200 dives)

Corrected spreadsheet, post #14, thanks @johndiver999

So, I chose a 19 cf pony. Using the above gas would leave me with about 425 psi from a full cylinder.

This is a simple calculation, anyone could make it using their own chosen variables.

 

[johndiver999]

1 min on bottom calculation needs a second look

Edit. Think there are other issues as well

 

[wetb4igetinthewater]

1 min on bottom calculation needs a second look

Edit. Think there are other issues as well

Such as? I’m looking at doing a second spin

 

[scubadada]

1 min on bottom calculation needs a second look

Edit. Think there are other issues as well

You are correct, here is the correct version of the spreadsheet, thanks you are quick

 

[johndiver999]

Looks better but why 72.5 feet?

 

[scubadada]

Looks better but why 72.5 feet?

Average depth between 130 and 15 feet, I often use 10 feet here rather than 15, makes it 70 ft

 

[johndiver999]

130 - 15 divided by 2 ??? Splitting hairs but if someone is following the logic

 

[scubadada]

130 - 15 divided by 2 ??? Splitting hairs but if someone is following the logic

(130-15)/2 +15

You ascend from 130 to 15 ft, 115 feet, half of that is 57.5 feet, but you are still 15 feet below the surface.

From the bottom 130-57.5=72.5
From the surface 15+57.5=72.5

 

[johndiver999]

I would have to think about the math but if you went
From 130 to surface the average depth is 65 feet. So you would multiply the ascent time by the sac by the average depth of 65 feet (after conversion of feet to atm). That is my understanding. Numerical difference is not large, but I’m looking at it conceptually.

 

[scubadada]

I would have to think about the math but if you went
From 130 to surface the average depth is 65 feet. So you would multiply the ascent time by the sac by the average depth of 65 feet (after conversion of feet to atm). That is my understanding. Numerical difference is not large, but I’m looking at it conceptually.

Yes, the average depth for a direct ascent to the surface from 130 ft is 65 ft. I believe we are on the same page