Minimum Gas chart for quick reference

[Ulfhedinn]

Aye this is based on GUEs mins gas calculations that Lorenzoid mentioned. Agree with maybe I should have put this in DIR forum but thought it might be helpful to new divers who are trying to get that safety margin in. And as @rob.mwpropane mentioned I think it wise to always talk it over and understand why and not just follow a chart.

@tursiops ("CAT" formula--Consumption x Avg depth in atm x Time) that Consumption of each diver is 0.75 cfm. If any calculation result is less than 500 psi, then GUE says to use 500 psi as the minimum.

 

[tursiops]

Aye this is based on GUEs mins gas calculations that Lorenzoid mentioned. Agree with maybe I should have put this in DIR forum but thought it might be helpful to new divers who are trying to get that safety margin in. And as rob mentioned I think it wise to always talk it over and understand why and not just follow a chart.

@tursiops ("CAT" formula--Consumption x Avg depth in atm x Time) that Consumption of each diver is 0.75 cfm. If any calculation result is less than 500 psi, then GUE says to use 500 psi as the minimum.

I'm asking how you calculate time....

 

[Ulfhedinn]

Lets use 100 on AL 80s

1.5X2.5x11
1.5 CFM
2.5 Average depth in ATM
11 time to accent to with average speed of 10ft/min +1
41.25CF = MG Volume
41.25/2.5x100=1650
1700PSI round off for scuba math

 

[tursiops]

Lets use 100 on AL 80s

1.5X2.5x11
1.5 CFM
2.5 Average depth in ATM
11 time to accent to with average speed of 10ft/min +1
41.25CF = MG Volume
1700PSI round off for scuba math

Thanks. So you are NOT using 30 ft/min in the deeper part of your profile?
And you are not considering that the 1 min at depth is at Full ATA, not the ATA at average depth?

 

[Ulfhedinn]

The CAT formula is intended only to be a conservative way of easily calculating minimum gas, and does not mirror the minimum decompression ascent profile perfectly.
Quoted from my notes.

I think UTD does use the 30ft/min idea but not GUE. I might be wrong with UTD... its been a moment.

The cool thing about this is.. It made me go back and look through all my old notes from years ago.

 

[Ulfhedinn]

Also from my old notes..

The ascent strategy in a gas sharing contingency (after 1 minute at operational depth to problem solve) is a 30 second hold/30 seconds to ascend for every delta 10'/3m from operational depth -for an ascent rate of 10 fpm

 

[tursiops]

@Ulfhedinn
One reason my calculations don't match yours is you are using too many roundoffs., especially your tank factors. Best to keep a few decimal places, then only round off once, at the end.....and only then round up to the nearest larger 100 psi.

 

[Ulfhedinn]

@Ulfhedinn
One reason my calculations don't match yours is you are using too many roundoffs., especially your tank factors. Best to keep a few decimal places, then only round off once, at the end.....and only then round up to the nearest larger 100 psi.

I hear ya.. just rounding off.. up.. for simple math in my head. I appreciate this though. Thanks for the reply's!

 

[tursiops]

I hear ya.. just rounding off.. up.. for simple math in my head. I appreciate this though. Thanks for the reply's!

You don't/shouldn't need to use tank factors -- for your calculation in your head -- when you are making a chart. And you don't need to round off more than once....at the end. For example, where you use 41.25/2.5x100=1650, you'd be better off using 41.25/77.4x3000=1599, and that row would become 1600 rather than 1700.

 

[tursiops]

Here is a calculation with no roundoff, but the same assumptions (C=1.5, 10 ft/min all the way, 1 min at the bottom NOT calculated at bottom pressure but rather at avg pressure); definitely not the same as yours!

If one (correctly) assumes that the one minute spent at depth before ascending is using gas at the ATA of the bottom -- not the ATA of the average depth -- then you get a different table: