PADI DSAT Gas Matching Formula Question Please Help

[tomcatbubba]

Hi all, I'm trying to figure out why PADI teaches the Gas Matching formulas a certain way, so if anyone can help with one of the nuances of their formula it would be appreciated.

I completely understand all the calculations and why to do them, except for the actual turn pressure calculation. Why on earth do you divide the final calculated reserve pressure by 2 then subtract from the working pressure of the tank to get the turn pressure? What is the significance of the number 2? By my calculations this takes the reserve volume required for reserve and divides it in half. Why do this?

Thanks in advance. I've emailed PADI as well but no luck yet. Trying to know exactly why we do this when teaching the class (eventually).

Thanks.

 

[Kevrumbo]

Sounds like usable gas Rule-of-Halves: first you calculate your Minimum Gas Reserve/"Rock Bottom" pressure value with a nominal ascent strategy to get you and a buddy in a gas share contingency from operational depth to the surface. Then take this MGR value and subtract it from the initial SPG value you expect to read at reaching operational depth. Finally take this difference and divide-it-by two, and you get your usable gas Turn Pressure delta value for Rule-of-Halves.

For example: Your usable gas for the dive is your starting pressure subtracted by the MGR/Rock Bottom Reserve --so a full tank at 200 bar minus 60 bar MGR equals 140 bar usable. Let's use 20 bar of this 140 to get squared away in good trim & buoyancy on the descent to 18meters which leaves 120 bar usable. Now if your dive plan calls for returning to near the vicinity of your original point of entry (like a beach dive for instance), then turn the dive back around when you use half of 120 bar or 60 bar consumed [Rule of Halves]. If at any time at 18meters you read "60 bar" remaining on your SPG, you are at MGR and should begin a normal ascent to the shallows, safety stop and eventual ascent to surface -or if your buddy's tank neck o-ring happens to blow at that exact moment, you know you've got enough gas to get both of you with a safe & sane ascent rate to the surface. . .

How To Calculate Minimum Gas Requirements

 

[rjack321]

I have no idea what they are teaching or trying to explain, but curious what they are teaching.