# Dis-similar tank/size pressure and determining thirds...

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Posted by Daryl on August 31, 2001 at 17:35:31:

In Reply to: Volume matching posted by New Tech Guy on August 31, 2001 at 15:04:11:

If your question is concerning the above, i.e two sets of tanks in a buddy team with dis-similar capacity or fill pressures, I'll risk the flames and take a stab at it.

In an overhead environment, the Rule of Thirds or similar dive specific gas management plan is used to determine a 'turn" of the dive, i.e. exit the cave or wreck. (Flow, or lack there of, DPV's and other factors may dictate a more conservative plan, 1/4, 1/5, 1/6, etc.) Use of dis-similar tanks involves an equation to calculate actual gas volume of each set. Multiply by 2 of course for doubles.

The rated size of the tank divided by the rated pressure = X

X multiplied by actual fill PSI = Actual Volume Cubic Feet.

To determine Thirds,

Actual Volume Cubic Feet divided by 3 = Y

Actual Volume Cubic Feet subtract Y = Z

Z divided by X = Turn Presure (PSIG)

Dive is called on Thirds (or whatever) based on diver with least amount of gas at start.

You probably know how to calculate SAC and RMV and how to apply it to the above equations. If not, let me know.

I hope I typed this correctly. Been a long day!
Regards,
Daryl

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