Is Rule of Thirds incompatible with decompression diving?

[Subcooled]

I want to pose a physics problem where a tech diver attempts to obey rule of thirds.

(Reminder, physics problems take place in physics land, where there is no friction or air resistance, no changes in SAC, no current, no changes in swim speed, etc. )

So, the diver plans a square profile with staged decompression using a 3000psi Twinset of aluminum 80's. He immediately descends to 135ft and upon reaching depth, swims into a long horizontal cave for 20 minutes before he hits 2000psi. He then turns his dive around because he was told to always obey rule of thirds to be safe.

So he retraces his steps at exactly the same swim speed until reaching the ascent/descent line and starting decompression. But due to the decompression burden, he needs to use more gas for the total return trip than he used on the initial descent and penetration.

He therefore surfaces with far less than 1,000 psi.

How is it possible to obey rule of thirds in decompression dives?

If it isn't possible, then what application does Rule of Thirds have in technical diving?

I would apply the rule of thirds for bottom gas.

I would also have separate gas for the mandatory decompression stops and ascent time required for the longest possible bottom time. So, let's assume that instead of gas sharing you have muscle cramps and you need 3/3 of your gas at the bottom. How much deco gas do you and your buddy need then?

Some related thoughts
- Due to the low pressure of 3000 psi only, the nonlinear compressibility of the gasses does not affect your planning in any meaningful way. For higher pressures (above 3600) the amount of gas is not ~100% relative to the pressure.
- You carry the third of you buddys gas. Remember this, if your consumption and/or cylinder sizes differ.

 

[Subcooled]

From "I am not returning with 0 bar tanks"

Returning with 0 bar tanks is a near death experience. It is not pleasant. The thought will haunt you for years. That's why.

 

[boulderjohn]

Others have already made the points about the rule of thirds of an overhead environment only applies to the portion of your total gas needed for the overhead, and it is a best case plan, meaning it should be adjusted for any anticipated complications.

An additional point is to remember that not all thirds are created equal, and it is wise to think of your reserved third as time--the time you need to solve a problem. Obviously, a third of gas in an overfilled LP 108 steel twinset will provide a lot more time than a third of the gas in an AL 80 twinset. Subtract the amount needed to descend to descend to 135 and the amount needed for ascent from AL 80s, and you don't have a whole lot left. Figure out how fast you will be going through your gas at that depth and you will see that your 1/3 for emergencies is giving you precious little time to solve a problem--and that is your best case scenario.