Physics of diving question...

[JointVentureDive]

Arcflash.   First I apologize if you think my post was intended to insult you.  If you knew me,  you would know that I would never try to insult you or anyone.   I have been around divers for over 12 years or slightly more than half my life.    I been on lots of dive trips, and boats, with great divers and “jerks.”

I’ve had plenty of times where I could have insulted someone for just cause and did not.    I have never been told that I have tried to insult any diver.  If anything, I am one of the most complimentary and congenial persons you will meet. Hopefully over time you too will agree.  So before I go off and cry, I think I will post the following:

I must admit the key argument in your dispute, is that I missed the word BEFORE  OOA.    I had just read a complex string of comments herein, from Smith and Dectorguy.   

HOWEVER, your scenario led me in a different and to me perhaps a logical and sequential set of an assumptive path that  I would have taken, and done. 

Let me explain.  In the scenario , when you indicated that you "know you are going to have to violate a compartment"  which is what you said.....

and I understand that you indicated under NORMAL conditions you know your rate of Consumption of air. BUT.. MOST PEOPLE WOULD REACT TO THE 20 MIN.of air Estimate,  28 MIN DECO TIME DIFFERENTLY. Chances are your entire situation and breathing RATE by default, will soon change for the worst.
Nevertheless,
IT TELLS ME THAT YOU ARE ASSUMING YOU ARE GOING TO BE OUT OF AIR, Because what else should I assume, if you know one of the two compartments that you listed are going to cause a decompression hit.   OTHERWISE, you would not have to be considering the scenario you did.  IE..   
Just for example.
Quick to 30ft then very slow - bend arm
very slow to 30, then quick - bend spinego up fast, 
[/quote]

In either case, if you are going to violate anything, arm,spin, etc.  whatever,   YOU ARE VIOLATING EVERYTHING   that was not planned.  It doesn't matter what compartment.  You don’t have enough gas to offgas, and as such, you are OOA.   
Basically, the scenario described is not your planned dive.   
The safest alternative is  to stay at 15 feet for as long as you can. Almost deplete all air, and Surface and get help with or from another diver.  Which is what I posted. 

In an ideal emergency (if you can call it that)  you may be able to get some  air with another diver get back into the water with a rescue diver together, and off gas to proper deco.   Of course, if you have to surface,  suck on O2;  get 911 going;   pure O2deal works wonders.
That’s all I was trying to say.  Hope your still my friend.  Also, my gills are cold  : but they still can't make  O2.    Milk shakes anyone!
Nicki.