(Warning - long post filled with some wild speculation)
Okay, a brief experiment with 1 chimney of charcoal and no meat in the cooker shows that I was able to maintain dome temps ~250 with a completely empty pan. Used basically the Minion Method and made sure to clamp down on temps before they got out of hand.
What prompted this whole thought process was really a term I've seen many use - "thermal mass". According to the definition I could find, thermal mass is mass (weight) times the specific heat of the material. It basically measures how much heat an object can take up and give off.
One gallon of water = 8 lbs. at a specific heat of 1.00. Thermal mass = 8.
Sand has a specific heat of about 0.3 according to my research. I don't know how much sand you put in the pan, but I doubt that it is near the 26 lbs. you'd need to accomplish a thermal mass of 1 gallon of water.
In addition, water has this nifty property called "heat of vaporization". Basically, it takes a certain amount of BTU's to raise the temp of water to 212, and then a lot more to actually get the stuff to boil. So from a heat absorption viewpoint, water has sand beat by a ton.
But sand works and works well according to numerous posts. That makes me think that the only real role that the pan plays is as a heat shield or deflector so that the meat is cooked via heated air (convection) instead of direct heat from the coals (radiant energy).
Sand would help keep from burning the bottom of the pan out, but I just don't see how it can suck up enough energy to make any difference when you're comparing it to the BTU output of 15 lbs. of Kingsford.
Well, my plan is to try a few long running cooks with an empty pan to see how difficult temperature control becomes. My theory is that if I keep the fire under control from the beginning, the empty pan shouldn't be an issue.
I am not an expert in any of this and someone who understands thermodynamics might be able to clear up the theoretical side. And I sure as heck don't have the practical pit experience that most of you have. I just want to try to understand what's really going on inside that beautiful black bullet.