Bob-
We went through the butane/propane discussion once before, and my chemistry/physics teacher background has to respectfully disagree with you about the comparative btu output.
Here's an excerpt from an Airstream RV discussion forum which I'll
LINK here.
"If your Gas Connection to the regulator fits a standard LPG Liquefied Petroleum Gas) bottle then don't worry about it.
Actually, the BTU values per pound are very close. Propane delivers about 21,600 BTUs per pound, and Butane will put out about 21,300 BTUs. Not enough of a difference to argue in real life.
In the case of standard commercial cylinder (LPG) gas, what you often get is a "mixture" of Propane and Butane varied by area and season (and now, unfortunately, by availability). At atmospheric pressure Butane has a freezing point of 0ºc while Propane has a freezing point of -42ºc. When liquified gas approaches its freezing point, it ceases to vaporize effectively and the gas available to the Airstream is dramatically reduced.
If you have a heavy draw on the regulator (total gas demand maximized) in cold temperatures (winter) as the vapor is drawn off, it causes the remaining liquid in the tank to become colder, and, as the temperature falls, butane becomes less and less effective. Butane remains a liquid, which means it will not give off any gas vapor at 31 degrees F at atmospheric pressure, while Propane continues to perform at a higher level at the same temperature - it liquifies totally (stops gassifying) at -48 degrees F at atmospheric pressure.
Now, to quote one of my professors from the Missouri School of Mines and Metalurgy, "It is intuitively obvious that as evaporation cooling affects the temperature of the cylinder, Propane will maintain a more stable flow than Butane, all other factors being constant".
What the above means is that, under most cases, economics aside, Propane is a better fuel than Butane.
Holy Crap!!! been carrying around the books with those numbers in them since the dark ages in college (pre-computer) - and this is one of the first times I have been able to use those bits of information."
__________________
Dennis
"Suck it up, spend the bucks, do it right the first time."
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Joe.