D_Hall wrote:I actually wasn't too concerned with day to day barometric changes as much as I was concerned with altitude-based barometric changes. I mean, just as an example I live at 2300 ft which is still pretty damned low altitude, but ambient pressure is nominally down 10%. Thus, if I use sea level calculations, I'm going to be fuel rich to the tune of about 10% more fuel than I'd planned on.
That is a valid concern. Knowing the base pressure, you can calculate the gas mass for your elevation. The calculation based on pressure rise would cause the most variation due to elevation at low chamber mix. At higher mix pressures, the base atmospheric pressure drops in significance.
A 4% rise for a 1X mix would have the 10%, but at a 5X mix it would drop to about 1/5th that or aprox 2%.
The compensation can be done with a calculator. The metering you have is gauge. The mix is on absolute. 1 ATM is the " base pressure, you calculat the pressure rise due to the addition of the fuel. At elevation if your absolute base pressure is down 10%, then the new pressure is the new gauge zero. The rise will simply be recalculated based on the new mass in the chamber, so yes for a proper calculated mix, the rise from adding fuel will need to be less.
A solution is to purchase a guage that reads absolute and work from there. Lacking the money for that, a regular guage can be installed in a bell jar to read absolute instead of guage. Working from a known zero refrence (vacuum) for both air and fuel makes metering more accurate. Using this method, the cannon total gas mass can be repeated with high accuracy for even combustion power at any elevation. Use a manometer on the bell jar to ensure it is pumped down to less than 0.05 atm. Going further is in the noise level.
Further reading on absolute pressure measurements is here.