The EC155 requires a barometer for several reasons. First, the EC155 calculates CO2 as a concentration, and these values must be multiplied by the air density to get the CO2 flux. Second, the closed-path analyzer measures the number of CO2 molecules in the path and converts them to a concentration. This conversion requires a measure of sample cell pressure, which is the sum of the barometer and differential sensor. Finally, the EC155 has been calibrated at the factory over a range of barometric pressures.