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 in older EC155 models (serial numbers less than 2000) is the sum of the barometer and differential sensor. In newer EC155 models (serial numbers 2000 or greater), this conversion still requires a measure of sample cell pressure, but it is done with an absolute pressure sensor in the sample cell. Finally, the EC155 has been calibrated at the factory over a range of barometric pressures.