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Altitude makes a difference when you are measuring CO2 levels. CO2 sensor modules are calibrated at sea level, and are not designed to automatically compensate for higher altitudes. CO2 sensors need either a manual offset using a barometer or lookup table for altitude compensation, or a built-in barometric pressure sensor to automatically compensate for the change. Altitude Compensation: Identifying the Problem A customer called recently and said we had sold him a defective CO2 sensor module. He had calibrated it precisely at 0.0ppm (zero parts-per-million), took it outside, and the sensor read 335ppm. “Where do you live?” we asked. “Colorado,” he...