Did you know that the calibration of carbon dioxide sensors is affected by changes in altitude?


The carbon dioxide sensors available on the market today are calibrated for operation at sea level. As the altitude increases, air density (and the number of carbon dioxide molecules) decreases in accordance with the ideal gas laws. Since the sensor is calibrated at standard barometric pressure it "sees" fewer carbon dioxide molecules at reduced pressures and therefore outputs a signal corresponding to the reduced number of molecules. This signal will indicate a lower carbon dioxide level than is actually present.

For example, assume a CO2 sensor (calibrated at sea level) operating at 3000 feet elevation reads 1000 ppm. Barometric pressure, and therefore air density, at this elevation is approximately 10% lower than the sensor's calibrated conditions. This means the sensor reading will be 10% lower than the actual CO2 level. The CO2 level will be about 1100 ppm, not 1000 ppm as indicated by the sensor.

This correction is for the physics associated with the ideal gas laws. Some types of infrared CO2 sensors (such as the 2001 Series) utilize a pyroelectric element which is also
affected by changes in pressure, so as the pressure decreases, corrections must also be made for the change in sensor performance. The chart on the opposite page shows the altitude correction factors for the carbon dioxide sensors offered by Kele .

To utilize this chart, simply multiply the sensor's indicated CO2 level by the correction factor selected from the chart.


by Steve Carney