The Altitude Compensation Needed for Gas Sensors

When measuring CO2 levels, altitude makes a difference. As CO2 sensor modules are calibrated at sea level, they are not manufactured to instantly compensate for higher altitudes.

When thinking about altitude, it is beneficial to visualize a vertical column of air rising to the outer limits of the atmosphere from the surface of the earth. Gas molecules present in air contain weight. At the earth’s surface, the weight is the heaviest but becomes lighter as it is measured higher up in the vertical column. The weight is zero at the highest point of the column.

Atmospheric Pressure

Scientists use the term ‘atmospheric pressure’ when discussing the ‘weight of air’. AP is commonly measured in hectopascals (hPa), inches of mercury (inHg), or millimeters of mercury (mmHg). At sea level, these are as follows:

  • 1,013.25 hPa
  • 29.921 inHg
  • 760 mmHg

The higher a measurement is taken in the atmosphere, each of these numbers become smaller. Therefore, to take into account atmospheric pressure when measuring CO2 levels, the atmospheric pressure at sea level must be divided by the atmospheric pressure where the CO2 measurement is taken.

The type of scale used does not matter as the solution will always be the same ratio. If this fraction is multiplied by the CO2 measurement at sea level, the new CO2 level at any altitude is found.

Application of CO2 Sensor

The following is an example of this calculation:

The city of Denver, Colorado, is around 5,000 feet above sea level. The atmospheric pressure is 632 mmHg (barometric pressure table) at this altitude. A CO2 sensor calibrated at 400 ppm at sea level would show a reading of 400(623/760) or 332 ppm.

A credible suggestion is that the CO2 level at altitude will adjust by around 3% for every 1,000 feet (300 m). Therefore, at 5,000 feet, this would be 15% less than 400 ppm, or around 340 ppm.

This kind of correction can have consequences in the real world. As an example, if a grower in Denver wished to maintain exactly 1,000 ppm CO2 in their greenhouse, they would need to set the sensor to 15% above 1,000 ppm, or around 1,150 ppm.

It is important to note that although CO2 sensor modules do not account for variations in altitude from sea level, certain CO2 meters have built-in barometers that can compensate this instantly. As such, those who work at high altitudes and require highly precise CO2 level readings should identify this feature before making a purchase.

This information has been sourced, reviewed and adapted from materials provided by GasLab.

For more information on this source, please visit GasLab.

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