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AN131 – CO2 Sensor Calibration: What You Need to Know
All carbon dioxide sensors need calibration. Depending on the application, this can be accomplished by calibrating thesensor to a known gas, or using the automatic baseline calibration (ABC) method. Both have pros and cons you shouldknow.
Why Calibrate?
Most of our products use non‐dispersive infrared (NDIR)
carbon dioxide sensors. These rely on an infrared light sourceand detector to measure the number of CO2 molecules in thesample gas between them. Over many years, both the lightsource and the detector deteriorate, resulting in slightly
lower CO2 molecule counts.
To combat sensor drift, during calibration a sensor is exposed
to a known gas source, multiple readings are taken, anaverage is calculated, and the difference between the newreading and the original reading when the sensor wasoriginally calibrated at the factory is stored in EPROM
memory. This “offset” value is then automatically added orsubtracted to any subsequent readings taken by the sensor
during use.
Calibration Using Nitrogen
The most accurate method of CO2 sensor calibration is toexpose it to a known gas (typically 100% nitrogen) in order toduplicate the conditions under which the sensor wasoriginally calibrated at the factory. ISO certification isavailable on our high‐end sensors to verify this process for
medical or scientific purposes. Nitrogen calibration is also
required if CO2 levels between 0‐400 ppm will be measured.
The problem with calibrating using nitrogen is the expense. A
sealed calibration enclosure, a tank of pure nitrogen, andcalibration software is required to match the original factory
testing environment. Otherwise, the accuracy of thecalibration cannot be ensured.
Calibration Using Fresh Air
Where maximum accuracy is less important than cost, a CO2
sensor can be calibrated in fresh air. Instead of calibrating at0ppm CO2 (nitrogen), the sensor is calibrated at 400ppm CO2(outdoor air is actually 390ppm), then 400 ppm is subtracted
from the newly calculated offset value.
Fresh air calibration is best for sensors in manufacturing
settings or greenhouses where the sensor is constantly
exposed to different CO2 levels. It should also be used inproducts like the TIM10, if the unit is never exposed to fresh
air, and our low‐cost, hand‐held CO2 meters, which can easily
be taken outdoors.

AQ500 ‐ MODBUSABC Calibration
Manufacturers of early CO2 sensors used in buildings tomeasure occupancy or indoor air quality (IAQ) levels realized
the difficulty of calibrating wall‐mounted units. Removing theunits from the wall to bench calibrate was expensive,
required trained staff, and with budget cuts calibration
schedules were often ignored.
To solve the problem of CO2 sensor calibration for IAQ,
SenseAir developed Automatic Baseline Calibration (ABC).
The theory behind ABC calibration is that for IAQ use, at somepoint each day a room is unoccupied, and the CO2 levelshould return to 400ppm, the same as outdoor air. By storing
the lowest CO2 readings taken over time (typically severaldays) in EPROM memory, an offset to 400ppm could be
calculated, then added or subtracted from the actual CO2readings.
The advantage of ABC is that the CO2 sensor is self‐calibratingover the life of the sensor. The disadvantage is that if thesensor never “reads” normal 400ppm air, over time it will
display inaccurate CO2 levels.
When To Use ABC Calibration
ABC calibration is best suited for HVAC or any situation wherefresh air CO2 levels can be recorded by the sensor every few
days. Otherwise, known gas (either nitrogen or fresh air)calibration should be used. Many of our products have theability to turn ABC on or off in the software, depending on
how the sensor will be used.
How Often Should A CO2 Sensor Be Calibrated?
The more accurate CO2 level reading required, the more
often it should be calibrated.
 Scientific Experimentation – Before each test
 Personal Safety – Weekly to monthly
 Greenhouse – After each growing season
 Manufacturing – Bi‐Annually to Annually Indoor Air Quality – Annually, or not required if ABCis used
These are general guidelines. Calibration schedules may also
be dictated by experimental protocols or by particularindustrial standards. Consult your specific sensor
requirements for your application.

Revision 1.1 – March 20131