Issue
How to read and understand the accuracy and repeatability on CO2 sensors.
The sensors shows 380 ppm, is it defect?
Product Line
Field Devices
Environment
CO2 Sensors
Cause
Accuracy +- 30ppm +-2% of measured value
Repeatability +- 20ppm +-1% of measured value
Resolution
Accuracy
The accuracy of a unit is defined as the tolerance between the actual absolute gas concentration and the measured value. For example, if a unit has a specified accuracy of ±30ppm, this implies that the displayed value will be within 30 parts‐per‐million of the actual gas concentration under standard ambient operating conditions. If the gas concentration in a room is 400ppm, then the sensor is operating within rated parameters if it reads anywhere between 370ppm and 430ppm on average.
Repeatability
Repeatability is closely related to precision. Repeatability is defined as the variance in measured values of a known, unchanging gas sample upon repeat measurements. To put it another way repeatability is the range of acceptable values the sensor will read, relevant to a previous measurement, when measuring a gas, independent of the actual gas concentration.
For example, if the gas concentration in a room is 400ppm and the sensor has a rated repeatability of ±10ppm, then if originally the sensor reads 420ppm, upon future measurements the sensor will read between 410ppm and 430ppm.
Repeatability is generally specified as having a tighter tolerance than accuracy. Even if a sensor is reading a value that is slightly high when compared to the actual gas concentration, it is likely that it will read close to the same slightly higher value upon repeat measurements.
How to understand the accuracy and repeatability
Accuracy +- 30ppm +-2% of measured value
Repeatability +- 20ppm +-1% of measured value
Accuracy at 400ppm:
- Min = – (30 + 400*0.02) = -38
- Max = + (30 + 400*0.02) = 38
Repeatability at 400pm:
- Min = - (20 + 400*0.01) = -24
- Max = (20+400*0.01) = 24
This means that in a 400ppm environment, the sensor output can vary between
- 400-38-24 = 328 ppm
- 400+38+24 = 462 ppm
Finally, based on the explanation given above, 380ppm would still be in spec.
However, another reason could be that the calibration was done in an environment that was not the “purest” and by doing so, an offset was created on the 400-baseline explaining why values below 400ppm are reported.
As said 380ppm is still in spec but the customer could try to recalibrate the CO2 sensor to see if it changes anything.
