Measuring CO₂ level

Measuring CO₂ level

Many methods and devices are available to measure CO₂ concentration in the air. Some sensors are not reliable, the electro-chemical type, for the chemical reaction varies as CO₂ level increases and Oxygen lowers. A method using test tubes provides a rough estimate. Precision CO₂ measurement devices use the NDIR technology (Non Dispersive Infra Red).

A CO₂ NDIR meter or controller is a fragile device (Figure 3). A shock to the device may cause the loss of calibration, thus resulting in erroneous measures. The NDIR sensor is built with a light source and mirrors that can easily be affected by dusty air or gaseous adverse conditions. So, the sensor accuracy drifts during the use in the garden and measures grow farther from the real level with time.

Figure 3: Three CO₂ level environmental controllers using NDIR sensors from the same manufacturer.

A CO₂ NDIR meter or controller should be calibrated in conditions near its normal operating environment. From our experience, for best results, a CO₂ NDIR meter or controller should be calibrated every 3 months or at every crop start. We have seen measures drift in excess of 100 ppm relative to the actual level after only one month of use for a renowned meter. CO₂ level measures are as precise as the sensor's calibration is accurate (Figure 4).

Figure 4: Three CO₂ level environmental controllers showing three different measures although using the same manufacturer's sensor. The two on left have just been calibrated and show a negligible difference in the readings by only 3 ppm. The unit on the right has been calibrated 6 months before the picture was taken. Since this last calibration, the drift to the real measure is about 140 ppm: it is time to re-calibrate!