You expect a sensor to deliver reliable measurements, but that is not a given. What you may not consider is that the raw figures produced by sensors are not necessarily the actual values. This is also the case for CO2 sensors. A measurement can easily deviate by more than 10% from reality — which does not necessarily mean the sensor is broken. It does, however, mean the sensor needs to be calibrated.
Straight from the factory, a sensor can give (considerably) deviating readings. Calibration means adjusting the value so you know the deviation relative to a reference measurement.
However, calibration is a tricky job that can be carried out in several ways. A common method is to expose the individual sensor to nitrogen or argon and make a zero-point adjustment. On top of that, almost all sensors develop a drift as they age — including CO2 sensors. So you need to keep calibrating periodically.
A Smart Solution: ABC
The calibration dilemma has existed for years and, as is often the case, the industry has come up with a solution: Auto Baseline Correction (ABC), a low-cost option to automatically calibrate a CO2 sensor. A large number of CO2 sensors use ABC.
How ABC Works
The ABC algorithm continuously tracks the lowest reading from the sensor over a pre-configured time interval — for example, one week. It assumes that the lowest measured value equals the CO2 level of fresh (outdoor) air, which is often fixed at 400 ppm.
By observing the minimum reading over a fixed period and assuming that value should have been 400 ppm, a correction for the next period can be calculated and all new measurements will be adjusted automatically.
In simple terms: if the sensor last reads 700 ppm, this is corrected to 400 ppm after the period. The interval between corrections depends on the sensor.
That Sounds Clever — What Is the Problem?
Whether automatic correction is a good idea depends on the environment being measured. At the very least, this automatic calibration cannot be fully applied during the first week of use, because the sensor has not yet established its lowest point.
Furthermore, the assumption is that the actual CO2 level will reach 400 ppm at some point during the measurement period. In environments that are not adequately ventilated with outdoor air for extended periods, CO2 levels will not drop to outdoor values, and a CO2 sensor with ABC will not produce reliable results.
Are There Sensors Sold Without ABC?
Almost all common professional brands include ABC. This system is usually built into the sensor's firmware and cannot normally be adjusted or disabled. For specific applications, CO2 sensors without ABC are available, but these are more expensive due to their higher requirements.
How Does This Work with Cobra Climate Sensors?
Cobra sensors do not use ABC, but IBC (Intelligent Baseline Calibration). This means the sensors transmit raw, unfiltered data. The choice of which algorithm to apply is only made at the software level. This provides the flexibility to make the best choice per situation — and even on the fly.
Features of Intelligent Baseline Calibration
- Calibration takes place in the software, not in the sensor — making calibration far more flexible.
- Dozens of sensors can be configured simultaneously at the touch of a button.
- A calibration plan is possible in which a limited number of sensors are 'smartly' calibrated.
- ABC can still be applied, but within the software, with the option to choose a baseline.
- Raw, unfiltered sensor data can be consulted at any time.