Understanding Your CO2 Meter

You have purchased a reliable CO2 meter — great! But what do those numbers actually mean? The CO2 level is expressed in parts per million (ppm). Assuming you have the right sensor, it should at least be able to display values between zero and 5,000 ppm.

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.

Most people are unable to do this themselves. So how can sensors be sold if we know the values may not reflect actual readings?

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 then be adjusted automatically.

In simple terms: if the sensor last reads 700 ppm, this is corrected to 400 ppm after the period. From that point on, 300 ppm is subtracted from all readings. The interval between corrections depends on the sensor — sometimes it is set to one day by default; for other sensors it may be eight days.

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. This is a drawback for one-off or short-duration building scans.

Furthermore, the assumption is that the actual CO2 level will reach 400 ppm at some point during the measurement period — meaning the building momentarily has air as fresh as outdoors. This is far from always achievable.

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. There are many locations where people or animals are present for most of the day, resulting in a background level higher than the assumed 400 ppm.

It is not always possible to determine in advance whether a sensor with ABC is suitable, so many CO2 sensors create a false sense of security. This can apply to schools, offices, or public spaces — but also to a greenhouse where elevated CO2 levels benefit plant growth. If such an environment maintains a minimum of 600 ppm over an extended period, a CO2 sensor with ABC will display 400 ppm. A considerable error.

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. The actual measurements are not changed, but an algorithm is applied within the device itself that makes it appear as though the sensor has been calibrated — which is not the case.

For specific applications, CO2 sensors without ABC are available. The downside is that these sensors need to be calibrated periodically — roughly every one to two years — and are more expensive due to their higher requirements.

There is a real chance that some cheap consumer sensors bought online have no proper calibration at all.

How Does This Work with Cobra Climate Sensors?

There is a better and future-proof solution for dealing more intelligently with recurring calibration. 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. By using a CO2 sensor that does not require ABC for reliable measurements, combined with Cobra Climate's proprietary IBC, periodic calibration is no longer necessary in many situations.

By using higher-quality components in combination with the smart IBC software, the goal is to deliver reliable measurements at the lowest possible total cost of ownership.

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, enabling far greater accuracy at considerably lower cost.
• 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.

Currently the available options are: ABC or manual calibration. In the future, additional options will be added, including AI-based baseline correction — without needing to replace the sensors!

More Ventilation Is Better

• The most important indicator of fresh air is the CO2 level.
• Humidity plays a role.
• Fewer visitors per space means fewer infections → the Cobra Interaction Platform can measure and report on this.
• If an infection occurs, it is advisable to have a record of prior visitors → the Cobra Interaction Platform includes a contact tracing solution.

In addition, we recommend taking any physical measures that can optimise the measured values, such as purchasing a humidifier or simply opening a window.