What does an air quality sensor measure, and what does it not show?

What is the main difference between CO₂ and eCO₂?

CO₂ measurement requires a sensing principle responsive to the target gas. In the Sensirion SCD4x example, a photoacoustic NDIR approach provides CO₂ concentration in ppm. eCO₂ or CO₂eq can instead describe an equivalent output derived from other gas signals and a model. Showing the same unit on a display does not make the methods interchangeable.

This distinction matters when reusing a purchased module. Check whether the CO₂ label on the existing display matches the sensor's actual output. Add the channel type and sensor model to the measurement record. Where actual CO₂ is required, do not select a module using an equivalent output; the data schema and interface label must reflect that decision.

Does a VOC index indicate the amount of a particular gas?

VOC is a collective term for volatile organic compounds. Depending on the model, a gas sensor's processed output may be a TVOC signal or a VOC index. Sensirion's migration guide explicitly distinguishes the SGP30's TVOC and CO₂eq outputs from the SGP40's VOC index produced by an external algorithm; this is a software change as well as a component change.

If the project is intended to monitor changes caused by cleaning, production or device use, record example events first. Store the index name, initial conditions and software version with the result. If a selective concentration measurement for a particular chemical is required, do not assume a general index will meet the need; investigate a method specifically for that target.

Why is PM measurement separate from gas measurement?

Particulate sensors examine airborne particles; they do not replace CO₂ or VOC channels. The SPS30, for example, uses laser scattering and provides separate mass and number concentration outputs. Manufacturer documentation also states that some size groups rely on a calculated distribution. The output type and unit must therefore be understood alongside the PM label.

Using two sensors in a filter trial does not by itself define the test method. First determine inlet and outlet positions, simultaneous logging, matching operating conditions and the sensor comparison setup. Recheck results after changing airflow or adding an enclosure. The report should explain this arrangement as well as show graphs.

How are the airflow path and sensor location determined?

The EPA recommends unrestricted air access when positioning monitors and considering the effects of nearby sources and air currents. A device monitoring general room trends and one placed immediately beside a source answer different questions. Without a documented placement purpose, two correct measurements may be wrongly regarded as contradictory.

The design review maps sensor compartments, air openings, fan operation and maintenance access. Prototype tests record at several locations to assess whether the chosen position represents the target. The operating state of electronics beside the sensor is also logged. The open prototype and enclosed device are compared before the mechanical design is finalised.

Explore humidity, temperature and enclosure effects →

What should a multisensor board and software record?

It is useful to define a separate data contract for each channel: quantity, unit, scale, reading time, readiness and error state. Since different modules may behave differently at startup, we do not treat all readings as valid immediately after power-on. Sensor drivers, measurement timing and state machines are developed according to the selected components' documentation.

The user display distinguishes old data, invalid data and zero values. We plan how local recording continues during network loss and how chronological order is preserved when data is transferred later. Filtering and averaging settings can remain traceable without deleting raw data. This allows investigation of whether a field change originated in the sensor, communication or presentation settings.

What should the calibration and comparison plan include?

The CO₂ module manufacturer's support page provides separate testing and design-in guides. This distinction matters in practice: communicating with a development board does not validate measurements in the finished device. Device records should identify which manufacturer correction is used, the conditions it requires and the version of any change.

Project tests plan for candidate devices to operate simultaneously in the same environment, comparison against a selected reference and logging of repeated events. The comparison range, reference method and acceptable deviation are defined before testing. Results include data loss, transition behaviour, startup time and environmental changes, rather than only an average value.

Common interpretation errors and an effective project brief

Key errors include labelling eCO₂ as actual CO₂, treating a VOC index as the ppm concentration of one gas and interpreting PM data as gas concentration. If a combined air quality score is provided, its calculation is explained and access to individual channels is retained. A colourful display should not conceal the method underlying the measurement.

For an initial discussion, share the quantities to monitor, how the space is used, device position, power and communication conditions. Logging duration, target quantity, service expectations and the reference setup also define the scope. VOMRO can address sensor selection, custom boards, embedded software, enclosures and pilot validation together according to the project; product outputs are defined through a technical acceptance plan.

• Distinguish CO₂ and eCO₂ labels.
• Show units and data validity for each channel.
• Compare at the same time and under the same environmental conditions.