How accurate are low-cost optical PM sensors?

Modern optical particle counters are stable to roughly ±10–15% under typical indoor conditions but can be biased by humidity and aerosol composition. They are useful for trends and intercomparison; gravimetric sampling remains the reference method.

What causes sensor drift?

Electrochemical cells gradually lose sensitivity through electrolyte depletion, MOX semiconductor sensors drift with humidity and contamination, and optical sensors collect dust on the chamber optics. Drift is normal — scheduled calibration is how it is managed.

How should air quality sensors be placed?

At breathing-zone height, away from direct sunlight, supply diffusers, doorways and heat sources. Placement is as important as sensor choice — a calibrated sensor in the wrong location will report misleading data.

Sensor technology

Air quality sensors — principles, accuracy and calibration

A sensor system is only as honest as the science behind each module. This is a working reference to the sensor technologies used in modern indoor and environmental air quality monitoring.

Modern indoor air quality sensor with cyan status indicator

AirQualityTech.uk · Editorial

Principles

The four sensing methods that matter indoors

Most indoor monitors combine four sensor types. Knowing how each one works explains its accuracy, its drift profile and where it can mislead.

METHOD 01

NDIR — CO₂

Non-dispersive infrared measures the absorption of CO₂-specific infrared wavelengths. Stable, selective and the reference method for indoor CO₂.

METHOD 02

Optical — PM2.5 / PM10

Laser light scattering counts and sizes particles in a sampled airflow. Excellent for trends; influenced by humidity and aerosol composition.

METHOD 03

Electrochemical — NO₂, O₃, CO

A target gas reacts at a sensing electrode, producing a measurable current. Sensitive but ages with cumulative exposure.

METHOD 04

PID & MOX — VOCs

Photoionisation detectors ionise volatile compounds for broad VOC response. Metal-oxide semiconductors offer cheap indicative TVOC.

Specification

What separates a credible sensor from a marketing claim

Documented accuracy at defined conditions. A sensor datasheet should declare accuracy, linearity, response time and operating range — and the temperature and humidity envelope across which those numbers hold.

Calibration traceability. Calibration should be traceable to a recognised reference standard. For air quality sensors that means scheduled comparison against laboratory-calibrated instruments, not a one-off factory tick.

Stable firmware and OTA updates. Sensor processing changes over a deployment. Devices that accept remote firmware updates allow correction of bugs, calibration coefficients and reporting formats without site visits.

Reference table

Indicative accuracy by sensor type

Parameter	Common method	Typical accuracy	Drift / lifetime
CO₂	NDIR	±50 ppm	Years with auto-baseline
PM2.5	Optical light-scatter	±10–15%	Cleaning every 6–12 months
NO₂	Electrochemical	±15–20 ppb	12–24 month cell life
TVOC	MOX / PID	Indicative trend	Recalibrate annually
Humidity / Temp	Capacitive / thermistor	±2% RH / ±0.3°C	Stable for years

Deployment

Designing a sensor system that holds up over time

Placement protocol

Breathing-zone height, away from supply air, sunlight and heat sources. Documented per location for repeatability.

Calibration cycle

Annual recalibration against a reference instrument, with logged adjustments and pre/post-cal data retained.

Environmental compensation

Sensors corrected for temperature and humidity at the firmware level — and validated against reference data in situ.

FAQ

Sensor technology questions

Non-dispersive infrared (NDIR) is the established standard for indoor CO₂ measurement. Quality units offer ±50 ppm accuracy, automatic baseline calibration and stable output over years of operation.

Discuss an Air Quality Monitoring Project

Independent advice on sensor selection, calibration and deployment for commercial UK buildings.

Discuss sensor deployment