29.09.2026
Two optical dust monitors placed side by side in the same dust can show different readings. Data sheets rarely explain why, because no uniform standard exists for specifying particulate matter sensors. Dust monitor calibration is therefore not a single step but a sequence of checks. Each check answers a different question: whether every unit measures in the same way, whether the reading agrees with a reference method, and whether it stays stable over years of use. Dustlight addresses these questions through individual calibration of every sensor unit, independent validation at an accredited laboratory and a documented calibration chain. This article describes each step, the tolerances that apply and the limits of what a traceability statement can claim.
Discover every test, tolerance and reference instrument behind each reading in the full technical statement on accuracy, precision, calibration and traceability.
Dual-track approach to dust monitor calibration
Dust monitor calibration at Dustlight runs on two tracks: algorithm development and reference validation at an independent accredited institute, and calibration of every sensor unit in production. The first track defines how the optical signal is converted into mass, and the second ensures that every unit applies that conversion in the same way.
The separation follows from the optical measuring principle. A laser diode sends light through the drawn-in air, particles scatter part of it, and a photodiode converts the scattered light into signals. An algorithm then calculates the mass concentration in µg/m³, and this conversion is influenced by the refractive index, shape and material of the particles.
Gravimetric sampling avoids this dependence because it weighs the collected mass directly, which is why it remains the reference method for formal exposure assessment, with EN 482 and EN 689 setting the requirements. NIOSH notes that factory-calibrated optical monitors can differ from filter-based respirable samplers by up to a factor of ten. The algorithm is therefore validated gravimetrically across real industrial dusts, as the comparison of gravimetric and real-time measurement explains.
The dual-track approach reduces the discrepancy between optical and gravimetric results, but it does not remove the aerosol dependence that applies to every optical method.
Algorithm development in the IGF laboratories
The measurement algorithms are developed and refined at the Institute for Hazardous Substances Research (IGF), an institute at Ruhr University Bochum run by the German Social Accident Insurance Institution for the Raw Materials and Chemical Industry (BG RCI). Gravimetric reference values are determined under DAkkS-accredited procedures in accordance with DIN EN ISO/IEC 17025:2018, accreditation number D-PL-14436-01-00.
The IGF dust channel test rig combines controlled aerosol generation, a defined dust channel, a sedimentation chamber and adjustable airflow. Reference values come from a Grimm Aerosol Spectrometer Model 11-D, with a size range of 0.25 to 35 µm across 31 channels, running alongside gravimetric sampling with calibrated pumps. The algorithm is exposed deliberately to a wide range of industrial aerosols:
- wood dusts, welding fumes and diesel particles
- amorphous silica, including fumed and precipitated grades
- Eskal dusts and several grades of Arizona test dust
- mixtures of different dust types
Tests last one to two hours, typically at 125, 625, 1,250, 2,500 and 5,000 µg/m³ and an air velocity of 2 m/s. For repeatable verification, Arizona Test Dust A1 ultrafine is used. It is defined in ISO 12103-1, and its quartz content of approximately 69 to 77 % makes it relevant to silica-exposed workplaces. A single reference dust cannot represent the complexity of workplace aerosols.
Factory calibration of every sensor unit
Every particulate matter sensor unit used in Dustlight is calibrated and verified individually in production, batch by batch, against reference instrumentation under identical conditions. Units that do not meet the specification are not used in a device.
The verification aerosol is an atomised potassium chloride (KCl) solution of approximately 3 %. A TSI DustTrak DRX 8533 serves as the reference instrument, supported by a TSI Optical Particle Sizer 3330. All units of a batch are tested simultaneously along a decaying concentration curve, and values are averaged over at least 150 seconds to filter noise.
The reference instrument itself is maintained and calibrated at regular intervals, because any error in the reference is passed on to every unit calibrated against it. Reference instruments from different manufacturers may differ by more than 20 % when they are not consistently maintained. The tolerances below therefore apply to the KCl aerosol only.
Accuracy and unit-to-unit variability limits
Two separate figures describe a calibrated sensor unit: accuracy, the deviation from the reference instrument, and precision, the agreement between units and between repeated measurements. A batch of sensor units is accepted only when both limits are met.
What is the difference between accuracy and precision in a dust monitor?
Accuracy describes how closely a reading agrees with a reference method. Precision describes how closely devices agree with each other, and how closely repeated measurements agree. Monitors can agree closely with each other and still deviate from the reference. Data sheets giving one percentage without saying which is meant are hard to compare. The verification limits are as follows.
| Fraction | Concentration range | Permissible deviation |
|---|---|---|
| PM1 and PM2.5 | 0 to 100 µg/m³ | ±5 µg/m³ and ±5 % |
| PM1 and PM2.5 | 100 to 5,000 µg/m³ | ±10 % |
| Respirable dust and PM10 | 0 to 100 µg/m³ | ±25 µg/m³ |
| Respirable dust and PM10 | 100 to 5,000 µg/m³ | ±25 % |
The wider tolerance for the coarser fractions reflects the physics of optical detection. Larger particles contribute disproportionately to mass, so small counting and sizing errors in a few of them affect the result far more than in the fine fractions.
Why do two Dustlight devices show the same reading side by side?
Two devices show comparable readings because each production batch is accepted only if its unit-to-unit variability, also called device-to-device variation (D2D), fits within the specification band. This figure describes how far a single unit deviates from the batch mean. All units are measured together in one chamber, so their deviations are evaluated as a distribution around that mean.
For correction factors, NIOSH notes that applying one field-calibration factor to several monitors of the same type depends on their intra-unit variability, to be verified by the user. Both limits are set for the verification aerosol, and field deviations can additionally depend on the optical properties, shape and density of the actual dust.
See the calibrated sensor technology up close and explore the full measurement specifications for every dust fraction on the device page.
Respirable dust convention in the Dustlight algorithm
After the sensor units have been calibrated, each assembled device receives a device-specific respirable dust calibration. Respirable dust is defined not by a sharp cut-off but by a size-dependent sampling convention, set out in EN 481 and ISO 7708.
The respirable fraction is the part of airborne dust small enough to reach the alveoli. The larger a particle, the lower its probability of getting there. The algorithm reproduces this convention, so the displayed value corresponds as closely as possible to that fraction. In the Mixed Dusts setting, the calibration is based on a defined dust mixture. Settings for specific materials are explained in the article on correction factors and material configurations.
In the validation with the calibration dust mixture, the respirable dust curves of Dustlight and the Grimm 11-D reference follow the same course, including short peaks and drops. The calibration defines which size fraction is reported, but it does not determine what the particles are made of, as the article on detecting silica in real time explains.
Traceability of the calibration chain
Behind the factory calibration stands a calibration chain that is traceable to recognised national measurement standards, including those realised at the National Physical Laboratory (NPL), the Physikalisch-Technische Bundesanstalt (PTB) and the National Institute of Standards and Technology (NIST). The references used in calibration are linked to these standards through an unbroken chain of documented comparisons.
The sensor manufacturer in Switzerland calibrates the units against transfer standards, whose own reference is calibrated within an ISO 9001:2015 registered system. The accreditation body is a full member of the International Laboratory Accreditation Cooperation (ILAC), whose Mutual Recognition Arrangement means that accredited certificates are accepted by all signatories.
What does NIST-traceable mean for a dust monitor?
Traceability describes the calibration chain, not an accredited device certificate. It is not a claim that an individual device carries an accredited calibration certificate in accordance with ISO/IEC 17025.
In aerosol measurement, traceability runs through the underlying physical quantities. Mass, volumetric flow, particle size and ambient conditions are traceable, as is the gravimetric determination of collected mass. There is no primary national standard for airborne dust mass concentration, a limit shared by every optical dust monitor on the market. Arizona A1 is therefore a standardised test dust for reproducibility, not a certified reference material.
Long-term stability and accelerated ageing tests
Calibration describes a unit at delivery, so long-term stability is verified through accelerated ageing tests on samples of sensors, and every sensor in a sample must pass. The specified drift is 1.25 % per year, and after a load test simulating 10 years of service the permissible accuracy limit is ±12.5 %.
Four tests are applied:
- High Temperature Operating Lifetime (HTOL): for example 60 °C for 1,000 hours, per JESD22-A108F
- Temperature and humidity: for example 60 °C and 90 % relative humidity for 860 hours, per JESD22-A101D
- Temperature cycling: for example 380 cycles between −40 °C and +60 °C, per JESD22-A104E
- Resistance to dust: exposure corresponding to approximately 45 µg/m³ over 10 years
| Parameter | Value |
|---|---|
| Long-term drift per data sheet | 1.25 % per year |
| Simulated test duration | 10 years |
| Permissible accuracy limit after load test | ±12,5 % |
No calibration is therefore needed between the prescribed maintenance intervals, but the tests do not replace them.
Calibration intervals and field functional checks
Annual calibration and maintenance of Dustlight are recommended, depending on the application and on regulatory requirements. The device calculates maintenance from service life and dust concentration, at least every 12 months. A field functional check with a test spray confirms that the sensor responds, but it does not verify measurement accuracy.
Does a test spray replace calibration?
No. A test spray shows that the reading reacts, but not whether the displayed concentration agrees with a reference. Only residue-free test sprays should be used, because sprays containing oils or solvents can contaminate the optical path. NIOSH makes a similar distinction for zeroing, which it describes as the first step rather than a calibration.
A correction factor derived on site from a parallel gravimetric sample is a separate step that adjusts the conversion for a specific dust. Neither step turns an optical reading into a formal compliance measurement, for which EN 482 and EN 689 continue to apply.
Experience how calibrated real-time measurement brings clarity to exposure assessment. Book a personal demo and see Dustlight at work.
Frequently asked questions about Dustlight calibration
How often should Dustlight be calibrated?
Annual calibration and maintenance are recommended, and the device schedules maintenance at least every 12 months. Between intervals, a test spray can confirm that the sensor responds.
Does calibration make optical readings equal to gravimetric results?
No. Calibration ensures that every unit measures to specification, while agreement with a gravimetric sample depends on the dust present. A correction factor from a parallel sample can close that gap for a specific material.
Which reference dust is used to verify Dustlight?
Arizona Test Dust A1 ultrafine, defined in ISO 12103-1, is used for repeatable verification. Production verification uses a KCl aerosol.