We don’t, but the light scattering technique we use allows our instruments to be calibrated gravimetrically with sufficient accuracy. Any calibration errors are normally much less than the range of correction factors the DETR has suggested for the TEOM.

We believe our instruments provide the best and most cost-effective means of real-time particle monitoring. They are sufficiently accurate to determine when an incident has occurred and at what time. Inter-instrument agreement is excellent, allowing groups of them to be used for comparison studies. We have numerous graphs showing our instruments tracking TEOMs over many weeks. Generally, they are easy to use and cheap to run and maintain. They are also small, unobtrusive, quiet, easy to install, and can be battery powered.

Yes. Numerous instruments can be linked together using any combination of fixed wiring, licence-free radio telemetry, telephone or cellular modems, fixed-line or 3G/4G mobile internet connection. They can either be controlled and data collected online, or manually or automatically called periodically to upload results. We have desktop software AirQ, the web-based system AirQWeb, and phone apps which do all this, and automatically send email and SMS alerts if high concentrations occur. Alternatively, any of the instruments can be used as a stand-alone monitor.

Besides particle concentration, they can also be used to measure wind speed and direction at the same time. This allows AirQ to produce a pollution rose. They also have two analogue inputs which can be used for gas sensors or sound meters. A traffic counting input is also available.

Because we measure individual particles by analysing their scattered light at angles where it is virtually independent of composition. The light scattered by a particle can be thought of as made up of reflected, refracted and diffracted components. The first two depend on its material composition, the last is independent of composition. We measure mainly the diffracted component.

Yes, we have over 20 years of in-house technical expertise in dust measurement using light scattering.

None. We size the particles electronically within the instrument. This allows it to measure PM1, PM2.5, PM10 and TSP mass fractions simultaneously. Alternatively we can measure the inhalable, thoracic and respirable fractions.

From about 0.5 microns in diameter up to 20 microns in diameter. One micron is one millionth of a metre, or one thousandth of a millimetre. A 20-micron particle is about one fifth the diameter of a human hair. Any particle smaller than about 0.5 micron diameter scatters too little light to be detected. At the other end, it is quite difficult to get very large particles (greater than 20 micron diameter) into an instrument’s inlet; in fact, anything bigger than this is measured as 20 microns.

Because particles 20 micron in diameter have a falling speed in free air of about 1 cm per second, they are only really significant very close to the dust source and then only in the TSP or inhalable mass fractions. This falling speed increases with particle diameter squared, and is due to the balance of the downward gravitational force and the buoyancy caused by the viscosity of the air.

Imagine holding black or white plates between you and a powerful light source. They will both reduce the light intensity equally giving an equivalent response, but only if they are between you and the light. If you look at other angles you will see them differently. This is exactly what happens to the airborne particles in our photometer, because we only look at light very close to the laser beam. If we looked at wider angles (or even 90 degrees) their colour or absorption (i.e. reflection and refraction) would be important.

Not strictly speaking. The signal we measure is predominately the diffracted component of the scattered light, which is independent of material composition. Because the particle’s size is the same order of magnitude as the wavelength of the light source, the diffraction lobe spreads a few degrees either side of the laser beam.