Condensed from an OPSIS AB article by Bengt Löfstedt, first published 2024-02-23.
A triboelectric probe measures dust concentration in a duct from the charge the particles carry. As dust flows past an insulated probe, particles touch it or pass close by and transfer charge, which the probe records. The signal is amplified and consists of two parts: a DC component from particles in direct contact with the sensor — the classical triboelectric signal — and an AC component produced by particles both touching and passing near the probe. In either case the charge signal is multiplied by a calibration factor to give a dust concentration. The method depends on the electrostatic behaviour of the particles, that is their ability to carry charge, and on changes in that charge, for instance the state of charge inside an electrostatic precipitator. Those factors are hard to estimate and can vary widely in a running process, which is why triboelectric instruments were long used for indicative measurement only. According to the article, advanced algorithms that analyse the signal using inductive electrification technology now make very accurate dust concentration measurement possible.
For stationary sources in China the reference method for particulate matter is still manual gravimetry — GB/T 16157 for sampling and determination, HJ 836 for low concentrations — with HJ 75 and HJ 76 setting the CEMS requirements and HJ 212 for data upload. Whatever the principle, a reading has to be shown valid by comparison with the reference method; measuring correctly and producing data that survives a compliance assessment are two different questions. A triboelectric probe sits in the duct and has no optical window, so there is no light transmission to lose to fouling. It is, however, sensitive to the cleanliness of the probe, its position, and the gas velocity: deposits change the charge transfer, and a different velocity changes how many particles pass the probe per second. That makes such instruments good at trends and relative change — spotting an upset at the outlet of a bag filter or an electrostatic precipitator — provided they are mounted in a stable straight section with good insulation between probe and wall, because moisture or contamination on the insulation adds a leakage current directly to the signal. The method is relative: the calibration factor comes from a comparison with manual gravimetry and is tied to the dust type, size distribution and humidity at the time, so a change of fuel, load, cleaning method or moisture calls for a new comparison.
Inside the Particulate Monitor: the Triboelectric Method — read the original at opsis.se ↗
Written by Bengt Löfstedt, OPSIS AB, first published 2024-02-23. The summary and the A.Johnson note above are our own text; the OPSIS article remains the authoritative version. Chinese reader? See the full Chinese article.
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