Abstract: A particle detector for detecting particles flowing in a duct has a first triboelectric probe (5) for detecting the particles and a second triboelectric probe (3) for detecting a signal generated by contaminants on the particle detector. The presence of signal-generating contaminants on the particle detector will result in an error signal on the first probe (5) and a signal on the second probe (3). Therefore, the presence of a signal on the second probe (3) will indicate that the signal appearing on the first probe (5) includes an error signal. If the ratio of the magnitude of the signal generated on the second probe (3) to the magnitude of the signal generated on the first probe (5) exceeds a certain pre-defined limit, cleaning of the detector will be required.