The output of a wheel speed sensor is sampled at dithered time intervals and the samples used to construct a pair of histograms representing high and low sample values. The burner assembly includes a burner block formed to include a flame chamber having inlet and outlet openings, a bypass structure for conducting oxygen outside of the flame chamber to the outlet opening of the flame chamber and structure for discharging fuel into the flame chamber formed in the burner block. The body of the filter column has a number of surfaces to accommodate placement within variously sized collection tubes. A straight coupler is fitted to the front member for hitching the assembled trailer to a tow vehicle.