Trapped soot measurement

A method for use in a regeneration control strategy which evaluates, against a soot limit, quantity of soot trapped in a particulate filter which traps soot in flow of products of combustion through an exhaust system of an internal combustion engine, the method comprising:

• drawing electric current from an electric power supply through a circuit which comprises in series circuit relation an internal portion of a particulate filter containing trapped soot and an electric impedance which is external to the particulate filter;
• providing, from at least one electric measurement device connected to the circuit, circuit-related data sufficient to enable temperature-adjusted electric resistance of soot trapped in the internal portion of the particulate filter to be calculated;
• providing temperature data from the source of temperature data indicative of an internal temperature of the particulate filter;
• calculating temperature-adjusted electric resistance of soot trapped in the internal portion of the particulate filter by calculating, from the circuit-related data, non-temperature-adjusted electric resistance of soot trapped in the internal portion of the particulate filter and then temperature-adjusting a calculation of non-temperature-adjusted electric resistance of soot trapped in the internal portion of the particulate filter based on temperature data from the source of temperature data indicative of an internal temperature of the particulate filter;
• and
• using a temperature-adjusted calculation of electric resistance of soot trapped in the internal portion of the particulate filter as a measure of quantity of soot trapped in the particulate filter.