Abstract: A deposit measuring device, useful for improving combustion processes especially kraft mill recovery boilers, determines the temperature at the windward and leeward sides of a probe tube positioned in the flue gas stream transverse thereto and then determines the rate of build-up of deposits on at least the windward side and the temperature of the flue gas stream. This information is used by an operator or automatically to control boiler operations. The measuring device has a deposit removal means associated therewith periodically to remove deposits from the tube. The ability to control the boiler operation enables considerable economic benefits to be achieved.