a method for measuring ash / slag deposition in an operating utility boiler . the method has the following steps : i ) providing a probe for the boiler wherein the probe has at least one thermocouple therein or thereon for measuring temperature ; ii ) measuring the temperature at the thermocouple at a baseline time ; iii ) measuring the temperature at least one thermocouple at a pre - determined time later than the baseline time ; and iv ) comparing the temperature at the baseline time to the temperature at the pre - determined time to correlate to a level of deposition . there is also a utility boiler system .