A method for characterizing a process fluid comprising applying electrical power to a first resistance temperature detector (RTD) in contact with a fluid to increase the temperature of the first RTD; allowing the first RTD to cool toward a fluid equilibrium temperature; analyzing the temperature decay profile of the first RTD over time to determine thermal characteristics; applying electrical power to a second RTD in contact with the fluid to increase the temperature of the second RTD; allowing the second RTD to cool toward the fluid equilibrium temperature; analyzing the temperature decay profile of the second RTD over time to determine thermal characteristics; comparing the thermal characteristics of the first and second RTD to determining one or more characteristics of the fluid based; and performing a corrective action. The first RTD has a first coating and the second RTD has a second coating different than the first coating.