Abstract: A conductivity sensor for use in a steam turbine in operation to monitor corrodents being deposited therein. The sensor comprises a bottom substrate on which an electrode pair is located. The bottom substrate also includes a means for measuring the temperature at the bottom substrate. A first layer of dielectric material is bonded to the top of the bottom substrate. The first layer has an aperture formed therein so that an end of the electrode pair is located in the aperture. A second layer of dielectric material is bonded to the top of the first layer. The second layer defines a crevice for entrapping steam-transported corrodents that are deposited in the turbine. A portion of the bottom of the crevice includes the aperture so that corrodents are also deposited in the aperture. By passing a current between the ends of the electrodes located in the aperture, a measure of the conductivity and thus the concentration of corrodents deposited in the aperture and crevice is given.