Displacement measurements at high temperatures with capacitive transducers

Measurements have been made on a generating plant including rotary-hood air heaters and a nuclear boiler. Although the air heaters were not an ideal environment due to the presence of dust and water vapour, capacitive transducers were successfully used. In one experiment, coplanar-electrode transducers were mounted on the rotary hood in the hot air stream, necessitating the use of specially screened slip rings. In another, they were mounted on the stator and were subject to alternate flows of hot flue gas and cool air. The nuclear boiler had a more suitable atmosphere, namely CO2 at 2800C and 2500 kN m-2, but long mineral-insulated cables were need to bring the signals out of the pressure vessel. Coplanar transducers similar to those for the air heaters were used to one measurement, but parallel plates with a moving inter-electrode screen were used at a position where more range was required. Another application was on an oxidation test rig designed to monitor the rate of oxidation of stainless steels in different atmospheres. The test piece is a thin-walled tube of about 20 mm diameter which is allowed to oxidize externally while the bore is kept free from corrosion by an atmosphere of inert gas. The different mechanical properties of the oxide set up a hoop stress causing a change in internal diameter of the tube of up to 100 μm. The capacitance variation between the inner wall and an insulated concentric electrode is measured. The temperature range required is 6000C to 10000C and ionization of the gas gives problems from the high shunt conductance. Methods of overcoming these have been successful to to 8000C. (author)