Some Applications of Capacity Micrometers to High Speed, High Temperature Measurements

A capacity micrometer for the measurement of blade clearance in a turbine is described. The calibration of the instrument is non-linear with displacement, and clearances of the order of .030 to .001 inch may be measured with a nominal accuracy of ± 10 percent of the indicated reading. Measurements may be made at temperatures up to 1500°F. by the use of water-cooled pickup probes, and with a frequency response from DC to 200,000 cycles per second. The stability of the measurement depends on the temperature stability of the pickup dielectric material. Pickup drift may be measured during operation and may be counteracted by a control on the instrument. Stable reference capacitors provide a means for checking zero drift and calibration independent of the pickup and cable. Examples of the application of the instrument to a variety of other displacement measurements are provided.