| 摘要 |
Capacitance transducer apparatus, for example for measuring blade tip clearance in a jet engine of gas turbine, comprises an oscillator (14) connected by a cable (6) to a capacitance means such as a probe (4). The cable is tri-axial comprising an inner conductor (22), an outer protective layer (24) and an intermediate layer (26) between the outer layer and inner conductor, the inner conductor, intermediate layer and outer layer all being electrical conductors. The intermediate layer (26) consists of a material that is different to that forming the outer layer and is of relatively high electrical conductivity. The resistance of the intermediate layer is preferably less than 1 Ohm metre<-1>, and especially les than 0.1 Ohm metre<-1>, which may be achieved by forming the intermediate layer from a nickel based material, and especially from pure nickel. The reduction in the intermediate layer resistance can lead to higher sensivity of the apparatus by enabling any guard driver amplifier (165) in the apparatus to drive the capacitance over the full length of the cable. |
