Most of the turbomachine manufacturers are using a non-contact measurement system to measure blade vibration by two probes located in the casing radially outside a blade row. All blades of a row can be observed by this system, therefore it may be considered to be an ideal solution for measuring the vibration response of mistuned blades which have quite different vibration levels from blade to blade in resonance conditions.
By a simple calculation model a non-contact blade vibration measurement was simulated. It was found that for mistuned coupled blades the non-contact measurement results show some deviations from the actual vibration amplitudes. Different mistuning conditions have been investigated by a parameter study to estimate the inherent measurement error of the non-contact principle. Measurements at a circumferentially coupled gas turbine blading performed by a non-contact system and by strain gauges did also show deviations in the results obtained by the two measurement methods.