Precision interferometric dilatometer

An improved double Michelson laser interferometer dilatometer is described. The unit is suitable for the study of materials of arbitrary size or shape with time-dependent and near zero coefficients of thermal expansion (CTE) over the range 100 to >450 K. Direct recording of four amplified photodetector signals gives a ΔL/L resolution of 5×10−8 and an average CTE error in the 10−10–10−9 K−1 range. An inexpensive automatic digital counter–microprocessor combination permits recording of instantaneous ΔL/L and CTE values at >30-s intervals. The resolution of ΔL/L in this case is <8×10−7 while the instantaneous CTE (4 K interval) has an uncertainty of <2×10−7 K−1. The latter values are based on a counting unit of λ/2 and interpolating. Smaller counting units reduce errors proportionately to the system noise level and the limitations of the temperature measurement.