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State-of-the-art CME measurement

The coefficient of moisture expansion (CME) of a material needs to be well known to accurately predict moisture induced dimensional changes and account for them is the design of large, stable optical benches. Historical methods for measuring CME are identified and discussed. The uncertainties associated with those earlier methods are due to using two different coupons to measure the length and weight change, or the interruption of the coupon environment to perform the measurements. The state-of-the-art CME apparatus described has eliminated these uncertainties by measuring the weight and length change simultaneously on one coupon. The patented design uses two opposing linear variable differential transformers to precisely measure the coupon length change while suspended from a Cahn precision balance to accurately measure the coupon weight change. The resulting determination of CME is accurate to better than +/- 10 ppm/% moisture. The apparatus is described in detail and the innovative design attributes that enable precise CME measurements to be made are discussed. Typical test data is also presented and discussed.