Assessment of calibration procedures for accurate determination of thermal parameters of liquids and their temperature dependence using the photopyroelectric method

We present a systematic theoretical and experimental investigation on the accuracy of thermal diffusivity alpha and thermal effusivity e of liquids measured by the photopyroelectric (PPE) method in back-detection configuration (BPPE). Special cases corresponding to different cell structures are analyzed in terms of error determination of alpha and e for water and ethylene glycol. We propose a new normalization procedure allowing for estimation of these parameters with accuracy of 2% on alpha and 5% on e over extended frequency range. The normalization eliminates the frequency-dependent influence of the transducer impedance and associated electronics, reduces the errors due to coupling fluid between cell components, and reduces the number of temperature-dependent parameters that must be known in order to characterize the sample. Technical solutions for improving the performances are suggested. Another goal of the study was to demonstrate the possibility of the BPPE method to yield small variations of thermal parameters as a function of temperature. We found good agreement with the literature data for the temperature coefficients (Deltaalpha/alpha)/DeltaT=0.267+/-0.015%/K for water and (Deltae/e)/DeltaT=0.10+/-0.05%/K for ethylene glycol, between 20 and 60 degreesC. The special case implying the normalization to a reference material and with a glass substrate for the pyroelectric sensor is shown to be the best adapted to the determination of both thermal parameters and of their temperature dependence. (C) 2002 American Institute of Physics.