The operation characteristics (the Peltier cooling current i and the cooling period tc), the range of measurement of water potential, the calibration sensitivity, and the accuracy are evaluated for an improved psychrometer and compared with data in the literature, and with a recent theory. The range increases with increasing Peltier cooling current up to the optimal value of i ≈ 9 mA and has been found to agree fairly well with the predictions of theory. A maximum range of 0 to about –13 000 J/kg is obtainable with the improved psychrometer. This is nearly twice that previously reported for a Spanner-type psychrometer. The experimental calibration sensitivities C′s increase with increasing water potential, cooling period, and temperature and, with tc = 60 s, are in general larger than the theoretical values at high water potentials but less than the theoretical values at low water potentials. They are mostly larger than those reported by Monteith and Owen but about the same as those of Lang and Trickett. The accuracy of the improved psychrometer is significantly greater than that previously reported for a Spanner-type psychrometer. While the theory predicts a linear relationship between water potential and peak output, the observed calibration curves are non-linear and vary between psychrometers. Because of these discrepancies between observation and theory, calibration of each psychrometer is necessary.