Calibration and sample-measurement techniques for flow heat-capacity calorimeters

Abstract

A new redesigned flow heat-capacity calorimeter which facilitates measurements on aqueous solutions near the critical point of water is described. Traditional methods of using flow heat-capacity calorimeters require corrections for heat losses and there is no consensus on the best way to make these corrections. A more complete heat-loss model has been derived which includes the exponential decay in temperature of a fluid as it moves along a tube, as well as the effects of surroundings which are not at the same temperature as the input stream. Experiments with the rebuilt calorimeter show that this model predicts the heat-loss correction factors over a wide range of sample flow rates and reference flow rates, with only three adjustable parameters. Using this heat-loss model, the traditional methods of calibration and measurement are assessed and their strengths and-weaknesses discussed. A method of making measurements with these calorimeters that does not require any correction for heat losses has been developed. This method involves changing the sample flow rate to match the temperature rise of the reference fluid with the heaters at constant power. The heat-loss model predicts that, under these circumstances, the heat losses are constant so that no correction is needed. Experiments were performed on 3 mol · kg−1 and 1 mol · kg−1 NaCl(aq) at 598.11 K and 17.68 MPa over a wide range of flow rates and heater powers to confirm this method. The results agreed within 0.3 per cent, even under conditions where the heat-loss corrections were as large as 20 per cent.