Absolute calibration of flow calorimeters used for measuring differences in heat capacities. A chemical standard for temperatures between 325 and 600K

Two methods for the absolute calibration of flow calorimeters (used for measuring the differences in heat capacities of two fluids) have been investigated. In the recommended method of calibration, a change in the flow rate of the fluids is used to minic a change in the heat capacity of the fluid. In the other method of calibration, heat loss is measured using a fluid of known heat capacity, and it is assumed this heat loss is constant. This calibration method is not recommended because the heat loss is, in general, not constant. For some calorimeters the difference between the two methods of calibration is negligible, while for others erros as high as 40% are caused by choosing the wrong method. A detailed analysis of the heat losses in this kind of calorimetry shows why the two calibration methods give different results and leads to various methods of improving calorimeter construction and operation. Because chemical standardization is far more convenient for routine use, the recommended absolute calibration method has been used to establish 3.00 mol-kg^–1 aqueous NaCl as a chemical standard for temperatures between 325 and 600K at 17.7 MPa.