An automated flow calorimeter for the determination of liquid and vapor isobaric heat capacities: Test results for water and n-pentane

An automated flow calorimeter has been developed for the measurement of highly accurate isobaric heat capacities and enthalpies of fluids at elevated temperatures and pressures. The instrument was successfully tested over the range 300-600 K and 0-12 MPa and is readily extensible to 700 K and 30 MPa. The flow calorimeter relied on an adiabatic design with a two-chambered cell design incorporating several concentric shells of active and passive shields designed to minimize heat losses, facilitate easy component replacement, and eliminate any external temperature regulation baths. A pair of miniature standard platinum resistance thermometers were built to determine the differential calorimeter temperatures. A precision metering pump eliminated the need for direct mass flow-rate determinations while a complete automation system supplied all of the required data acquisition, monitoring, and control (except for pressure) resources necessary to operate the calorimeter and make measurements from a single personal computer. Measurements of isobaric heat capacities were performed on water (liq.) and n-pentane (liq. and vap.) to test the new calorimeter. These experiments compared well with the published values, indicating an overall measurement precision of 0.1% and uncertainty in the range of 0.2%-0.3%. The total calorimeter heat leak was small affecting the experimental uncertainty as much as the combined mass flow-rate and calorimeter temperature difference errors.