Net power output of geothermal power plants depends on thermal heat, which is delivered by the geothermal water flow. In order to determine the potential, knowledge of temperature, mass flux and heat capacity of thermal water is important. On-site heat capacity of thermal water is only roughly estimated because dissolved gases and minerals influence heat capacity significantly. In order to address this knowledge gap, a new flow calorimeter was constructed which allows for on-site detection of specific heat capacity of geothermal water. It operates at high maximum temperatures (170 degrees C) and pressures (3 MPa), and can be installed as a bypass loop in geothermal power plants. The flow calorimeter was tested using water and aqueous sodium chloride solutions at a range of temperatures (20-160 degrees C) and absolute pressures (0.2-2.6 MPa). The experimental results deviate by about 1% from existing data reported in the academic literature. (C) 2014 Elsevier Ltd. All rights reserved.