The test method is designed to measure the heat of hydration of cement when mixed with water. The measurement takes place at essentially constant temperature, if the instrument and the measurement are well designed, therefore it is assumed to be the “isothermal heat of hydration of cement”. An isothermal heat conduction calorimeter (here called calorimeter) consists of a thermostatic heat sink upon which two heat flow sensors are placed. The sample is placed in an ampoule that is placed in an ampoule holder that is in contact with one of the heat flow sensors, and an inert reference is placed in contact with the other. The sample ampoule and the reference ampoule are thermally connected by heat flow sensors to a thermostatic heat sink. The output from the calorimeter is the difference between the outputs from the sample heat flow sensor and the reference heat flow sensor.