calorimetry

calorimetry , measurement of heat and the determination of heat capacity . Heat is evolved in exothermic processes and absorbed in endothermic processes; such processes include chemical reactions, transitions between the states of matter, and the mixing of two substances to form a solution (see thermodynamics ). A number of different units are used in heat measurement, e.g., the calorie , the British thermal unit (Btu), and the joule . The apparatus used in heat measurement is called a calorimeter. The measurement given by the most common type of calorimeter depends upon the temperature change in a fixed quantity of water (or some other liquid whose heat capacity is known) when heat is transferred between the water and an exothermic or endothermic process. If the temperature change is not too large, then the heat transferred is equal to the heat capacity of the water times the mass of the water times the change in temperature. The accuracy of this method of heat measurement depends on the assumption that all the heat transferred in the process passes into or out of the water in which the temperature change is measured, no heat being lost to the environment and none being absorbed by the walls of the container. The amount of heat given off by the combustion of a fuel can be determined very accurately in the so-called bomb calorimeter, which consists of a combustion chamber (the "bomb" ) set in another chamber filled with water. Heat generated by combustion of the fuel is transmitted to the water, raising its temperature. The calorie content of food is tested this way. Calorimeters are also employed to measure the energies of elementary particles .

calorimetry The measurement of energy expenditure by the body. Direct calorimetry is the direct measurement of heat output from the body, as an index of energy expenditure, and hence energy requirements. The subject is placed inside a small, thermally insulated room, and the heat produced is measured.

Indirect calorimetry is a means of estimating energy expenditure by either measurement of the rate of oxygen consumption, using a spirometer, (each litre of oxygen consumed is equivalent to 20 kJ energy expenditure) or estimation of the total production of carbon dioxide over a period of 7–10 days, after consumption of dual isotopically labelled water (i.e. water labelled with both ²H and ¹⁸O). See also isotopes.