A ballistic bomb calorimeter.

The classical methods for determining the calorie value of foodstuffs are both long and tedious, and a number of alternative methods have been suggested for assessing the energy value of foods based on an analysis of the foodstuff and the use of factors with each of the chemical components to give energy content. None of these methods is really satisfactory, and when a high level of accuracy is required direct calorimetry must be used. An improved method which shortens the time taken for a single determination without loss of accuracy would be clearly of great value. The adiabatic method used by Raymond, Canaway & Harris (1957) reduced the time taken for a determination to approximately 30 min, but the equipment is complicated and expensive. A fundamental feature of this and the classical procedure is that the quantity of heat produced is measured by a small rise in temperature of a large mass of material, resulting in long equilibration times and the need for great accuracy of temperature measurement. A simple method proposed by FCry (1912) partly overcomes these disadvantages by making use of a light bomb casing of a standard heat capacity and eliminating the water calorimeter. In this way the equilibration time is reduced to 5 min and the temperature rise is increased to 20-30'. In the method now described no attempt is made to measure the equilibrium temperature of the bomb casing; instead, advantage is taken of the fact that on combustion of a sample most of the heat is transmitted to the upper parts of the bomb casing, which thus rapidly (50 sec) warm up to a high temperature (about 70°) and rapidly cool. We have found that the peak temperature is an accurate measure of the heat released, and we have termed this method of measuring heat quantity 'ballistic ', by analogy with the ballistic method of measuring electrical quantities. In an earlier communication (Fox, Miller & Payne, 1959) a commercially available bomb, modified for this technique by attaching a thermocouple to the lid, was described. Though it was satisfactory in operation, it was thought that some improvement in performance would result from a reduction in thermal capacity of the upper parts of the vessel. Also the conventional lid, with its gas inlet, electrode and thermocouple attachments, was difficult to dry after cooling with tap water between determinations.