The values are conventionally measured with a bomb calorimeter. They may also be calculated as the difference between the heat of formation Δ"H" of the products and reactants (though this approach is somewhat artificial since most heats of formation are typically calculated from measured heats of combustion). For a fuel of composition C"c"H"h"O"o"N"n", the (higher) heat of combustion is usually to a good approximation (±3%), though it can be drastically wrong if (for instance in the case of nitroglycerine () this formula would predict a heat of combustion of 0). The value corresponds to an exothermic reaction (a negative change in enthalpy) because the double bond in molecular oxygen is much weaker than other double bonds or pairs of single bonds, particularly those in the combustion products carbon dioxide and water; conversion of the weak bonds in oxygen to the stronger bonds in carbon dioxide and water releases energy as heat.