In Prime - Bomb Calorimeters A known amount of the sample is burnt in Fuel cups (crucible) of Stainless Steel in a sealed chamber (Bomb). The air is replaced by pure Oxygen. Sample is ignited electrically. As the sample is burnt, heat is produced. The rise in temperature is measured.
Since barring the loss of heat, the amount of heat produced by burning the sample must be equal to the amount of heat absorbed by the Bomb Calorimeter assembly.
A knowledge of the Energy Equivalent of the Bomb Calorimeter assembly and of the rise in temperature enables one to calculate the heat of combustion of the sample.