What is the working principle of a bomb calorimeter (constant volume or constant pressure)?

What is the working principle of a bomb calorimeter (constant volume or constant pressure)?

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  1. The answer of Thomas Strickland Jr. is mostly correct, but I think what I am going to tell you answers your question in full. A bomb calorimeter is for heavy fuels, solid or liquid, and consequently, it works with a constant amount (mass) of sample. As a consequence, what is really measured is the internal energy u, NOT the enthalpy h. Perhaps this is what you mean with constant v and constant p, respectively; please note also that volume doesn’t need to be constant, in fact, it is not because the fuel transforms itself into combustion gases that expand. Only for fuel gases, the calorimeter can be in the form of a continuous flame whose power divided by its mass flow gives its enthalpy (and this time, yes, the pressure is almost constant).
    We engineers normally understand that the heating value of a fuel is h and not u. This is so because large combustion equipment are open systems, continuously accepting fuel plus air, and rejecting gases and ash; only small, domestic apparatus function in batch mode, loading a charge of fuel and emptying of ashes periodically. Therefore, measurements of heating value must be corrected, and this is normally done by the laboratory or even the apparatus itself. The correction uses the definition of enthalpy for any substance h = u + p*v, the elemental analysis of the fuel to calculate which gases are formed, and the fact that the volume of the solid is negligible compared with that of gases.

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  2. Since, as you know, one calorie is the equivalent to raising 1 gram of water 1 degree Celsius, a bomb calorimeter is immersed in a given amount of water, with the food in question, mixed with an oxidizer. The mixture is ignited, electrically, and burned completely. The temperature is measured, continuously, during the process. When the water temperature reaches its maximum, the total heat generated can be calculated, based on initial and max temperatures of the known quantities of water. The material is burned in the bomb, under constant volume. The pressure is irrelevant.

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