Oxygen bomb calorimeters measure the heat of chemical reactions or physical changes. This is done by performing the reaction in a container that maintains constant volume and they are thus designed to withstand the pressure that builds up as the reaction occurs. In an oxygen bomb calorimeter, the sample is loaded in the reaction chamber (called the bomb) along with oxygen; this is submerged in water within an insulating container. Once the sample is ignited, a thermometer which is partially submerged in the water, records the temperature changes that occur. Level of precision, capacity, and price are important considerations when purchasing a bomb calorimeter.