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Max Meneveau asked the Naked Scientists:Hey everyone, My name is Max Meneveau from Baltimore Maryland in the US, and my question is about measuring calories.How do companies measure the amount of calories found in their food products? I know they use a machine called a bomb calorimeter that burns the item and measures the heat. My question then I guess is, how do they burn the item without adding heat from, lets say a match, and how do they know they have burned the whole piece of food.Love the podcast!!Max MeneveauWhat do you think?
The Mahler bomb calorimeter is a device used to determine the high calorific power of a solid or a liquid; for gaseous combustibles it's used the Junkers calorimeter.The heat generated from the combustion reaction of the analyzed combustible is absorbed from a water, or other liquid, mass of which is observed the temperature increase.It consists of a cylindrical container of about 500 cm^3, closed with a screw cap which has 2 holes. In one of them it's introduced an iron bar which sustaines a platinum crucible, in the other hole it's introduced another iron bar which sustaines a small iron curl connected to the first iron bar and put in the crucible.In the crucible it's introduced the sample of combustible (usually 40 cm^3) and then pressurized oxygen is introduced in the bomb [my note: I assume this is the reason it's called "bomb"]. The bomb is immersed in a calorimeter full of water which have a stirrer and a thermometer.The stirrer is started and when the thermometer reads a constant temperature, a voltage is created between the two iron bars which heats the little curl which burns by oxidation; the heat released burns the sample. The thermometer reads the temperature which arises suddenly to a certain value and then decreases slowly. It's then possible to compute the high calorific power with the formula:Qs= [(T2-T1)*(P+A)]/pwhere T2 and T1 are, respectively, the maximum and the minimum temperature, P is the amount of water introduced in the calorimeter, A is the water equivalent of the calorimeter and p is the amount of combustible used.