Direct Calorimetry Vs Indirect Calorimetry

When you are going to calculate the heat involved in certain physical changes and chemical reactions, then you are studying the field of calorimetry. Taken from the word ‘calor,’ a Latin word that literally translates as heat, calorimetry was pioneered by a Scottish scientist named Joseph Black, who first noted the difference between temperature and heat. Using a calorimeter, he also classified two different forms of calorimetry namely direct and indirect calorimetry. So how do these two concepts differ?
The subject may sound too technical but the underlying principle is pretty basic. Indirect calorimetry involves measuring the heat that living things create from manufacturing carbon dioxide (CO2) and nitrogenous wastes, that are usually coming from the ammonia in aquatic creatures and so as the urea from terrestrial organisms.  Indirect calorimetry also deals with calculating the heat from O2 (oxygen) consumption.
Direct calorimetry also has the same goal of measuring the heat but it uses another approach ‘“ the organism under study is contained inside a calorimeter for direct observation and calculation of values.
Heat is basically calculated using the formula q = ms∆T where ‘m’ stands for the mass, ‘s’ for the specific heat, and ‘∆T’ for temperature change. Their product leads to the ‘q’ which is the heat or energy. Take note, this is just one of the many formulas used to estimate energy expenditures.
Of course, because of the advancement in human technology, humans are able to compute for the heat energy, while incorporating other variables in the formula. This is achieved by using modern calorimeters like the constant volume calorimeter, in the concept of Bomb calorimetry.
Many say that indirect calorimetry is the more accurate measurement tool. It gives a precise calculation of heat, because it involves actual oxygen uptake to give the caloric burn rate. It uses the principle of requiring 208.06 mL of O2 per burning of 1 calorie. Thus, there is a more direct relationship between calorie burning and consumed O2. To obtain such values, indirect calorimetry usually uses other devices, such as the incentive spirometer and other equipments that measure values of inspiration and expiration. It is also very feasible, most likely, because of the omission of the need of containing the subject under study.
Direct calorimetry aims to measure the actual heat getting out from the body. If you think about the concept itself, it is somewhat impractical and difficult to monitor, unless of course, you contain the entire subject inside a calorimeter for a certain period of time. This is easy if the test subject is little but what if it’s man-size?
All in all,

1. Direct calorimetry measures the heat output by the subject, through direct observation inside a calorimeter.
2. Indirect calorimetry measure heat by using the variable of O2 consumption and manufactured CO2.
3. Indirect calorimetry gives a more feasible and accurate measure of heat or energy, compared to direct calorimetry.