Calorimetry: Direct & Indirect Methods
The word calorimetry is derived from the Latin word “calor”, meaning “heat” and the Greek word “metron” meaning “measure“. Calorimetry is the process of measuring the amount of heat released or absorbed during a chemical reaction. Calorimetry is used to measure the heat of a reaction, it is a crucial part of thermodynamics. By knowing the change in heat, it can be determined whether or not a reaction is exothermic (releases heat) or Endothermic (absorb heat).
Calorimetry also plays a large part of everyday life, controlling the metabolic rate in human and consequently maintaining such functions like body temperature. The principal of Calorimetry indicates the law of conservation energy the total heat lost by the hot body is equal to the total heat gained by the cold body.
The equation of Calorimetry is Q=mc∆T, Where Q=heat evolved, m= mass, c= specific heat capacity and ∆T.
Calculation of energy:
Basal metabolism is the energy expended in the cellular processes necessary to the maintenance of life. Total energy expenditure is defined as the total amount of energy humans need to function total energy expenditure can be subdivided into basal energy expenditure. Diet induced thermogenesis and physical activity related energy expenditure. The Harris-benedict equations are commonly used for calculation of the BMR in adults:
- BMR for men (kcal)=66 +13.7(weight in kg)+ 5(height in cm)- 6.8 (age in years)
- BMR for women (kcal)=655 + 9.6 (weight in kg) + 1.85 (height in cm)- 4.7(age in years)
Direct calorimetry measure the heat production of an individual in calories when placed in an insulated chamber, the heat is transferred to surrounding water. This is a very accurate method of measuring energy expenditure. Heat released into the walls, is measured and used to assess energy expenditure using the relationship between energy expenditure and heat production, similar to a bomb calorimeter.
Human metabolic efficiency:
ATP= 40% of substrate energy
Heat=60% of substrate energy
Based on known concentrations of gases in atmospheric air. Indirect calorimetry measure respiratory gas exchange to estimate energy metabolism. On a cellular level, metabolism entails the production of adenosine triphosphate (ATP) with carbon dioxide and water as by products, by consuming oxygen and the burning substrate such as glucose, free fatty acids, and amino acids as the energy produced equals the energy consumed. Indirect calorimetry measure Oxygen consumption and carbon dioxide production represent real-time energy metabolism.
Measures respiratory gas concentrations estimates total body energy expenditure based on oxygen used carbon dioxide produced. Heat production can be calculated from a measurement of VO2and CO2.
- Accurate over long recording period
- Good for resting metabolic measurements
- Take a long time
- Error because exercise equipment
Heat production can be calculated from a measurement of VO2 and/or CO2
Calorimetry are used to measure the amount of heat released or absorbed by a process. The process could be a reaction, the cooling of a solid the melting of ice any process that involves a transfer of heat. The heat causes a change in temperature in the calorimeter. Since the change in temperature can be measured the heat released by the reaction can be determined.