Calorimeter

A calorimeter a device to measure the amount of heat released or absorbed in any physical, chemical or biological process.

Modern calorimeters operate in the temperature range of 0.1 to 3500 Kelvin and allow you to measure the amount of heat with an accuracy of 0.01-10%. The arrangement of calorimeters is very diverse and is determined by the nature and duration of the process under study, the range of temperatures at which measurements are made, the amount of heat measured and the accuracy required.

In thermodynamics, calorimeters are used to measure the enthalpy of a thermodynamic process.

Calorimeters and solar energy

One of the applications of calorimeters in the field of solar energy is found in solar thermal energy systems. These devices are important for calculating thermal efficiency in heating systems and generation of domestic hot water.

The calorimeter, in a heating system, is a device that is installed in each radiator and measures two temperatures: that of the surface of the same and the environment of the room, calculating the consumption with these data and based on the characteristics and radiator size

Types of calorimeters

A calorimeter designed to measure the total amount of heat Q released during the process from its beginning to its end is called an integrating calorimeter.

The calorimeter is used to measure the thermal power and its changes at different stages of the thermodynamic process, by means of a power meter or an oscilloscope-calorimeter. The design of the calorimetric system and the method of measurement distinguish between liquid and massive calorimeters, single and double (differential).

Liquid integrating calorimeter

A variable temperature liquid calorimeter integrator with an isothermal cover is used to measure dissolution heats and heats of chemical reactions, or chemical energy. It consists of a container with a liquid (usually water), in which there is: a chamber to carry out the process under study ("calorimetric pump"), a stirrer, a heater and a thermometer. The heat released in the chamber is then distributed between the chamber, the liquid and other parts of the calorimeter, whose totality is called the device's calorimetric system.

In liquid calorimeters, the cover's isothermal temperature remains constant. When determining the heat of a chemical reaction, the greatest difficulties are often associated not with considering secondary processes, but with determining the integrity of the reaction and the need to take into account several reactions.

Calorimetric measurements

A change in the state (for example, the temperature) of the calorimeter system allows you to measure the amount of heat introduced into the calorimeter. The heating of the calorimetric system is recorded with a thermometer. Before taking measurements, the calorimeter is calibrated: the temperature change of the calorimetric system is determined when a known amount of heat is communicated to it (by a calorimeter heater or as a result of a chemical reaction in the chamber with a known amount of a standard substance).

As a result of the calibration, the thermal value of the calorimeter is obtained, that is, the coefficient by which the change in the temperature of the calorimeter measured by the thermometer must be multiplied to determine the amount of heat introduced into it. The thermal value of such a calorimeter is the heat capacity of the calorimetric system. The determination of the unknown calorific value or other reaction of the chemical thermodynamics Q is reduced to measure the temperature change Δ t of the calorimetric system caused by the process under study: Q = c Δt. Typically, the Q value refers to the mass of the substance in the calorimeter chamber.

Secondary processes in calorimetric measurements

The calorimetric measurements allow to directly determine only the sum of the heats of the process under study and several secondary processes, such as mixing, water evaporation, rupture of a vial with a substance, etc. The heat of the secondary processes must be determined empirically or by calculation and excluded from the final result.

One of the inevitable secondary thermodynamic processes is the heat exchange of the calorimeter with the environment through radiation and thermal conductivity. To take into account the secondary processes and, above all, heat transfer, the calorimetric system is surrounded by a shell whose temperature is controlled.

Isothermal Integrating Calorimeter

In the study of thermodynamics there is another type of integrative calorimeter: isothermal (constant temperature), the heat introduced does not change the temperature of the calorimetric system, but causes a change in the state of aggregation of the body that is part of this system (for example , the ice melting in the Bunsen ice calorimeter).

The amount of heat introduced is calculated in this case by the mass of the substance that changed the state of aggregation (for example, the mass of melted ice, which can be measured by the change in the volume of the mixture of ice and water) and the heat of the phase transition.

Mass Integrative Calorimeter

A massive integrative calorimeter is used more frequently to determine enthalpy of substances at high temperatures (up to 2500 degrees Celsius). The calorimetric system for this type of calorimeter is a metal block (usually copper or aluminum) with holes for the vessel in which the reaction takes place, for the thermometer and the heater.

The enthalpy of a substance is calculated as the product of the calorimeter's thermal value by the difference in the temperature increase of the block, measured after dropping a blister with a certain amount of substance in its nest, and then an empty blister heated to the same temperature

Labyrinth calorimeter flow

The heat capacity of gases, and sometimes of liquids, is determined in the so-called. Flow labyrinth calorimeters: according to the difference in temperature at the entrance and exit of a stationary flow of liquid or gas, the power of this flow and the heat in joules emitted by the calorimeter electric heater.

Calorimeter - power meter

A calorimeter that functions as a power meter, in contrast to an integrating calorimeter, must have a significant heat exchange so that the amounts of heat introduced into it are quickly eliminated and the calorimeter status is determined by the instantaneous value of the power of the thermal process. The thermal power of the process is found in the heat exchange of the calorimeter with the housing.

These calorimeters, developed by the French physicist E. Calvet, are a metal block with channels in which the cylindrical cells are placed. In the cell, the investigated process is carried out; a metal block plays the role of a shell (its temperature remains constant with an accuracy of 10 −5 –10 −6K) The temperature difference between the cell and the unit is measured by a thermopile that has up to 1000 joints. The heat exchange of the cell and the EMF of the thermopile is proportional to the small temperature difference that arises between the unit and the cell when heat is released or absorbed.

Very often, two cells are placed in the block, which function as a differential calorimeter: the thermopile of each cell has the same number of joints and, therefore, the difference in its EMF allows you to directly determine the difference in power of the heat flow entering the cells.

This measurement method eliminates the distortion of the value measured by random fluctuations in the block temperature. In general, two thermal batteries are mounted in each cell: one allows you to compensate the thermal power of the process under study based on the Peltier effect, and the other (indicator) is used to measure the uncompensated part of the heat flow. In this case, the device works as a differential compensation calorimeter. At room temperature, calorimeters measure the thermal power of processes with an accuracy of 1 μW.