EntranceWhat is the name of the device for measuring the amount of heat? Device for measuring the amount of heat
In this article we will answer the question: “What is a calorimeter?” Let us determine the general characteristics of this mechanism, its operating principle and areas of application, functionality and measuring quantities. We will also pay attention to the classification and description of some specific species.
Introduction
Answering the question of what a calorimeter is, in general terms it can be characterized as a device by which the amount of heat that is released or absorbed during the course of physical, chemical or biological processes is measured.
The introduction of a new terminological unit “calorimeter” was proposed in 1780 by P. Laplace and A. Lavoisier. A similar device is also used in the nuclear branch of physics, which studies elementary particles, and it is called an ionization calorimeter. However, the function of this device is to measure the energy potential of particles.
Modern mechanism
Determining the heat capacity with a modern calorimeter makes it possible to record the value under study with an accuracy of ten to one hundredth of a percent. The range in which this device can operate ranges from 0.1 to 3500 Kelvin. The type of calorimeter device is very diverse. It can be determined by the nature of the process being studied, as well as its duration. Another important parameter for determining the type of mechanism is the temperature range in which measurements take place, as well as the amount of heat being measured.
Determining the energy equivalent with a calorimeter can show the subject the amount of body that is released during the combustion of a fuel resource. This can be done thanks to the expression Q = C∆T, in which C is the thermal (energy) equivalent indicator. Set determination parameters by calibrating the device. Another quantity, ∆T, is a function of the known output signal of the calorimeter.
Distribution by type
It is impossible to answer the question of what a calorimeter is without familiarizing yourself with its types.
One of the most common representatives of such devices is the integrator calorimeter. It is intended to determine the total amount of heat Q that is released at the beginning of the reaction and its end.
Another well-known calorimeter is a device for measuring heat power, that is, the rate at which heat is released - L. They can also be divided according to the design of the mechanism and measurement methodology, approach. There are also liquid and solid calorimeters. Single and differential devices are also available.
Heat measurement
What is a calorimeter in physics? The definition says that it is a device for measuring the amount of heat generated. In this case, the heat released during a chemical reaction can only be determined using a liquid integrator calorimeter.
The design is presented in the form of a vessel filled with liquid (usually water). It contains a chamber for conducting an experiment (“calorimetric bomb”), a stirrer, a thermometer and a heating device.
Measurements of calorimetric systems
Making adjustments to the natural course of heat release of the system can be detected when any of its states change. They, in turn, are determined by analyzing the amount of heat that is introduced into the device. The calorimeter constant is determined before the start of measurement work and is compared with the specified and adjusted value. The devices are calibrated, thanks to which the coefficient is determined. It must be multiplied with the change in temperature of the device measured by the thermometer.
Presence of side effects
In fact, calorimetric data directly shows only the total number of heats that are studied in the process. You can also find out by the presence of a side process (or processes) that could cause the phenomenon of mixing, evaporation of liquid, as well as breaking of an ampoule with substances, etc. Determining the calorimeter constant allows a person to gain access to a comparison of indicators of changes against the background of something. It is with its help that information is analyzed.
The heat of a secondary series of processes must be determined using experience or calculation, which is excluded from the research results. An example of a side effect is the inevitable heat exchange between the calorimeter and the surrounding space and matter.
Isothermal observations
There is an isothermal type integrator calorimeter that allows you to introduce changes in the aggregate states of bodies that form the main part of the system. An example is the melting of a mass of ice in the ice chamber of a Bunsen calorimeter. You can find out the change in heat, which affects the state of aggregation, but does not cause a change in temperature, by calculating the mass of the substance and the amount of heat that would be required to expend for this.
To determine the specific heat capacity of a calorimeter, you need to know that it is numerically equal to the amount of heat that is expended to heat a unit mass of matter. Its unit is J/kg▪K.
It is important to remember that specific heat capacity is an ambiguous characteristic. There is a relationship between the conditions for heat transfer and the value of work that accompanies this process.
Massive type
To determine the enthalpy value of a substance at temperatures up to 2500 degrees Celsius, massive integrators are used. The weight of this type of calorimeter can vary depending on the weight of the substance being measured, since the structure consists of metals. In fact, this is a block with a number of recesses for vessels. They contain reactions intended for a heating device and/or thermometer. The product of the heat value measured by the calorimeter and the difference in temperature rises in the block shows us the enthalpy of the substance(s).
Flow
You can determine the heat capacity of a gas or liquid using a flow labyrinth calorimeter. It records the temperature difference that enters and exits stationary flows of the substance under study. It also determines the power of such a flow and the strength of the heat generated by the electric heater, in joules.
Power Measuring Tool
When answering the question of what a calorimeter is, it will be important to mention the purpose of this device to determine power. Such a device, unlike an integrator, must be endowed with significant heat transfer capacity. This is necessary so that it can remove the amount of heat that is introduced into it. It follows from this that the state of the calorimeter is in instantaneous measurement.
The thermal value of the process power is found using shell calorimeters. The invention was made by a physicist from France, E. Calvet. Initially, the mechanism was presented in the form of a metal block equipped with channels. Special cylindrical cells intended for carrying out the process under study were laid out on them. The metal used in the construction of the camera is the shell. Its temperature should be kept at a constant level with an accuracy of five to six Kelvin.
The difference between the temperature of the cell and the block is measured using a thermopile with up to a thousand solders. The indicators of its EMF and heat transfer of the cell are quantities proportional to the small difference in temperature that occurs between components such as the block and the cell. In this case, heat must be released or absorbed in the cell itself. Very often, such blocks contain a pair of cells that will operate differentially.
Name and classification
Common names for calorimeters are:
- for chemical reactions;
- bomb;
- isothermal type;
- low temperature type;
- ice type.
All of them have data on historical origin. They usually owe their name to the area in which they will be used. However, these names do not refer to comparative or complete characteristics.
The general type of classification of calorimeters is constructed using as a basis consideration of one of the three main quantities, separately or together. It is the approach to the analysis of indicators that determines the temperature measurement technique, which has:
- calorimetric system Tc;
- shell That;
- the amount of heat released L per unit time (thermal power).
Calorimeters with constant values of Tc and To are of the isothermal type, and devices in which Tc = To are called adiabatic. If the device operates in conditions with a constant difference between temperatures, then it is called a calorimeter with a constant flow of heat exchange. The isoperibol mechanism has a constant To, and Tc is a thermal function of power L.
Final results
There are a number of factors that can affect the final measurement result. One of them is the presence of changes that affect their final result. It is due to the reliability of the automatic set of temperature controllers for an isothermal or adiabatic shell. In the last of them, the temperature value is determined by its proximity to the changing conditions of the entire calorimetric system. This design has the lightness of a metal screen and is equipped with a heating device that reduces the flow and value of heat transfer to a certain level at which the temperature of the calorimeter will change only by decimal parts of a degree per minute. This can make it possible to reduce the heat exchange occurring during a calorimetric experiment to extremely low values that can be neglected.
The devices discussed in the article play a huge role in human life and are one of the very significant achievements of science. The main function of the calorimeter is to study temperature change data and determine the presence of defects in the heat transfer process. There are various ways to classify these devices, associated with specific parameters that differ sharply from each other. A wide variety of metals can serve as materials for manufacturing, for example, there are copper calorimeters, lead, steel and others. In addition to pure substances, alloys can also be used.
Calorimeter, m. [from Latin. calor – warmth and Greek. metron – measure] (physical). A device for measuring the amount of heat. Large dictionary of foreign words
The need for prompt determination of heat consumption and heat loss has become particularly acute recently, when the requirement to save fuel and energy resources has come to the fore. Heat consumption is measured using heat meters . In this case, the problem is solved in accordance with the algorithm:
Q = V T C T t (T 1 -T2) MJ,
Where: Q- the amount of heat given off by the coolant during time t;
V T- volumetric flow rate of coolant (water), m 3 /s;
T 1;T 2- coolant temperature at the consumer inlet and outlet, 0 C;
S T- specific heat capacity of the coolant, MJ/m 3.
In the simplest case, the heat meter circuit should contain a coolant flow sensor (water meter), two temperature sensors and a computing unit that implements the above algorithm (Fig. 5.7).
Industrial systems require more complex heat meter circuits that take into account the change in enthalpy of the coolant and heat consumption for hot water supply.
Individual heat meters, widespread in a number of European countries, estimate the heat consumption of individual consumers, for example, central heating radiators (see Fig. 5.8). They contain a sensitive element - a glass graduated tube filled with tetralin. Its systematic heating leads to the evaporation of the liquid, which is used to judge the heat consumption. Heat consumption is calculated using a computer, into the database of which the characteristics of each radiator are previously entered.
Rice. 5.7. Diagram of the simplest heat meter: 1-supply (straight) pipeline; 2-outlet (return) pipeline; 3-water meter; 4-coolant temperature sensor; 5-temperature sensor of used coolant.
Rice. 5.8. Individual heat meter on a central heating radiator.
There are three main ways to control the processes of heat supply and electricity supply - manual, automatic and semi-automatic. To implement them, various instruments and devices are used. The most common ones are: manual taps, valves, switches, switches and regulators of various parameters. Let's look at some of them.
Solenoid valves - these are devices in which the valve movement is ensured by an electromagnet (Fig. 5.9). The solenoid valve provides, as a rule, two positions - open and closed.
The valve opens when there is control electrical voltage from the control circuit. In its absence, the valve is held closed by the action of a spring.
Rice. 5.9. Solenoid valve: 1-pipe; 2-valve; 3-movable core; 4-solenoid; 5-spring.
Electromechanical valve (valve) contains a tap or valve as a working element. The working element is driven by an electric motor, rack, cam or worm gear. To increase the force and reduce the speed of the crane, a gearbox is used (Fig. 5.10).
By changing the polarity of the supply voltage supplied to the DC motor, you can close or open the valve. If an AC motor is used, it must be reversed by switching the windings.
Electromechanical devices allow you to smoothly change the position of the working body (valve, tap) from completely open to completely closed. This ensures smooth regulation of the flow rate from υ f =0 before υ f = υ max . .
Rice. 5.10. Electromechanical valve: 1-pipeline; 2-valve; 3-rack mechanism; 4-gearbox; 5-electric motor.
Electric pumps allow the movement of liquids and gases through pipelines from areas of low static pressure to areas of higher pressure. It is effective to use adjustable (for example, frequency regulation) electric drives, which improve the operation of the control system and provide noticeable energy savings.
Automatic regulators - these are devices that ensure that an object’s parameter is maintained at a constant level or changes according to a given law. An automatic regulator must contain: a sensitive element or sensor of the controlled parameter; regulatory body; a master device that determines the required parameter value. An example of such a device is an automatic thermostat installed in front of the heating devices of the heating system (Fig. 5.11).
It works as follows. The valve shutting off the pipeline is mechanically connected to a sealed harmonic sensitive element-temperature sensor, the inside of which is filled with a heat-sensitive mass that can expand as the temperature rises. As the air temperature in the room increases, the temperature sensor expands and blocks the pipeline, thereby limiting the flow of coolant into the radiator. The required room temperature can be set manually by turning the protective cap with a spring.
According to new energy-saving standards, owners of apartment buildings must worry about how to organize heat metering. General building heat meters must be installed, and if you want to take into account heat in apartments, individual heat meters must be installed. Heat meters are a device for measuring the amount of heat. As a result of measurements, we obtain the amount of heat. Depending on the heat meter, the amount of heat is measured in:
1. In gigacalories (Gcal/h).
2. In kilowatt-hours (kW/h)
Scope of application of heat meters
Heat meters are installed starting from central, district, etc. thermal power plants. to the consumer. In this way, the consumption of thermal energy and its distribution are controlled. Heat meters are installed in systems where the coolant is water or steam. These are multifunctional microprocessor devices. They derive their readings based on measurements of temperature, pressure, and coolant flow. They include heat calculators. The devices are programmed to calculate the amount of heat according to current standards. Such devices have reliable protection against unauthorized access.
How is the amount of heat calculated?
The program for calculating the amount of heat specified to the heat calculator takes into account the type of coolant and the structure of the heat supply system. In different heating systems, the amount of coolant may change due to its removal into the hot water supply system or as a result of leaks. Calculation algorithms will be different depending on whether the heating system is open or closed, that is, whether the amount of coolant in it changes or not.
The amount of heat is measured indirectly. The error of this value depends on the error of the means of measuring primary quantities, the error of calculations, the error of the heat calculator, which depends on the error of the calculated ratios reflecting the properties of steam and water. The formula for calculating the amount of heat is:
Q=Q m ×k×(t 1 -t 2)×t, in Gcal\h, Q=V×k×(t 1 -t 2), in kW\h
where Q m (t) is the mass of the passed coolant,
V (m 3) = volume of coolant passed through,
t 1, t 2 (°C) – inlet and outlet temperature of the coolant,
t (h) - time,
k is the thermal coefficient of the coolant (GOST R EN 1434-1-2011 Appendix “A”).
Accordingly, to determine the amount of heat, readings from temperature sensors at the input and output of the system are required. Such sensors are installed on the supply pipe of the heating system and on the return pipe. From them and from the flow meter, data for calculation is supplied to the computing processor. In most cases, the calculated data is displayed on the LCD screen. As a result of calculations, a data archive is formed that can be opened for viewing.
The heat metering device also includes pressure converters and shut-off valves. The price of the device depends on the device and the type of meter. The main difference lies in the type of flow meter. However, regardless of what type of flow meter is used, each heat meter stores data on average hourly and average daily flow rates. The heat meter can be located not only in the heating station, but also outside it, providing the ability to take readings remotely.
Methods for measuring coolant flow
Different types of heat meters differ in the method of measuring coolant flow. According to the measurement method, flow meters are distinguished:
- variable pressure drop;
- turbine (vane);
- ultrasonic;
- electromagnetic.
The variable pressure differential method is outdated and is now practically not used in devices. Turbine flow meters have major drawbacks, although they are the most economical options for flow measurement. Ultrasonic flowmeters are the most common today due to their high reliability and accuracy. Electromagnetic flowmeters, which are quite modern and reliable, are about as popular.
The flow meter is usually installed on the supply pipe. If the system is being disassembled or there are leaks, they are recorded using the readings of a flow meter installed on the return pipeline.
Requirements for heat meters
Regardless of the design features of the meter, they are all quite accurate. The flow measurement method does not affect its readings. Basic requirements for meters that must be met when installing them:
- The brand of the heat meter must be included in the register of devices acceptable for use in the commercial sphere;
- the heat meter must have a conclusion from the metrological government service;
- installation of such a meter is licensed work, so you must contact the appropriate services.
The general principles for calculating the amount of heat spent on heating depend on the temperature difference, and not on the temperature of the coolant at the entrance to the heating system.
Device for measuring particle energy.
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Modern calorimeters
Modern calorimeters operate in the temperature range from 0.1 to 3500 and allow one to measure the amount of heat with an accuracy of 0.01-10%. The design of calorimeters is very diverse and is determined by the nature and duration of the process being studied, the temperature range at which measurements are made, the amount of heat measured and the required accuracy.
Types of calorimeters
Calorimeter designed to measure the total amount of heat Q, released in the process from its beginning to completion, is called integrator calorimeter
Calorimeter for measuring thermal power (rate of heat release) L and its changes at different stages of the process - power meter or calorimeter-oscilloscope. Based on the design of the calorimetric system and the measurement method, a distinction is made between liquid and solid calorimeters, single and double (differential).
Liquid calorimeter-integrator
A variable temperature liquid integrator calorimeter with an isothermal shell is used to measure the heats of solution and heats of chemical reactions. It consists of a vessel with a liquid (usually water), which contains: a chamber for carrying out the process under study (“calorimeter bomb”), 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, the totality of which is called the calorimetric system of the device.
For liquid calorimeters, the isothermal temperature of the shell is maintained constant. When determining the heat of a chemical reaction, the greatest difficulties are often associated not with taking into account side processes, but with determining the completeness of the reaction and the need to take into account several reactions.
Calorimetric measurements
Changing the state (for example, temperature) of the calorimetric 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 making measurements, the calorimeter is calibrated - the change in temperature of the calorimetric system is determined when a known amount of heat is imparted to it (by the 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 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 should be multiplied to determine the amount of heat introduced into it. The thermal value of such a calorimeter is the heat capacity (c) of the calorimetric system. Determining an unknown heat of combustion or other chemical reaction Q comes down to measuring the temperature change Δ t calorimetric system caused by the process under study: Q=cΔ t. Usually the value Q refers to the mass of the substance located in the calorimeter chamber.
Side processes in calorimetric measurements
Calorimetric measurements make it possible to directly determine only the sum of the heats of the process under study and various side processes, such as mixing, evaporation of water, breaking of an ampoule with a substance, etc. The heat of side processes must be determined experimentally or by calculation and excluded from the final result. One of the inevitable side processes is the heat exchange of the calorimeter with the environment through radiation and thermal conduction. In order to take into account side processes and, above all, heat transfer, the calorimetric system is surrounded by a shell, the temperature of which is controlled.
Isothermal integrator calorimeter
In an integrator calorimeter of another type - isothermal (constant temperature), the introduced heat does not change the temperature of the calorimetric system, but causes a change in the aggregate state of the body that is part of this system (for example, the melting of ice in a Bunsen ice calorimeter). The amount of heat introduced is calculated in this case from the mass of the substance that changed the state of aggregation (for example, the mass of melted ice, which can be measured by a change in the volume of the mixture of ice and water), and the heat of phase transition.
Massive integrator calorimeter
A massive integrator calorimeter is most often used to determine the enthalpy of substances at high temperatures (up to 2500 °C). The calorimetric system of this type of calorimeter is a block of metal (usually copper or aluminum) with recesses for the vessel in which the reaction occurs, a thermometer and a heater. The enthalpy of a substance is calculated as the product of the thermal value of the calorimeter and the difference in the temperature rise of the block, measured after dropping an ampoule with a certain amount of substance into its nest, and then an empty ampoule heated to the same temperature.
Flow labyrinth calorimeters
The heat capacity of gases, and sometimes liquids, is determined in the so-called. flow labyrinth calorimeters - by the temperature difference at the inlet and outlet of a stationary flow of liquid or gas, the power of this flow and the Joule heat released by the electric heater of the calorimeter.
Calorimeter - power meter
A calorimeter operating as a power meter, as opposed to an integrator calorimeter, must have significant heat exchange so that the amounts of heat introduced into it are quickly removed and the state of the calorimeter is determined by the instantaneous value of the power of the thermal process. The thermal power of the process is found from the heat exchange between the calorimeter and the shell. Such calorimeters, developed by the French physicist E. Calvet, are a metal block with channels into which cylindrical cells are placed. The process under study is carried out in the cell; the metal block plays the role of a shell (its temperature is maintained constant with an accuracy of 10 −5 -10 −6 K). The temperature difference between the cell and the block is measured by a thermopile with up to 1000 junctions. The heat transfer of the cell and the emf of the thermopile are proportional to the small temperature difference that occurs between the block and the cell when heat is released or absorbed in it. Most often, two cells are placed in a block, operating as a differential calorimeter: the thermopiles of each cell have the same number of junctions and therefore the difference in their EMF allows one to directly determine the difference in the power of heat flows entering the cells. This measurement method makes it possible to exclude distortions of the measured value by random fluctuations in the temperature of the block. Two thermopiles are usually mounted on each cell: one makes it possible to compensate the thermal power of the process under study based on the Peltier effect, and the other (indicator) serves to measure the uncompensated part of the heat flow. In this case, the device operates as a differential compensation calorimeter. At room temperature, such calorimeters measure the thermal power of processes with an accuracy of 1 μW.
Names of calorimeters
The usual names of calorimeters - “for a chemical reaction”, “bomb”, “isothermal”, “ice”, “low-temperature” - are of historical origin and indicate mainly the method and area of use of calorimeters, without being either a complete or comparative description of them.
General classification of calorimeters
A general classification of calorimeters can be constructed based on consideration of three main variables that determine the measurement technique: temperature of the calorimetric system Tc; sheath temperature T o, surrounding the calorimetric system; amount of heat L, released in the calorimeter per unit time (thermal power).
Calorimeters with constants Tc And T o called isothermal; With Tc = T o- adiabatic; calorimeter operating at a constant temperature difference Tc - T o, called a calorimeter with constant heat exchange; for an isoperibol calorimeter (also called a calorimeter with an isothermal shell) the constant T o, A Tc is a function of thermal power L.
Factors influencing the final measurement result
An important factor influencing the final measurement result is the reliable operation of automatic temperature controllers for isothermal or adiabatic shells. In an adiabatic calorimeter, the temperature of the shell is controlled so that it is always close to the changing temperature of the calorimetric system. The adiabatic shell - a light metal screen equipped with a heater - reduces heat transfer so much that the temperature of the calorimeter changes only by a few ten-thousandths of a degree/min. Often this makes it possible to reduce heat transfer during a calorimetric experiment to an insignificant value that can be neglected. If necessary, a correction for heat transfer is introduced into the results of direct measurements, the calculation method of which is based on Newton’s law of heat transfer - the proportionality of the heat flow between the calorimeter and the shell to the difference in their temperatures, if this difference is small (up to 3-4 ° C).
For a calorimeter with an isothermal shell, the heats of a chemical reaction can be determined with an error of up to 0.01%. If the dimensions of the calorimeter are small, its temperature changes by more than 2-3 °C and the process under study is long, then with an isothermal shell the correction for heat transfer can be 15-20% of the measured value and significantly limit the accuracy of measurements. In these cases, it is more expedient to use an adiabatic shell.
Using an adiabatic calorimeter, the heat capacity of solid and liquid substances is determined in the range from 0.1 to 1000 K. At room and lower temperatures, an adiabatic calorimeter, protected by a vacuum jacket, is immersed in a Dewar flask filled with liquid helium, hydrogen or nitrogen. At elevated temperatures (above 100 °C), the calorimeter is placed in a thermostatically controlled electric oven.