RU2655459C1 - Method of measuring heat capacity of materials - Google Patents

Abstract

FIELD: test technology.

SUBSTANCE: invention relates to thermal tests, namely to devices for determining the heat capacity of materials, and can be used to determine their thermal properties. Method for measuring material heat capacity is proposed, which is carried out by means of a differential calorimeter comprising two calorimetric cells placed inside a common heat-insulating body equipped with temperature meters, temperature fluctuations and electric power sources, is that the sample under study is placed in one of the cells and the temperature in each of the cells during their heating is measured. In this case, the heat capacity of each cell without a sample is preliminarily determined, for which an amount of energy is supplied to the cells having the same temperature, cells temperature is recorded when they reach thermal equilibrium, and their heat capacity is determined by the formula

where c_i(T) – heat capacity of the i-th cell at temperature T, J/(kg⋅°C); ΔT is a change in the cell temperature, °C; E_i – energy, heated cell, J. Sample is placed in one of the cells. For each cell, taking into account their heat capacity, energy necessary to heat it by a given amount is supplied. Cell containing the sample is supplied with additional energy until the cell temperatures equalize and the specific heat of the sample is determined by the formula. Value of the energy supplied to the cells is determined from the number of electrical pulses per heater of each cell from the discharge on them common for both heaters of the condenser cells, measuring the capacitor voltage before each pulse.

EFFECT: increased determination accuracy of the required parameter.

1 cl, 2 dwg

Description

The invention relates to thermal tests, and in particular to devices for determining the heat capacity of materials, and can be applied to determine their thermal properties.

A known method of measuring the heat capacity of materials by continuous heating by an electric heater with a constant speed of a sample in adiabatic conditions (SU 262438, G01K, 1970 [1]). At the same time, to increase the accuracy of determination, several samples are simultaneously heated, continuously measuring the heating power, and the arithmetic average of them is found, which is proportional to the specific heat. The disadvantage of this method is the low accuracy of determining the desired parameter, due to the low accuracy of determining the power used to heat the samples. The method involves measuring several parameters during the experiment — temperature, current, voltage. The use of standard current or voltage sources does not provide the necessary accuracy of stabilization and power measurements. In fact, the result is affected by a change in the internal resistance of the source and the resistance of the heater, so just measuring the current in the load and voltage for accurate measurements is not enough.

There is a method of measuring the heat capacity of materials, which consists in heating according to the same law two cylindrical samples of the material under study, identical in external dimensions and surface condition, but different in mass, with a measurement of the temperature change of the samples and the difference in the energies supplied to them (SU 685966, G01N 25/20, 1979 [2]). After establishing a temperature field close to stationary in the samples, the desired heat capacity can be determined from the time difference in the energy consumption spent on heating the samples and the measured increment of their temperature. The disadvantage of this method is the low accuracy of determining the desired parameter, due to the low accuracy of determining the power used to heat the samples. The use of standard current or voltage sources does not provide the necessary accuracy of stabilization and power measurements, since the result is affected by a change in the internal resistance of the source and the resistance of the heater.

Therefore, simply measuring current in a load and voltage for accurate power measurements is not enough.

Closest to the claimed in its technical essence is a method of measuring heat capacity, known from SU 1516926, G01N 25/20, 1989 [3]. The method for measuring heat capacity is carried out by means of a differential calorimeter containing two calorimetric cells located inside the equalizing temperature of the unit, equipped with temperature difference meters and sources of electrical power. Two tests are carried out, placing a comparison sample in one of the cells and a reference and test samples in succession, bring equal powers to the cells and record the change in the temperature difference of the cells after the power supply is stopped, and the desired parameter is calculated using the mathematical dependence, which includes experimentally measured values.

The disadvantage of this method is the low accuracy of determining the desired parameter, due to the low accuracy of determining the power used to heat the samples. The use of standard current or voltage sources does not provide the necessary accuracy of stabilization and power measurements, since the result is affected by a change in the internal resistance of the source and the resistance of the heater. Therefore, simply measuring current in a load and voltage for accurate power measurements is not enough. The inventive method of measuring the heat capacity of materials is aimed at improving the accuracy of determining the desired parameter.

This result is achieved by the fact that the method of measuring the heat capacity of materials is carried out by means of a differential calorimeter, including two calorimetric cells placed inside a common heat-insulating casing, equipped with temperature meters, temperature drops and sources of electrical power, and consists in the fact that the studied sample is placed in one of the cells measure the temperature in each of the cells during their heating. In this case, the heat capacity of each cell without a sample is preliminarily determined, for which a certain amount of energy is supplied to cells having the same temperature, the cell temperature is recorded when they reach thermal equilibrium, and their heat capacity is determined by the formula

where: c _i (T) is the heat capacity of the i-th cell at temperature T, J / (kg⋅ ° C);

ΔT is the change in cell temperature, ° C;

E _i is the energy that heated the cell, J,

a sample is placed in one of the cells, each cell, taking into account their heat capacity, is supplied with the energy necessary for its heating by a predetermined amount, additional energy is supplied to the cell containing the sample until the cell temperatures are equalized and the specific heat of the sample is determined by the formula

,

,

where ΔЕ is the energy required to heat the cell with the sample to a given temperature, J;

m is the mass of the sample, kg;

ΔT is the difference between the initial and final after heating the cell temperatures, ° C,

and the amount of energy supplied to the cells is determined by the number of electrical impulses to the heater of each cell from the discharge on them of the capacitor cell common to both heaters, measuring the capacitor voltage before each pulse.

Distinctive features of the proposed method are:

- preliminary determination of the heat capacity of each cell without a sample, for which a certain amount of energy is supplied to cells having the same temperature, the temperature of the cells is recorded when they reach thermal equilibrium, and their heat capacity is determined by the mathematical dependence given above;

- a sample is placed in one of the cells, each cell, taking into account their heat capacity, is supplied with the energy necessary for its heating by a predetermined amount, additional energy is supplied to the cell containing the sample until the cell temperatures are equalized and the specific heat of the sample is determined by mathematical dependence, the above;

- the amount of energy supplied to the cells is determined by the number of electrical impulses to the heater of each cell from the discharge on them of the capacitor cells common to both heaters, measuring the capacitor voltage before each pulse.

A preliminary determination of the specific heat of each cell without a sample allows one to extract from the obtained data those parameters that are related to the true specific heat of the sample, and thereby increase the accuracy of determining the desired parameter.

Placing a sample in one of the cells and supplying to each cell, taking into account their heat capacity, the energy necessary to heat it by a predetermined amount, and additional energy to equalize the cell temperatures, also increases the accuracy of determining the desired parameter to the cell containing the sample. Indeed, such actions make it possible to determine the energy spent only on heating the sample, since the additional energy is numerically equal to the energy spent on heating the sample from the initial to the final temperature. Thus, the heat capacities of the cells are excluded from the formula for calculations, which increases the accuracy of measurements.

Determining the amount of energy supplied to the cells by the number of electrical pulses to the heater of each cell from the discharge on them is common to both heaters of the capacitor cells, by measuring the capacitor voltage before each pulse, it is possible to very accurately determine the energy spent on heating. Usually, to achieve it, it is necessary to have a power source that provides not only high stability of power on the heaters, but also knowledge of its magnitude. The use of standard current or voltage sources does not provide the necessary accuracy of stabilization and power measurements. In fact, the result is affected by a change in the internal resistance of the source and the resistance of the heater, so just measuring the current in the load and voltage for accurate measurements is not enough. Features of the power supply according to the proposed algorithm can provide benefits due to the following factors:

- the energy of a single pulse, the feed heater of the calorimeter cell can be accurately determined by the formula E = C ₂ U ^2/2 where U - voltage read meter V ₁ (see the power supply circuit.). This energy does not depend on the value of the load resistance if the discharge time of the capacitance to the load is sufficiently large;

- in order to supply a predetermined amount of energy to a calorimeter cell, it is necessary to apply the necessary number of power pulses to it, summing their energies until a preset value is reached.

The essence of the proposed method is illustrated by an example implementation and drawings. In FIG. 1 shows a diagram of a device (differential calorimeter) that implements the proposed method. In FIG. 2 shows a simplified diagram of a pulsed power system for cell heaters.

A device for implementing the method comprises a housing 1 filled with heat-insulating material 2, calorimetric cells 3, sample 4, leads of heaters and thermocouples 5, contacts for connecting a measurement system and a power supply for heaters 6. The device contains a switching power supply with computer control, the diagram of which is presented in FIG. 2. The unit contains a capacitance C ₁ , which is charged through the rectifier, transformer and current limiting resistor R ₁ from the network. From this capacity, which is a buffer energy storage, through the resistance R ₂ and a normally-closed contact of relay k _{1, the} working capacity is charged.

To supply an energy pulse to the load (R _H1 or R _H2 ), the contacts of the relay k _{1 are} opened, the voltage across the capacitor C _{2 is} counted using a meter V ₁ , and then the contacts of the relay k ₂ or k _{3 are} closed, depending on which cell the calorimeter needs to give a boost. After the capacitor C _{2 is} completely discharged, the circuit is returned to its original state, which leads to recharging of the capacitor C ₂ . Capacity C ₁ is charged continuously as energy is taken from it to recharge C ₂ . The computer control unit calculates the number of pulses, calculates their energy and the allocated power on the heaters.

The method is implemented with the following sequence of actions:

1. Calibrate the calorimetric cells. To do this, a certain amount of energy is supplied to cells without a sample having the same temperature and their temperature is recorded when thermal equilibrium is reached. If after supplying a predetermined energy the temperature of the cells differs, additional pulses are applied until the temperature is equalized. The heat capacities of the cells are calculated by the formula

where c _i (T) is the heat capacity of the i-th cell at temperature T, J / (kg⋅ ° C);

ΔT is the change in cell temperature, ° C;

E _i is the energy that heated the cell, J.

2. Prepare a sample of the test material, weigh it. Place the sample in one of the calorimetric cells.

3. For each cell, taking into account their heat capacity, the energy necessary for its heating by a predetermined amount is supplied.

4. An additional energy is supplied to the cell containing the sample until the cell temperatures are equalized.

5. Calculate the specific heat of the sample by the formula

where ΔЕ is the energy required to heat the cell with the sample to a given temperature, J;

m is the mass of the sample, kg;

ΔT is the difference between the initial and final after heating the cell temperatures, ° C.

To implement the method, an installation was created in accordance with FIG. 1 and 2.

The dimensions of the inner glass of the cell of the differential calorimeter are 8 × 40 mm, the heater winding is made of nichrome and has a resistance of 4.5 Ohms, the cell jacket is made of stainless steel and has a wall thickness of 15 mm. The glass and cover of the heater are made of corundum ceramics.

The differential calorimeter case is sealed and can be evacuated and filled with protective gas. All wired connections are made using pressure glands.

The power supply system of the installation has an input isolation transformer 220/220 V, 400 watts. All wire resistors have a resistance of 4.5 ohms. The capacitor C _{1 is} electrolytic, 1000 mF, 450 V, C ₂ - starting, 25 mF, 450 V. The power supply uses 5P40.10PA1-75-4-D68 high-speed electronic relays, controlled from the controller via optocoupler isolation. A sample of steel 20 with a diameter of 6 and a length of 30 mm was placed in the second cell of the calorimeter. The weight of the sample was 6.6 g.

At a temperature of 612 ° C, the heat capacity of the first cell of the calorimeter is 124 J / ° C, the second - 131 J / ° C. To heat the cells by 10 degrees, the energy of 1.240 kJ was applied to the first cell, and 1.310 kJ to the second. To equalize the temperature, an additional 39.7 Joules were required for the second cell. The specific heat of steel 20 was 601.5 J / (kg (° C).

Claims (11)

A method for measuring the heat capacity of materials by means of a differential calorimeter, including two calorimetric cells placed inside a common heat-insulating casing, equipped with temperature meters, a temperature differential meter and electric power sources, which consists in placing a test sample in one of the cells and measuring the temperature in each cell in the process of heating, characterized in that the heat capacity of each cell without a sample is preliminarily determined, for which temperature-hand cell is fed a certain amount of energy, temperature recorded cells when they reach thermal equilibrium, and their heat capacity is determined by the formula

, where c _i (Т) is the heat capacity of the ith cell at a temperature of T, J / (kg⋅ ° C); ΔT is the change in cell temperature, ° C; E _i is the energy that heated the cell, J, a sample is placed in one of the cells, each cell, taking into account their heat capacity, is supplied with the energy necessary to heat it by a predetermined amount, and additional energy is supplied to the cell containing the sample until the cell temperatures are equal and the specific heat of the sample is determined by the formula

, where ΔЕ is the energy required to heat the cell with the sample to a given temperature, J; m is the mass of the sample, kg; ΔT is the difference between the initial and final after heating the cell temperatures, ° C, the amount of energy supplied to the cells is determined by the number of electrical pulses to the heater of each cell from the discharge on them common to both heaters of the capacitor cells, measuring the voltage of the capacitor before each pulse.

Images