RU136894U1 - DEVICE FOR MEASURING PHASE TRANSITION TEMPERATURES - Google Patents

Abstract

A device for measuring phase transition temperatures, mainly working substances of thermoelectric generators, containing a measuring circuit connected via an analog-to-digital converter with an electronic computer, a thermostat filled with water, in the lower part of which there is a heater, characterized in that it is equipped with a thermocouple block with electrical leads, a container with a test substance, heat transfer plates, a radiator with a fan, while the electrical leads of the unit thermocouples are connected to the measuring circuit, the thermocouple block has an upper and lower surface, which contact, respectively, with a radiator, blown by a fan, and with the lid of the tank, moreover, heat-conducting plates immersed in the test substance undergoing a phase transition are fixed on the cover of the tank, and the tank with the investigated working substance is placed in a thermostat filled with water.

Description

The utility model relates to measuring technique and can be used to measure the temperature of phase transitions and the choice of working substances in the development of thermoelectric generators.

An analogue is a differential scanning calorimeter (DSC), which can be used to measure the temperature of phase transitions and analyze them (Emelina A.L., DIFFERENTIAL SCANNING CALORIMETRY, Laboratory of the Faculty of Chemistry, Moscow State University 2009).

The disadvantages of the known device are low accuracy and low measurement sensitivity. The DSC error is ± 1%, but at the ends of the range it approaches ± (2-3)%.

In addition, phase transitions in DSCs often cannot be visually observed, which limits the capabilities of researchers.

The prototype is a device that implements a method for measuring the temperature of substances during phase transitions according to the patent of the Russian Federation for invention No. 2300097, IPC G01N 25/02 from 05/27/2007.

The method consists in using temperature sensors with the conversion of an electrical signal, identifying the type of phase transition. In this case, the electrical signal of the temperature sensor is adjusted by the magnitude of the electrical signal generated by the phase transition of the substance. Moreover, a correcting electrical signal is obtained using an additional probe.

The method is carried out using a device containing a sample of a substance in a dielectric container, for example water, installed in a refrigerator. A probe and a temperature sensor are fixed on the sample - the first thermocouple.

In the Dewar vessel there is a second thermocouple immersed in a solution of water with ice (thermostated and designed for comparative temperature measurement with the first thermocouple). The probe, the first and second thermocouples are connected via wires to the corresponding contacts of the adapter block, which is connected in series to a universal board for collecting and controlling the input and output of digital and analog information located inside the system unit of the computer with a monitor and printer. The probe is made in the form of a copper plate to which a shielded wire is soldered. The first temperature sensor, made in the form of a welded chromel-kopel thermocouple in a shielded braid, is connected to the adapter block.

A disadvantage of the known device is the low accuracy of measuring the temperature of phase transitions, the low sensitivity of the measurements (their thermoEMF reaches only a few tens of mV) and the low information content of the analysis regarding the structure of the phase transition, as well as the bulkiness of the equipment. These disadvantages are caused by the presence of a probe that is not protected against interference, long connecting wires, the use of thermocouples and the separation of the volumes of the measuring and comparative substances. The separation of the measured substances into different capacities can give a temperature gradient and introduces an additional error. Due to the absence of filtering circuits, the probe and connecting wires most likely pick up interference from the mains supply (this is indicated by a signal oscillation frequency of 200 Hz and a multiple of the network frequency of 50 Hz. In the prototype, the oscillation amplitude on a thermocouple reaches 10-15 mV giving an error of up to 0.8 ° C. In addition, our nuclear magnetic resonance studies show that phase transitions combine first-order and second-order phase transitions, they have a complex structure, their identification is often impossible and it is necessary to measure the phase transition in a significant temperature range.

The objective of the utility model is to increase the accuracy, sensitivity and information content of the analysis regarding the structure of the phase transition. The technical result is achieved by the fact that a device for measuring phase transition temperatures, mainly of working substances of a thermoelectric generator, containing a measuring circuit connected via an analog-to-digital converter with an electronic computer, a thermostat filled with water, in the lower part of which there is a heater, according to this useful models, equipped with a block of thermocouples with electrical leads, a container with the investigated working substance, heat transfer plates, radiator ohm with a fan, while the electrical terminals of the thermocouple block are connected to the measuring circuit, the thermocouple block has an upper and lower surface that contacts, respectively, a radiator blown by the fan and the lid of the tank, and heat-conducting plates immersed in the test a substance undergoing a phase transition, and the container with the investigated working substance is placed in a thermostat filled with water.

The essence of the utility model is illustrated by drawings, where figure 1 shows a functional diagram of the proposed device for measuring the temperature of the phase transition; figure 2 shows the dependence of thermopower at the terminals of the block of thermocouples U (mV), U (T) and temperature T, measured by a thermistor during heating of the investigated working substance. The curves in Fig. 2: 1 - U (T), 2 - T (t), 3 - U (mV)

In figure 1, the numbers denote:

1 - block thermocouples,

2 - radiator,

3 - heat transfer plates

4 - fan

5 - thermostat,

6 - water heater thermostat,

7 - analog-to-digital Converter (ADC),

8 - electronic computer (computer),

9 - printer

10 - precision ohmmeter,

11 - fan power source,

12 - ambient temperature sensor,

13 - thermistor

14 - capacity with the investigated working substance,

15 - container lid,

16 is a measuring circuit.

A device for measuring phase transition temperatures, mainly of working substances of a thermoelectric generator, contains a measuring circuit 16 connected via an analog-to-digital converter 7 to a computer 8, a thermostat 5 filled with water, in the lower part of which is a heater 6 that changes the temperature of the test working substance .

The device has a thermistor 13 for monitoring the total temperature of the test working substance, connected to a precision ohmmeter 10 connected to the ADC 7, as well as an ambient temperature sensor 12 connected to the ADC 7.

The difference of the proposed device for measuring phase transition temperatures is that it is equipped with a block 1 of thermocouples with electrical leads connected to the measuring circuit 16, with a capacity of 14 with the test substance, heat transfer plates 3, a radiator 2 with a fan 4, powered by a power source 11.

Block 1 thermocouples has an upper and lower surface.

Block 1 thermocouples the upper surface is in contact with the radiator 2, blown by the fan 4, and the lower surface is in contact with the cover 15 with heat-conducting plates 3, immersed in the investigated working substance, undergoing a phase transition. Capacity 14 with the investigated working substance is placed in a metal thermostat 5 filled with water. Ammeter A _T and voltmeter V _T measuring circuit 16 connected to the electrical terminals (junctions) of block 1 of the thermocouples measure the current and voltage of the thermoelectromotive force EMF, the values of which are transmitted to the ADC 7, computer 8 and printed by the printer 9.

To demonstrate the operation of the device, we used the CaCl ₂ · 6H ₂ O crystalline hydrate salt, which undergoes a phase transition at a temperature T _Psr = 29.7 ° C with a specific heat of fusion Q = 170 kJ / kg. For research, the device shown in Fig. 1 was used.

Measuring circuit 16 connected to electrodes (junctions) of a single-stage thermocouple type TEC-12706 (TU 6349-001-79789858-2007, R = 1.5 Ohm; temperature difference up to ΔT = 73 ° C, current up to I = 6 A, sizes of junctions 40 × 40 mm) measured the thermopower voltage and current.

The temperature in salt and water is controlled by thermistors with resistance of 50 Ohms.

In figure 2. the dependence of the voltage at block 1 of the thermocouples and the thermistor on the heating time (curve 2) is shown, from which it is seen how low the steepness of the dependence is on the thermistor.

The volume of the investigated working substance is at least 25 cm ³ . The transit time through the phase transition is usually 15 minutes, but the process can be analyzed in a wider temperature range and in a longer time interval.

An analysis of the metrological parameters for measuring temperature T shows the following.

When measuring temperature with a thermistor in the temperature range -50 ° C≤T≤ + 180 ° C, the error of temperature measurements t is determined by the expression:

and actually amounts to ± 0.6 ° C, which corresponds to δ = S / ΔT = 0.6 / 40 = 0.015 in the measured range of the reduced error.

For block 1 of the thermocouples of the claimed device, the connection between U (T) and T, as can be seen from Fig 2 is given by the equation:

and is described with a correlation coefficient of R ² = 0.9 in the range up to 750 mV. The given error of the proposed device in this temperature range of the FP is δ = S / ΔU = 7.86 / 750 = 0.0105, where S is the standard deviation, ΔU is the range of variation of Uv mV.

The presented error of temperature measurements by the proposed device using block 1 thermocouples is 1.5 times lower than that of the analog device using a thermistor, and the relative sensitivity coefficient of block 1 thermocouples, as can be seen from figure 2 (curve 1), is k = U ( T) /T=27.74, which is significantly higher than k = 0.047 of the thermistor. As can be seen from the graphs, the phase transition process cannot be characterized by a single temperature. Melting of CaCl ₂ · 6H ₂ O begins at T _P ≈28 ° C and ends at T _P ≈35 ° C.

Measurements with a thermocouple or thermistor do not reveal the phase transition structure, that is, important information about its features is lost.

The heat transfer plates 3 provide efficient heat transfer to the lower junction of the thermocouple block 1, and the radiator 2 with fan 4 provides heat removal to the environment, providing a temperature difference on the surfaces of the thermocouple block 1. Placing in a single thermostat measuring and comparative volume of substances simplifies the proposed device in comparison with the prototype.

Thus, the introduction of thermocouples, heat transfer plates 3 and radiator 2 with fan 4 into the device of unit 1 gives a 1.5-fold reduction in error, a multiple increase in sensitivity (steepness k dependence), and the information content of the analysis.

Claims (1)

A device for measuring phase transition temperatures, mainly working substances of thermoelectric generators, containing a measuring circuit connected via an analog-to-digital converter with an electronic computer, a thermostat filled with water, in the lower part of which there is a heater, characterized in that it is equipped with a thermocouple block with electrical leads, a container with a test substance, heat transfer plates, a radiator with a fan, while the electrical leads of the unit thermocouples are connected to the measuring circuit, the thermocouple block has an upper and lower surface that contact, respectively, with a radiator, blown by a fan, and with the lid of the tank, moreover, heat-conducting plates immersed in the test substance undergoing a phase transition are fixed on the cover of the tank, and the tank with the investigated working substance is placed in a thermostat filled with water.

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