RU2213330C2 - Method of thermal measurement of levels of interface of media - Google Patents

Abstract

FIELD: measurement technology. SUBSTANCE: method can find use in various branches of industry to establish media interface in which sensitive element of measurement converter shows different convective heat transfer. Method includes formation of temperature field nonuniform in height in controlled media and positioning of temperature-sensitive elements in points of temperature field of controlled media. Values of temperature in points of temperature field of controlled media are taken with the aid of temperature-sensitive elements. Distribution of temperature field in height is reconstructed by measured vales of temperature by means of approximation. First derivative of temperature in height is found. Levels of interface of media are established by positions of local maxima of derivative. EFFECT: increased accuracy of measurement of level of interface of media, diminished sensitivity of measurement converter to variations of parameters of controlled media and to drift of its characteristics. 8 dwg

Description

 The invention relates to measuring equipment and can be used in various industries to determine the boundaries of the media, in which the sensitive element of the measuring transducer has different heat transfer.

 Known thermal method for measuring the level of the interface [1], based on the use of differences in thermal conductivity of controlled environments (thermistor level gauges, level gauges thermo-emf).

 A known method implemented in a device for measuring the level of the separation of media (AS USSR 673858, class G 01 F 23/22, bull. 26, 1979) and selected as a prototype based on measuring the output signal of a thermopile from series-connected thermocouples hot and cold junctions which are distributed in height and have thermal contact with the heater.

 The disadvantage of this method is the low accuracy of determining the level of media separation, which is due to the influence on the value of the output signal of the thermal battery of both the parameters of the controlled media (temperature, thermal conductivity, etc.) and the parameters of the measuring transducer (heater current, conversion characteristics of thermocouples).

 The technical task to be solved is to increase the accuracy of measuring the level of media separation and to reduce the sensitivity to changes in the parameters of controlled media and the drift of the characteristics of the measuring transducer.

The technical problem to be solved in a method for the thermal measurement of media separation levels, including creating a temperature field T (h) in the controlled media that is uneven in height h, and placing temperature-sensitive sensors at the points of the temperature field of the controlled media, is achieved by measuring temperature values using temperature-sensitive sensors T _i at the points of the temperature field of controlled environments, where i∈ {1,2, ..., N} , and N - the number of heat sensitive sensors, the measured values of the temperature T _i approximation vosstanavli ayut distribution of temperature field T (h) in height h, determining a first derivative dT / dh temperature T of the height h and the position of local maxima of the derivative dT / dh determine levels section h _j environments where j∈ {1,2, ... , M-1}, and M is the number of controlled environments.

 Figure 1 shows a diagram of a device that implements a method of thermal measurement of the level of the interface.

 In FIG. Figure 2 shows a graph of the temperature distribution over the height of the transmitter.

 Figure 3 shows a block diagram of the measuring part of a device that implements a method of thermal measurement of the level of the interface.

 Figure 4 shows an example of a measuring capacitance with controlled environments for the case of several levels of medium separation.

 Figure 5 shows a graph of the distribution of temperature T over the height h of the measuring transducer for the case of several levels of medium separation.

 In FIG. Figure 6 shows a graph of the distribution of the derivative temperature dT / dh over the height h of the measuring transducer for the case of several media interfaces.

 7 shows a variant of the excitation circuit of piezoresonance sensors.

 On Fig shows the temperature-frequency characteristic (TCH) of a quartz piezoresonance sensor RCTB 206.

 Appendix 1 (see Fig. 9) shows the algorithm of the spectrum analyzer.

 Appendix 2 (see figure 10) shows the algorithm of the processing and display unit.

The device (figure 1) that implements a method of thermal measurement of the level of the interface, contains a heater 1 with a current source 2, N heat-sensitive sensors 3 _i , where i∈ {1,2, ..., N}, a two-wire line 4 and a secondary recording device 5. Heater 1 and thermosensitive sensors 3 _{i are} installed in the measuring tank 6 with controlled environments. The number of controlled environments can generally be unlimited. Figure 1 shows an example for the case of measuring the separation level of two environments 7, 8. Figure 4 shows an example for the case of measuring the separation levels with the number of controlled environments M = 5. The number N of thermosensitive sensors 3 is determined by the required accuracy of the measurement of the level of separation of controlled environments. In the example implementation of the device shown in figure 1, N = 12.

The heater 1 is electrically connected to the current source 2. The temperature-sensitive sensors 3 _{i are} distributed in height, are in thermal contact with the heater 1 and are electrically connected via a two-wire line 4 to the secondary recording device 5.

As thermosensitive sensors 3 _i in a device that implements a method of thermal measurement of the level of the interface, we used quartz piezoresonance sensors 9 _i with different resonant frequencies ω _p1 , ω _p2 , ..., ω _pi , ... ω _pN .
The block diagram of the measuring part of the device (Fig. 3), including heat-sensitive sensors 3 _i , a two-wire line 4 and a secondary recording device 5, contains N quartz piezoresonance sensors 9 _i , a two-wire line 4, a secondary recording device 5, which includes an excitation circuit quartz piezoresonance sensors 10, a spectrum analyzer 11, a processing and indication unit 12.

Quartz piezoresonance sensors 9 _{i are} electrically connected using a two-wire line 4 to the excitation circuit of quartz piezoresonance sensors 10, the output of which is connected to the input of the spectrum analyzer 11. The output of the spectrum analyzer 11 is connected to the input of the processing and indication unit 12.

 The heater 1 and the current source 2 can be performed according to standard schemes published in information sources or manufactured by the industry.

 A possible version of the excitation circuit of the piezoresonance sensors 10, containing a chirp signal generator 13 and a current-voltage converter 14, is shown in Fig.7.

 The algorithm of the spectrum analyzer 11 is shown in Appendix 1 (Fig.9). The algorithm of the processing unit and display 12 is shown in Appendix 2 (Fig. 10).

Consider the implementation of the level measurement method using the device shown in figure 1. In controlled environments, a temperature field T (h) is created that is uneven in height h, and temperature-sensitive sensors are placed at points in the temperature field of controlled environments. Heater 1 and a chain of thermosensitive sensors 3 _i , connected in parallel by a two-wire line 4, are placed in a measuring tank 6 with controlled media 7, 8. When the current source 2 is turned on, heater 1 heats the thermosensitive sensors 3 _i . In this case, the sensors located in the medium 7 with lower thermal conductivity are heated more than the sensors located in the medium 8 with higher thermal conductivity. At level h ₁ of the media section (Fig. 2), the maximum temperature T will be observed. Using temperature-sensitive sensors 3 _i, the temperature T _i is measured at points of the temperature field of the controlled media, where i∈ {1,2, ..., N} , and N is the number of heat-sensitive sensors. Information on the temperature T _{i of the} temperature-sensitive sensors 3 _i enters the secondary recording device 5, where the temperature distribution T (h) with respect to height h is restored by approximation, the first derivative dT / dh of temperature T with respect to height h is determined, and the level h ₁ is found from its maximum position medium section In the case when it is required to determine several levels of media separation with the number of controlled media M more than two (see Fig. 4), the position of several (M-1) local maxima of the derivative dT / dh corresponding to M-1 media separation levels h _{j is} determined where j∈ {1,2, ..., M-1}.
In an example of a device for implementing the method of thermal measurement of media interface levels (Fig. 3), the temperature information T _i consists in the value of the serial resonance frequency ω _{pi of} quartz piezoresonance sensors 9 _i .

The device operates as follows. The excitation circuit of the quartz piezoresonance sensors 10 generates a signal with a spectrum that covers the frequency range of the quartz piezoresonance sensors 9 _i . This signal is fed to the input of the two-wire line 4. The output signal of the two-wire line 4, due to the resonance of the quartz piezoresonance sensors 9 _i , is fed to the spectrum analyzer 11, which allows to determine the resonant frequencies ω _{pi of the} sensors. Using the previously experimentally found or theoretically known dependence of ω _pi (T) of the resonant frequency of the quartz piezoresonance sensors 9 _i on temperature and the found amplitude spectrum in the spectrum analyzer 11, which operates according to the algorithm given in Appendix 1, the temperature values T _{i of the} sensors are calculated. Data on the temperature values T _{i of the} sensors are supplied to the processing and display unit 12, operating according to the algorithm given in Appendix 2. In the processing and display unit 12, the temperature distribution T (h) with respect to height h and the position of the local maxima of the derivative dT / dh are the levels h _j of the media section.

 We show that the proposed method for the thermal measurement of media separation levels and the devices that implement it make it possible to achieve the solution of the technical problem posed — to increase the accuracy of level measurement and reduce sensitivity to changes in the parameters of controlled media and the drift of the characteristics of the measuring transducer.

In the well-known thermal level meter that implements a method of thermal measurement of the level of the separation of media (AS USSR 673858, class G 01 F 23/22, bull. 26, 1979), a thermopile from series-connected thermocouples is used as a distributed heat-sensitive element. The output signal of the thermopile is the thermopower E, the value of which is a function of the measured level h _{1 of} the interface between two media. However, the thermo-emf E is also influenced by other factors, including the characteristics of heat transfer from the heater to the environment, depending on the temperature and composition of the measured media, the degree of contamination of the surface of the measuring transducer, and the amount of power supplied to the heater by the current source. To eliminate the ambiguity of the level gauge readings, it is necessary to significantly complicate its circuit, additionally introduce sensors of various physical quantities, calibrate the level gauge for the used types of media in the entire range of parameters that affect the form of the dependence E (h ₁ ).

 The sensitivity of the level gauge readings to changes in the parameters of the monitored media and to the drift of the characteristics of the measuring transducer leads to a decrease in the accuracy of level measurement.

 In the proposed method for measuring the levels of the separation of media and devices that implement it, the temperature field is measured along the entire length of the measuring transducer. In this case, the informative parameter, which depends on the level of the interface between the media, is the position of the local maximum of the derivative of the temperature in height. Changing the parameters of the controlled environments, the power consumed by the heater, the contamination of the surface of the measuring transducer affects the temperature value of the sensors of the level gauge, but does not lead to a significant change in the shape of the distribution of the temperature field of the measuring transducer (Fig. 2, Fig. 5). Thus, the level of media separation is determined with high accuracy and does not depend on changes in the parameters of these media and the characteristics of the elements of the measuring transducer.

 An additional advantage of the proposed method is the ability to simultaneously measure several levels of media separation with the number of controlled media M> 2 using one measuring transducer.

The use of temperature-sensitive quartz piezoresonance sensors with spaced frequencies as point sensors provides several advantages over other types of sensors. Such sensors have:
- increased accuracy of temperature measurement in the operating range from -50 to +370 ^o With [2];
- increased noise immunity due to the conversion of temperature into a frequency output signal.

 Using these sensors, it is possible to measure the temperature field of a distributed object at a number of points with an accuracy of 0.1% [2], and only one channel is required for transmitting measurement information to secondary equipment, provided piezo-resonant sensors with spaced frequencies are used.

 One of the types of quartz piezoresonance sensors manufactured by the industry is the tuning fork quartz piezoresonant temperature sensors RKTV 206. The main characteristics of the quartz piezoresonance sensor RKTV 206 are given in Table 1.

The quartz piezoresonance sensor RKTV 206 has a temperature characteristic described by the following expression [2]:
f (T) = f ₀ + A ₁ (T-T ₀₎ + A ₂ (T-T ₀₎ ² (1)
where f (T) is the frequency of the quartz piezoresonance sensor (Hz) at the current temperature T ( ^o С);
f ₀ is the frequency of the quartz piezoresonance sensor (Hz) at a reference temperature T ₀ ( ^o С);
T ₀ - reference value of temperature.

The coefficients for the quartz piezoresonance sensor RKTV 206 are [2]:
A ₁ = -1.76 ± 0.1 Hz / ^o C, A ₂ = -0.0031 ± 0.0001 Hz / ^o C in the operating temperature range -50 ... + 370 ^o C.

 A graph of the temperature frequency response of a quartz piezoresonance sensor with a central frequency of 34 kHz, calculated in accordance with (1), is shown in Fig. 8. By measuring the resonator frequency, the temperature of the sensor is calculated from the temperature-frequency characteristic.

A possible embodiment of a piezoresonance sensor drive circuit is shown in FIG. 7. The generator of the chirp signal 13 generates a linear frequency modulated signal, which is fed to the input of the two-wire line 4. The current-voltage converter 15 converts the current changes of the two-wire line 4, caused by the resonance of the quartz piezoresonance sensors 9 _i , into a voltage, which is then fed to spectrum analyzer 11.

Thus, when implementing the method of thermal measurement of the levels of the separation of the media as heat-sensitive elements operating in a wide temperature range (up to +350 ^o C), you can use quartz piezoresonant temperature sensors.

List of references
1. Bobrovnikov G.N., Katkov A.G. Level measurement methods. - M.: Mechanical Engineering, 1977. - P.74-91.

 2. Technical passport of the resonator of quartz thermosensitive high-temperature RKTV 206 TU 25-1862.0013-88. Special Design and Technology Bureau of Electronics, Instrument Engineering and Automation, Uglich.

 3. Kuznetsov O.A. Automatic control of the level of separation of two environments. - M. -L.: Energy, 1964. - 88 p.

 4. Malov V.V. Piezoresonance sensors. - 2nd ed., Revised. and add. - M.: Energoatomizdat, 1989.

Claims (1)

A method of thermal measurement of media separation levels, including creating a temperature field T (h) in controlled media that is uneven in height h and placing temperature-sensitive sensors at points of the temperature field of controlled media, characterized in that temperature values T _i are measured using temperature-sensitive sensors temperature field of controlled environments, where i∈ {1,2, ..., N} , and N - the number of heat sensitive sensors, the measured values T _i approximation reduced temperature distribution evap fi eld T (h) in height h, determining a first derivative dT / dh temperature T of the height h and the position of local maxima of the derivative dT / dh determine levels section h _j environments where j∈ {1,2, ..., M -1}, and M the number of controlled environments.

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