WO2022130210A1

WO2022130210A1 - A temperature sensing system with thermistor

Info

Publication number: WO2022130210A1
Application number: PCT/IB2021/061719
Authority: WO
Inventors: Cengiz YILMAZ; Sevda KAÇMAZ; Mücahit KORKMAZ
Original assignee: Tübi̇tak
Priority date: 2020-12-15
Filing date: 2021-12-14
Publication date: 2022-06-23
Also published as: TR202020541A1

Abstract

The invention relates to the temperature sensing circuit (18) and signal processing and data transfer unit (20) that can be used for high-accuracy temperature measurements with a temperature measurement range of -1.05 to +50.05 C, a resolution of 0.0001 K, short term instability of less than 0.5 mK, self-heating effect of less than 0.5 mK and measurement uncertainty of 2.0 mK. The design of the NTC thermistor temperature detection circuit (18) with four terminal shielded signal transmission cable connection, including invention, thermistor elements and linearization resistors, in the temperature sensing circuit (18) generated by the thermistor elements and linearization resistors, includes all steps for linearizing/conditioning the signal obtained using the constant current source (3) and the ratio metric signal sensing method.

Description

DESCRIPTION

A TEMPERATURE SENSING SYSTEM WITH THERMISTOR

Technical Field

The present invention relates to the negative temperature coefficient (NTC) thermistor temperature sensing circuit, signal processing and data transfer unit that can be used for high- accuracy temperature measurements.

The present invention relates to the linearization and/or conditioning of the signal by applying the ratio metric method to the temperature sensing circuit and constant current source, which have thermistors and linearization resistors in the same environment.

Prior Art

Thermistors (thermal resistors) are temperature- sensitive semiconducting ceramic devices, and NTC thermistors are often used in specific high-accuracy temperature measurements, temperature controls, temperature compensation and many applications such as thermal protection. The main advantages of thermistors are high precision, small size, fast response time and low cost. Disadvantages; non-linear temperature-resistance characteristic, a limited temperature range, the detected current (high resistance) connected to a self-heating and since they are mainly produced in two-terminal hardwired connections, the resistance effect of the cable sizes used in thermistors is the cause a temperature reading error for high-precision temperature measurements.

The linearization resistance(s) used to linearize the non-linear temperature-resistance characteristic of thermistor(s) causes reductions in the sensitivity of the thermistor(s).

In general, hardware linearization in a thermistor is done with external resistance that connects outside the temperature sensing circuit. The temperature sensing circuit will cause a measurement error in temperature measurements that require high accuracy because the temperature measurement environment and the ambient temperature of the external resistance are different. For multiple thermistors (two or three) produced in a single chip, the number of connection terminals is arranged to be one more terminal number. Since the temperature sensing circuit with multi -thermistor and linearization resistors operates on the basis of a voltage divider, an external voltage source is needed. By connecting the temperature sensing circuit to the voltage source via a serial resistor, a three-terminal temperature measurement system is created. The disadvantage of this system is that the changes in the voltage source and the dependence of the series resistance on the temperature change affect the accuracy level of the sensing signal.

In the state of the art, in the U.S. patent document numbered US3316765, filed on 02.05.1967, a multi -thermistor measurement circuit with a different and overlapping linear temperature response range and overlapping linearization resistors connected in parallel with each other can operate with a fixed voltage source or a fixed current source, however, it was mentioned that in order to achieve a constant current, relatively high potential sources or servo systems are used to regulate the current output, because the equipment required for both situations is more cumbersome or complex than desired, a constant voltage source such as nickel-cadmium battery may be used instead.

In the prior art, in a thermistor temperature sensing circuit, the use of a constant voltage source is preferable because the thermistor varies between ohms and mega ohms depending on the temperature, and large resistance variations cause impedance mismatching in current sources, the present invention has created a temperature sensing circuit that includes thermistor elements and hardware linearization resistors, and the need to improve the linearization/conditioning of the signal obtained using the constant current source and ratio metric method in this temperature sensing circuit.

Technical Problem that the Invention Aims to Solve

Typically, in order to linearize the characteristics of temperature-resistance, a linearization resistance suitable for the thermistor is used separate from the temperature sensing circuit (probe). A temperature sensing circuit with thermistors and linearization resistors is designed in the present invention, ensuring that the errors generated by temperature variations from different environments remain constant on the measuring elements, thus turning random errors into systematic errors, resulting in repeatable results for temperature measurements that require high accuracy in different areas of use of metrology. As a result, even if materials with different temperature coefficient are used depending on the design of the temperature sensing circuit, repeated measurements for each measurement point show the same characteristic, due to the equivalent resistance of the circuit. Thus, the temperature sensing circuit therefore has a more stable temperature-resistance change, such as a single temperature sensor.

In a thermistor temperature sensing circuit, it is often preferable to use a constant voltage source instead of a constant current source because the temperature-resistance variation varies between ohms and mega ohms and the system becomes a very complex structure.

With the present invention, the maximum resistance variation is limited in the temperature sensing circuit with the thermistor elements using the linearization resistors to enable the constant current source to be used. In addition, to obtain a constant current from the current source, the appropriate voltage reference and power supply voltage levels are adjusted to prevent impedance mismatching that can occur at the current supply output. For high accuracy temperature measurements, it is not sufficient to use only a constant current source because of sudden current changes at the constant current source output, adverse effects of ambient conditions on the current source, and other disruptive effects that prevent a reliable high- accuracy analogue signal from being obtained. This invention is used in conjunction with the constant current source ratio metric method to eliminate parameters that adversely affect the measurement mentioned above ensuring measurements are taken with high accuracy.

Objectives and Brief Description of the Invention

The purpose of this invention is to design a temperature sensing circuit that includes NTC thermistor elements and linearization resistors for high accuracy temperature measurements, and to perform the linearization/conditioning of the signal obtained in this temperature sensing circuit using the constant current source ratio metric method.

Another purpose of the invention is to achieve a constant linear temperature response at the maximum level, narrowing the temperature-resistance change range of the temperature sensing circuit according to the technical data of the thermistors and the linearization resistor circuit selected according to the measurement range.

Another purpose of the invention is to place the temperature sensing circuit in a stainless metal case, where the three NTC thermistors and linearization resistors, each separated, are at 120° angles in the same location and with the bottom of the base on the same plane, and to provide thermal balance between thermistors, to measure the temperature at the same point.

Another purpose of the invention is to perform a large resistance change in the thermistor temperature sensing circuit with the voltage reference and supply voltage of the current supply, so that there is no impedance mismatching at the current source output.

Another purpose of the invention is to achieve a high temperature resolution, with a positive slope temperature-to-voltage signal from the current source, which is obtained by removing the negative slope linearized signal voltage on the temperature sensing circuit (prob) from the reference temperature starting point voltage covering the entire measurement range.

Another purpose of the invention is to eliminate measurement errors caused by sudden current changes using the ratio metric method of current applied to the temperature sensing circuit, a more linear and reliable analogue signal voltage and ADC (Analogue Digital Converter) voltage reference voltage is obtained and the maximum accuracy reading of this signal from the ADC is performed.

Another purpose of the invention is to perform a voltage drop over the reference resistors, which produce a constant current in the application of the ratio metric method, to produce the reference initial temperature voltage and the ADC voltage reference voltage.

Another purpose of the invention is to perform the applicability of current cut-off and currentreversing methods to eliminate the impact of thermal errors caused by temperature changes, as all ports (Connector and PCB (Printed Circuit Board)) of the temperature sensing circuit create thermal offset error (thermal EMF) in ambient temperature variations.

Another purpose of the invention is to reduce the self-heating error of the temperature sensing circuit to a negligible level with a temperature-resistance change determined by the measurement range and a specified current, as well as to increase the sensitivity by increasing the output current so as not to have a self-heating effect on the thermistors.

Another purpose of the invention is to perform a temperature-resistance calibration by the temperature sensing circuit at the four-terminal resistance measurement stage with high resolution (at least 8 ’A digits) a multimeter, both directly without the need for external resistance and voltage supply as well as without using the signal processing and data transfer unit by narrow the temperature-resistance variation range of the temperature sensing circuit according to the technical data of the thermistors and linearization resistors selected according to the measurement range and measurement accuracy. And also, is to measure any ambient temperature.

Detailed Description of the Invention

A temperature sensing circuit with thermistor and linearization resistors to achieve the objectives of this invention and the linearization/conditioning of the signal using the constant current source and ratio metric method are seen in attached figures.

These figures;

Figure 1. Signal processing and data transfer unit

Figure 2. Temperature sensing circuit (probe)

Figure 3. Signal conditioning - slope reversing

Figure 4. Signal amplitude control circuit and temperature compensation circuit

Figure 5. Analog signal filtration circuit and ADC Ain drive circuit

Figure 6. Micro processor

Figure 7. Thermal EMF current reversing and ADC external calibration

The parts in the figures are numbered individually, and the numbers are given below.

1. Power supply

2. Voltage reference

3. Constant current supply, FB; feedback input of the current supply, RL; reference resistor connection output to which the stable current will be sent

4. Instrumentation amplifier

5. Instrumentation amplifier 6. Instrumentation amplifier

7. Signal conditioning and slope reversing circuit, 7-1, 7-2, 7-3, 7-6; Low pass filter, 7-4; ADC VREF driver, 7-5; Instrumentation amplifier,

8. Analog signal amplitude control circuit, 8-1; Amplitude control voltage

9. Temperature compensation circuit, 9-1; Temperature compensation

10. Analog signal filtering circuit, 10-1; High-order low-pass filter, 10-2; ADC Ain driver

11. Microprocessor, 11-1; Temperature sensor, 11-2; ADC, 11-3; Electro-mechanical switch control circuit, 11-4; LCD, 11-5; Serial communication

12. ADC external calibration circuit, 12-l;Zero calibration / Midpoint Calibration / Full scale calibration, 12-2;Internal temperature probe, 12-3; External temperature probe

13. Thermal EMF current reversing circuit, 13-1; Thermal EMF current reversing, 13-2; Electro-mechanical switch control circuit, 13-3; Function switching circuit

14. R_I ADJ Resistor (Reference Current Adjustment Resistor)

15. Thermal offset current cutting

16. The reference resistance “T0 THR R”, which determines the starting point of the reference temperature

17. The ADC R REF reference resistor, which determines the proportional reference voltage of the ADC

18. Temperature sensing circuit (probe)

19. “R RETURN" is the resistor that moves the level of the read signal in a positive direction from the ground voltage

20. Signal processing and data transfer unit The invention is a thermistor temperature sensing circuit and a system that performs the linearization/conditioning of the signal obtained using the ratio metric method with a constant current source, which includes:

- a temperature sensing circuit (18) with four terminals and shielded signal transmission cable, including 3 NTC thermistors and hardware linearization resistors,

- a stable voltage reference (2) and power supply (1) used to obtain a stable current from the constant current source (3) in the signal processing and data transfer unit (20),

- the feedback input (FB) (14,15,16,17) of the constant current supply and the reference resistance connection output (RL) (14,15,16,17,18,19) to which the stable current is to be sent,

- T0-THR R reference resistance (16), which determines the starting point of the reference temperature, and processing of the voltage generated by the current applied in the instrumentation amplifier (4),

- ADC R REF reference resistance (17), which determines the ratio metric reference voltage of the ADC and processing of the voltage generated by the current applied in the instrumentation amplifier (5),

- the current applied from the current source at any temperature measurement point, the processing of a voltage drop on the temperature sensing circuit (18) in the instrumentation amplifier (4),

- the signal processing and slope reversing circuit (7) in which the ratio metric method is processed, the acquisition of the ratio metric voltage differences and the reversing of the negative slope in the temperature-resistance curve to a positive slope as a temperature-voltage curve,

- the corresponding analogue voltage, The Analog Signal Amplitude Control Dircuit (8), which allows it to remain within the set high and low voltage limits,

- temperature compensation circuit (9), which determines the rate of change in temperature over the signal, - the analogue signal filtering circuit (10), where a filtering operation is carried out on the - signal that passes below the high range,

- the mathematical processes of the signal and the hardware controls are performed, the microprocessor circuit (11), in which the voltage values are converted to temperature information at the end of the operations,

- thermal EMF circuit (13) where thermal offset errors are corrected due to temperature variations in electrical connection points,

- ADC external calibration circuit (12) with external calibration of the ADC

Example Application

The temperature sensing circuit (probe) (18); 3 NTC thermistors and linearization resistors, which are appropriate for measuring at -1,05 °C to +50,05 °C, at 0,0001 K resolution and at 0,001 K accuracy, have been determined. Normally, a temperature sensing circuit with thermistor gives a nonlinear temperature response. To achieve a linear temperature response at the maximum level possible, the temperature-resistance variation of the temperature sensing circuit has been reduced and a temperature-resistance table corresponding to 1000 temperature points has been created according to the temperature measurement range.

Graph-1. Resistance- Temperature Change of Thermistors Tl, T2 and T3

Accordingly, the resistance-temperature variation for the thermistors Tl, T2 and T3, as shown in Graph-1, is T3, T2 and Tl respectively. As the temperature decreases, the difference in resistance between thermistors increases. To reduce this difference, the T1 thermistor has a serial R1 resistor and R2 resistor connected in series to the T2 thermistors as shown in Graph- 2.

Graph-2. (Rl+Tl), (R2+T2) and (T3) Resistance - Temperature Change

In addition, as shown in Graph-2, the lowest temperature change in resistance is T3, T2+R2 and Tl+Rl, from large to small. At the highest temperature, the resistance variation is Tl+Rl, T2+R2 and T3, from the larger to the smaller. The equivalent resistance in the parallel circuit with Tl+Rl, T2+R2 and T3 determines Tl+Rl at low temperature and T3 at high temperature.

Graph-3. (Rl+Tl) / (R2+T2) - (R3//T3) and R Temperature Sensing Circuit (probe) Resistance - Temperature Change The R3 parallel resistance is used to reduce the resistance range by changing the temperature of the temperature sensing circuit. The R3 parallel resistance makes the resistance value at low temperature smaller than itself. Graph-3 shows that the equivalent resistance of the temperature sensing circuit is smaller than the equivalent resistance on all parallel arms. Graph-2 shows that the smallest variation in resistance in the equivalent arms parallel at high temperatures is T3. Therefore, the equivalent resistance value of the temperature sensing circuit (18) at the high temperature is less than the resistance value of the T3 thermistor at this temperature.

Also Graphic-3, the first parallel arm (Tl+Ll) and second parallel arm (T2+R2) from parallelism with each other (Tl+Rl) // (T2+R2), the equivalent resistance-temperature curve from the parallelism of R3 T3 (T3//R3), the equivalent resistance-temperature curve and ((Tl+Rl) // (T2+R2)) // (T3//R3) parallelism from the temperature detection circuit (18) (R PROB) resistance-temperature curve has been established.

As a result of the linearization, the equivalent resistance-temperature variation (Temperature Sensing Circuit, R PROBE) of the four parallel arms has been linear at the maximum level, as shown in Graph-4. The linearization of thermistors results in losses in the sensitivity of the thermistor. However, these losses can be tolerated by increasing the value of the current so that they do not have a self-heating effect. This invention has shown that in applications, the output current of the constant current source is lOOpA, while the sensitivity of the thermistor is 16 mV/°C, the output current value of the current source is increased by 2,5 times, improving the sensitivity of the thermistor by 2,5 times (40 mV/°C).

As shown in Figure 2. a, according to the information mentioned above, the 3 NTC thermistors each separated are designed to be close to the linearization resistors in the same environment with a 120° angle between them and their subbases in the same plane.

Graph - 4. Resistance of Temperature Sensing Circuit - Temperature Change

This prevents measurement errors due to linearization resistances because the linearization resistors are in the same environment as the thermistors and creates a new temperatureresistance table for each measurement point. The temperature sensing circuit (18) is also designed to be used at the triple point of water (TWP) and Gallium (GA).

For each of the 4 temperature sensing circuits (probes) (18) produced, for example, in repeatability tests at the Ga point (29.7646°C), the maximum repeatability error of about 6000 data received every 13 seconds was determined to be 0.07 mK. As a result, it has been shown that the temperature sensing circuit described by the present invention exhibits the same temperature-resistance behavior of all materials during repeated measurements for the same temperature point.

The temperature sensing circuit is enclosed in a stainless steel case with a diameter of 6 mm and a length of 340 mm. Temperature detection takes place in the 5 mm area at the bottom of the temperature detection circuit (18).

As shown in Figure 2-b, the thermistors and the linearization resistors are electrically connected in series with T1 and R1 and in series with T2 and R2 so that they are in the same environment. T1 +R1 is the first parallel arm, T2+R2 is the second parallel arm, T3 is the third parallel arm and R3 is the fourth parallel arm. Each parallel arm is electrically connected to each other at points A and B with the other parallel arm. In addition, the connection of the temperature sensing circuit (18) was made via a special cable from ports A and B, four terminal measurement connections and a shielding terminal to reduce the effects of electromagnetic and radio frequency signals at a terminal.

The temperature sensing circuit (18) described by this invention is produced with four terminals, so without using the data transfer unit, external resistance and voltage source, by means of only using directly with a multimeter having high resolution (at least 8 /i digits), it was shown to be done not only calibration of resistance-temperature but also measurement of ambient temperature in accordance with calibration data.

In addition, the cable connection of the temperature sensing circuit (18) is 4-terminal, which also eliminates measurement errors due to cable length.

The impedance variation in the temperature measurement range of temperature sensing circuit (18)-1.05°C to 50.05°C is between 16 K and 8 K.

The impedance variation in the temperature measurement range of temperature sensing circuit (18) -1.05°C to 50.05°C is between 16 K and 8 K. Experimental studies to identify errors in the self-heating effect of the temperature sensing circuit (18) on the measurement, 4 temperature sensing circuits produced (18) were compared by a reference SPRT (standard plantinium resistance thermometer) and a reference thermistors in the oil bath of 22.998°C ± 0.0005°C, in the range 100 pA to 300 pA and in steps 50 pA. Each comparative measurement data was taken every 2.5 min and a total of 1580 measurement data was automatically saved. These experiments was shown that the self-heating error < 0.5 mK . In fact, the determined value is due to the bath's own change. Therefore the effect of the self-heating of the temperature sensing circuit (18) is therefore less than 0.5 mK.

In the performance tests of the 4 temperature sensing circuits (18) produced, the measurements were taken at the triple point of water and Galium fixed point using a traceable multimeter having a high resolution (at least 8,5 digits). The short time in stability of the temperature sensing circuits (18) has been analyzed for two months. During the measurement period, 60.000 data for each temperature sensing circuit (18) was automatically recorded. Each measurement data was taken every 13 s and one measurement was carried out in a total of 18 hours. These experiments have shown that the short-term instability of the temperature sensing circuit described by the invention is 0.5 mK and the expanded measurement uncertainty (k=2,0, 95%) is 2.0 mK. In the signal processing and data transfer unit (20), the most critical point of the system is the voltage reference (2) and its purpose is to ensure the stability of the system. The output voltage of the voltage reference (2) is applied to the voltage reference input (VREF) of the constant current source (3). The current output (RL) of the constant current source (3) is determined by the connection of I_ (REF) = VREF/ ((R14 + R15 + R16 + R17)). The stability of the constant current supply (3) is ensured via the feedback line (FB). The stability of the voltage between the RL and the FB is controlled by a negative feedback method, comparing the voltage reference (2) at the input of the voltage reference (3) to a voltage of the same stability. The filtration procedures have been carried out so that the noise from the feedback line does not affect the output stability of the constant current source (3). R_I ADJ (14) is a constant resistance and is used to adjust the desired value of the constant current source (3) and is determined by the resistance value TO THR R (16) corresponding to the initial temperature value of the operating range. The purpose of this resistor is to obtain the value of 0 Volts, which is the voltage Vo to be generated for the initial temperature value of the measurement range. The voltage Vo corresponding to the initial reference temperature is equivalent to the resistance value TO THR R (16) in a controlled bath of the temperature sensing circuit (18). Depending on the rate of current change applied from the constant current source (3), a ratio metric voltage drop is generated on the TO THR R (16) resistor and the temperature sensing circuit (18) corresponding to the initial temperature. The voltages generated on the TO THR R (16) resistance and temperature sensing circuit (18) are raised on amplifiers (4 and 6). The outputs of these two amplifiers (4 and 6) are applied to the input of the signal conditioning and the slope reversing circuit (7). These two signals are subject to the required pre-filtering operations and each filter output is applied to the corresponding inputs of the amplifier. The amplifier output in this circuit (7) is 0 Volts because each input voltage is the same value. This value is the voltage value at -1.05 °C, the initial temperature value of the temperature operating range. The resistance value of TO THR R (16) corresponding to the reference starting temperature is always equal to the resistance value of the temperature detection circuit (18) at -1.05 °C. Therefore, a maximum ratio metric initial reference (V REF T0) voltage has been created in relation to the initial reference temperature at the amplifier output (4). Depending on the rate of change of current applied for any measurement point of the temperature range, a ratio metric voltage drop is generated on the temperature sensing circuit (18). For example, on the temperature sensing circuit (18) of +50.05 °C, a resistance corresponding to that temperature and a voltage (V TEMP P) occurs. The resulting voltage signals (V REF T0) and (V TEMP P) are transmitted via the amplifiers (4) and (6) to the input of the signal conditioning and the slope reversing circuit (7). In this circuit (7), the output voltage corresponding to +50.05 0C is obtained by performing the ratio metric voltage differential acquisitions. At the end of this process, the negative slope in the temperature-resistance curve is converted into a positive curve as a temperature-voltage curve. The resistance of ADC Ain (17) ensures that the reference voltage of ADC voltage is obtained to be ratio metric, depending on the current source. Depending on the rate of change of current applied from the constant current source (3), a ratio metric voltage drop is generated on the resistor (17). This proportional voltage generated is increased by the amplifier (5), and the signal is transmitted to the conversion and tilt invert circuit (7). In this circuit (7), the voltage obtained by pre-filtering (10- 1) is applied to the ADC Ain Drive (10-2). The ADC ratio metric voltage reference voltage at the drive output is converted to +V REF and -V REF voltages and applied to the voltage reference inputs of the ADC. The signal amplitude control circuit (8) is limited to the level of the analogue voltage, ensuring that the appropriate analog voltage remains within the specified high and low voltage limits. This process checks the amplitude of the voltage applied to the analog input channels of the ADC and prevents damage caused by the high amplitude voltage. Unwanted changes in signal amplitude can occur when the analog signal applied to inputs is transmitted to the output due to the operating temperature and environmental temperature variations of the active electronic circuit elements. To prevent this change caused by ambient temperature, the rate of change in temperature change on the signal is determined in the temperature compensation circuit (9) and the effect of the signal changing with temperature is automatically corrected. By applying a filtration process that passes below the high level in the analog signal filtration circuit (10) to the signal that has been compensated for temperature, pollution from all undesirable frequencies on the analog signal is prevented and a more stable analog signal is generated. A high-resolution Analog to Digital converter (ADC) with analog and voltage reference inputs differential is used to provide maximum resolution from the measurement system, prevent common mode noise and noise, double the dynamic range and improve overall performance due to stable signaling. The mathematical processes and hardware checks of the reference and analog voltage signals, which are obtained to be ratio metric and applied to the inputs of the ADC, are performed in the microprocessor circuit (11). At the end of the operations, the voltage values are converted to temperature information. As a result of these conversions, temperature data is digitally transmitted by the serial port of the microprocessor to the user interface program on the computer. Inside the microprocessor, the temperature of the ADC is controlled via the temperature sensor. If the temperature change is outside the operating tolerance of the ADC, the ADC is automatically calibrated. The automatic calibration of the ADC is performed according to the three resistance values corresponding to the measurement range by disabling the temperature detection circuit (18) each time the system is switched on. When the deviations are outside tolerance, the new calibration factor is determined and these new values are automatically processed into the system. The calculations and simulation studies for the applicability of the method for signal processing and ratio metric signal detection with a constant current source in the data transfer unit are given in Table 1 and Graph-5. As shown, the coefficients of the ratio metric signal detection method for any temperature point are the same for the ±1% variation of the applied 250 pA current. Table 2 and Graphic-6 shows that the non- ratio metric signal detection method coefficients are different for any temperature point for the ±1% variation of the applied 250 pA current. The same coefficient for each current value is produced by the ratio metric signal detection method, as shown in the formula, enabling the same code to be generated from the ADC.

2²³. Gain. I_EXC. [(TO THR R - PR0B R)/2]

I_EXC. ADC_R_REF

Tablel. Ratio metric Signal Sensing Method Coefficients

Graph 5. Ratio metric Signal Sensing Method Coefficients - Temperature Change

When using the non-ratio metric signal sensing method with this invention, for example in a controlled environment of 20.0000 °C, when the current source is operated with an error of 0.1%, 1% and 10% at 2.5 V of the voltage reference value of the ADC, the temperature reading error is app. ± 2 °C, ±0.2 °C and ±0.02 °C. As a result, it has been found that using only a constant current source (3) is not sufficient for high-accuracy temperature measurements.

Table 2. Non- Ratio metric Signal Sensing Method Coefficients

Graph 6. Non-Ratio metric Signal Sensing Method Coefficients - Temperature Change

Table 3. Non-Ratio metric Signal Sensing Method Analog Signal Differences

Graph 7. Non-Ratio metric Signal Sensing Method Voltage Reading Error - Temperature Change

In addition, when applying the non-ratio metric signal sensing method with this invention, the effect of the ±1% variation of 250 pA current on the analog signal as shown in Table 3 and Graph-7, the linearity error has increased depending on the variation of current applied as the temperature increases.

Graph 8. Non-Ratio metric Signal Coefficients - Temperature Change

Graph 9. Non-Ratio metric Signal Sensing Method Voltage Reading Error - Temperature Change

A variation of 10% has been applied to the 250 pA current source to better understand the effect of the non-ratio metric signal sensing method on the temperature measurement. Graph-8 and Graph-9 also show that linearity error increases as temperature increases, depending on the variation of current applied.

The applicability measurements of the ratio metric signal sensing method with a constant current source were carried out at a resolution of 0.0001 °C, the triple point of water and the Ga fixed point. Accordingly, the measurement uncertainty (k=2.0 % 95) were determined as 2.0 mK.

At all ports of the system (probe internal ports, probe connector, and so on) thermal offset voltages occur due to ambient temperature change. These offset voltages will cause measurement errors, which are taken precautions during design phases (for example, material selection with less thermal EMF effect, PCB (Printed Circuit Board), removal of heat sources that could cause thermal EMF failure from sensitive areas, etc.). In addition, one of the current cut-off and current reversing methods, which is thermal offset elimination methods, depending on the measurement accuracy, can be applied to eliminate any EMF errors that may occur. In the thermal offset current cut-off method (15), the thermal offset error is corrected by taking the difference in the voltage readings from the signal processing and data transfer unit (20) in the off and on position of the switch. In cases where thermal offset error cannot be corrected due to measurement accuracy, the current reversing (13) method is applied. As shown in Figure 7, the current reversing circuit (13) is activated, the first measurement is taken in the normal position and the second measurement is taken by changing the terminals of the temperature sensing circuit (18). The average difference of the voltage readings from the signal processing and data transfer unit (20)

= V + V_EMF , V_M_ = -y₂ + V_EMF) is averaged. As shown in the formula V_M = p₀] _arj^y _of _t ,_e thermal offset error voltage is the

same. The considerations for the application of these methods are that measurements are taken in succession and as soon as possible, depending on the response time of the ADC. A different data acquisition time causes a change in thermal offset depending on the change in ambient temperature. In this invention, the current reversing method (T = 20.0000°C in a temperature controlled bath, I = 250 pA, temperature sensing circuit (18) resistance R Probe = 12646.714 and thermal offset voltage VEMF = 0.01 mV) was performed in two different measuring setup and the applicability of the current reversing circuit (13) was tested. Measurement Setup 1; temperature sensing circuit (probe) (18) is placed in a 20.0000 °C temperature controlled bath and V M+ and VM- voltage values obtained by applying the current reversing method as mentioned above; VM+ = 3.161688 Volts and VM- = 3.161668 Volts and the average value of the voltage differences is VM, VM = 3.161678 Volts. Measurement Setup2: An E-type thermocouple that can generate a noticeable thermal offset voltage is placed in this controlled bath and the output voltage value of the thermocouple is Vthemocoupie = 1.191504 mV. This thermocouple is connected to the temperature sensing circuit (18) so that it can generate thermal offset voltage, and V_M+and V_M_ voltage values are obtained by applying the current reversing method; V_M+ = 3.162880 Volts and V_M_= 3.160477 Volts and the average value of voltage differences is VM, V_M= 3.161678 Volts. As a result, when comparing the measurement device 1 and 2 data, it is found that the measurement results are the same and that the designed current reversing circuit (13) has eliminated both the thermal offset error caused by its own ports and the thermal offset effect created by an externally connected thermocouple. The current reversing circuit (13), designed with this invention, is available for troubleshooting thermal (EMF) offset errors. Industrial Applicability of the Invention

The invention mentioned can be used in space, defense, optical and opto-electronic and medical applications and so son, especially for all temperature measurements in metrology that require high accuracy and stability.

Claims

1. A temperature sensing system with thermistor and linearization resistors characterized in that comprising

- a temperature detection circuit (probe) (18) with four terminals and a shielded signal transmission cable, each with three separate NTC thermistors and thermistors, including hardware linearization resistors located in the same environment as the thermistors,

-a constant current source (3),

- signal processing and data transfer unit (20),

- a stable voltage reference (2) and power supply (1) used to obtain a steady current the current source (3) in the signal processing and data transfer unit (20),

- The feedback input (FB) (14,15,16,17) of the current source and the reference resistance connection output (RL) (14,15,16,17,18,19) to which the stable current is to be sent,

- Reference resistance T0-THR R (16), which determines the starting point of the reference temperature,

- ADC R REF reference resistance (17), which determines the ratio metric reference voltage of the ADC,

- The Instrumentation Amplifiers (4, 5), where the voltage generated by the applied current is processed,

- The Signal Processing and Slope Reversing Circuit (7), in which the ratio metric signal sensing method is processed,

- Analogue Signal Amplitude Control Circuit (8), which ensures that the compatible analogue voltage remains within the specified upper and lower voltage limit,

- Temperature Compensation Circuit (9), which determines the rate of change in temperature over the signal,

- An Analog Signal Filtering Circuit (10) in which a high-order low-pass filtering process is performed on the signal,

22 - Thermal EMF circuit (13), where thermal offset errors are eliminated due to temperature variations in electrical connection points,

- The ADC External Calibration Circuit (12), where the external calibration of the ADC is performed, and,

The use of the ratio metric signal sensing method for linearizing/conditioning of the obtained signal.

2. The system according to Claim 1 characterized in that the three NTC thermistors and linearization resistors, each separated, are positioned at the same location at an angle of 120° and at the same level at the bottom of the temperature sensing circuit (18).

3. The system according to Claim 1, wherein realization of a constant linear temperature response at the maximum level by narrowing the temperature-resistance change range of the temperature sensing circuit (18) according to the technical data of the thermistors and the linearization resistor circuit selected according to the measurement range.

4. The System according to Claim 1, characterized in that, based on the technical data of the thermistors and linearization resistors selected according to the measurement range and measurement accuracy, only high resolution directly without the need for external resistance and voltage source without the use of signal processing and data transfer unit (20) by narrowing the resistance-temperature change interval of the temperature sensing circuit (18) (at least 8 ’A rigid) is the ability to perform temperature-resistance calibration in the four-terminal resistance measurement range using a multimeter.

5. The System according to Claim 1, its feature is that the temperature sensing circuit (18) measures the ambient temperature according to the calibration data at the four-terminal resistance measurement stage using a high resolution multimeter directly without the need for external resistance and voltage supply without using the signal processing and data transfer unit (20).

6. The System according to Claim 1, the characterized in that the temperature sensing circuit (18) performs a sensitivity increase by reducing the self-heating error to a negligible level with the resistance-temperature variation specified by the measurement range, with the specified current and by increasing the output current so that there is no self-heating effect on the thermistors.

7. The System according to Claim 1 characterized in that elimination of the effect of thermal offset errors that occur in ambient temperature variations at connector and PCB (Printed Circuit Board) points of the temperature sensing circuit (18) via current cut-off and/or current reversing methods.

8.The System according to Claim 1 characterized in that obtaining a stable current from the constant current source (3), which will be used by the ratio metric signal sensing method in the signal processing and data transfer unit (20), using a stable voltage reference (2) voltage.

9. The constant current source (3) according to Claim 8 characterized in that a. Obtaining the current value to be used with the ratio metric signal sensing method by the ratio of the voltage reference (2) voltage to the sum of the resistance values on the feedback line (14, 15, 16, 17), b Depending on the rate of variation of the current applied, a proportional voltage drop corresponding to the initial temperature of TO is obtained with an equivalent resistance

(16) for the reference starting temperature. c. The reference voltage of the proportional ADC voltage is obtained with a resistor

(17), depending on the rate of variation of the current applied. d. Create a ratio metric voltage drop in the temperature sensing circuit (18) at any temperature measurement point, depending on the current rate of change applied e. It is the result of a linear and ratio metric voltage with a negative slope as a resistance- to-voltage curve through the high resistance temperature sensing circuit (18), depending on the rate of variation of the current applied.

10. A signal processing and data transfer unit (20) according to Claim 8; the application of the ratio metric signal detection method is to take the ratio metric voltage difference in the signal conditioning and slope reversing circuit (7), item b and d of Claim’s 9, and to convert the negative slope in the temperature-resistance curve to a positive curve as a temperature-voltage curve.

11. A signal processing and data transfer unit (20) according to Claim 8 characterized in that having a. the thermal EMF current reversing circuit (13), which can produce a signal that is fully insulated from high-accuracy, thermal effects, b. a thermal EMF current cut-off switch (15) that can produce a signal that is highly accurate, completely isolated from thermal effects. 12. A signal processing and data transfer unit (20) according to Claim 8 characterized in that eliminating the impedance mismatching that may occur in the high impedance resistors (14, 15, 16, 17, 19) and the temperature sensing circuit (18) in the acquisition of constant current from the current source (3) by applying the appropriate voltage reference (2) and the power supply (1) voltage values

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