Realize the compensation of the cold junction of the thermocouple in use
Because the thermocouple is a differential temperature measurement device, the cold junction is used as a reference point when processing the thermocouple signal. Considering the voltage of the cold junction at non-zero degrees Celsius, the cold junction must be compensated for the thermocouple output voltage. This article compares several cold junction compensation devices, and introduces three application design methods and measurement results using a silicon temperature sensor detection IC as an example.
There are many types of sensors in temperature measurement applications. Thermocouples are the most commonly used and can be widely used in automobiles, homes, etc. Compared with resistance temperature detectors (RTDs), thermoelectric regulators, and temperature detection integrated circuits (ICs), thermocouples can detect a wider temperature range and have a higher cost performance. In addition, the robustness, reliability, and fast response time of thermocouples make them the first choice in a variety of work environments. Of course, thermocouples also have some defects in temperature measurement, such as poor linear characteristics. In addition, RTD and temperature sensor ICs can provide higher sensitivity and accuracy, which can be ideally used for accurate measurement systems. Thermocouple signal levels are low and often require amplification or high-resolution data converters for processing. If the above problems are eliminated, the low price, easy use, and wide temperature range of the thermocouple can make it widely used.
Thermocouple and cold junction compensation
A thermocouple is a differential temperature measurement device. It consists of two different metal wires, one for the positive node and the other for the negative node. Table 1 lists the four most commonly used thermocouple types, the metals used, and the corresponding temperature measurement ranges. Two different metal wires of a thermocouple are welded together to form two nodes. As shown in Figure 1a, the loop voltage is a function of the temperature difference between the two nodes. This takes advantage of the Seebeck effect, which is often described as the process of converting thermal energy into electrical energy. The Seebeck effect is the opposite of the Peltier effect. The Peltier effect is the process of converting electrical energy into thermal energy. Typical applications are thermoelectric coolers. As shown in FIG. 1a, the measurement voltage VOUT is the difference between the junction voltage of the detection node (hot junction) and the junction voltage of the reference junction (cold junction). Because VH and VC are generated by the temperature difference between the two junctions, VOUT is also a function of the temperature difference. The proportionality factor α corresponds to the ratio of the voltage difference to the temperature difference and is called the Seebeck coefficient.
Figure 1b shows one of the most common thermocouple applications. This configuration introduces a third metal (intermediate metal) and two additional nodes. In this example, each open-circuit node is electrically connected to a copper wire. These connections add two additional nodes to the system. As long as the two nodes have the same temperature, the intermediate metal (copper) will not affect the output voltage. This configuration allows thermocouples to be used without a separate reference junction. VOUT is still a function of the temperature difference between the hot and cold junctions and is related to the Seebeck coefficient. However, because thermocouples measure temperature differences, in order to determine the actual temperature of the hot junction, the cold junction temperature must be known. The simplest case is when the cold junction temperature is 0 ° C (freezing point). If TC = 0 ° C, VOUT = VH. In this case, the thermal junction measurement voltage is a direct conversion of the junction temperature. The US National Bureau of Standards (NBS) provides lookup tables for the correspondence between voltage characteristic data and temperature of various types of thermocouples, all of which are based on 0 ° C cold junction temperature. Using the freezing point as a reference point, the thermal junction temperature can be determined by looking up the VH in the appropriate table.
In the early days of thermocouple applications, the freezing point was used as the standard reference point for thermocouples, but in most applications it is not practical to obtain a freezing point reference temperature. If the cold junction temperature is not 0 ° C, then the cold junction temperature must be known in order to determine the actual hot junction temperature. Considering the voltage at the non-zero cold junction temperature, the thermocouple output voltage must be compensated, so-called cold junction compensation.
Figure 1: a. The loop voltage is generated by the temperature difference between two junctions of a thermocouple . b. A common thermocouple configuration consists of two metal wires connected at a junction, and the open junction of each wire is connected to a copper thermostatic wire.
Figure 2: Local temperature detection IC (MAX6610) determines cold junction temperature. The thermocouple and cold junction temperature sensor output voltage is converted by a 16-bit ADC (MAX7705).
Figure 3: The far junction diode is installed near the cold junction to detect the temperature. The MAX6002 provides a 2.5V reference voltage for the ADC.
Figure 4: ADC with integrated cold junction compensation converts thermocouple voltage to temperature without external components.
Table 1: Several common thermocouple types.
Table 2: The measured values are taken from the cold junction and hot junction temperatures in different ovens. Cold junction temperature range: -40 ° C to + 85 ° C, hot junction temperature is maintained at + 100 ° C.
Table 3: The measured values are taken from the cold junction and hot junction temperatures in different ovens. Cold junction temperature range: -40 ° C to + 85 ° C, hot junction temperature is maintained at + 100 ° C. The thermal junction measurements in the table are compensated.
Table 4: The measured values are taken from the cold junction and hot junction temperatures in different ovens. Cold junction temperature range: 0 ° C to + 70 ° C, and hot junction temperature is maintained at + 100 ° C. The thermal junction measurements in the table are decimal numbers provided by the circuit.
Selecting a Cold Junction Junction Temperature Measurement Device In order to achieve cold junction compensation, the cold junction temperature must be determined, which can be achieved by any type of temperature detection device. In general-purpose temperature sensor ICs, thermoelectric regulators, and RTDs, different types of devices have different advantages and disadvantages, and need to be selected according to specific applications. For applications with very high accuracy requirements, a calibrated platinum RTD can maintain high accuracy over a wide temperature range, but its cost is high. When the accuracy requirements are not very high, the use of thermistors and silicon temperature sensor ICs can provide higher cost performance. The thermistors have a wider temperature range than silicon ICs, and the temperature sensor ICs have higher linearity, so performance The indicators are better. Correcting the non-linearity of the thermistor will take up more microcontroller resources. Temperature-sensing ICs have excellent linearity but a narrow temperature range.
Therefore, the cold junction temperature measurement device must be selected according to the actual needs of the system, and precision, temperature range, cost, and linearity indicators need to be carefully considered in order to obtain the best cost performance.
Lookup Table Method Once you have established a cold junction compensation method, the compensated output voltage must be converted to the corresponding temperature. A simple method is to use a lookup table from NBS. Implementing lookup tables in software requires memory to store them, but these tables provide a fast and accurate solution when continuous testing is required. Two other methods for converting thermocouple voltage to temperature require more than a lookup table. These two methods are: a linear approximation using a polynomial coefficient and an analog linearization of the thermocouple output signal.
Software linear values are popular because they do not need to be stored except for the pre-defined polynomial coefficients. The disadvantage of this method is the processing time related to multiple-order polynomial. For polynomials of more orders, the processing time is further increased. For temperature measurement applications that require multiple polynomials, lookup tables may be more efficient and accurate than linear approximation methods.
Before software was used to convert the measured voltage to temperature (in addition to manually searching the look-up table), analog linearization methods were commonly used. This hardware-based method uses analog circuits to correct the nonlinearity of the thermocouple response. Its accuracy depends on the order of the approximate correction. This method is still widely used in multimeters that receive thermocouple signals.
Application Circuits Three typical applications for cold junction compensation using silicon sensor ICs are discussed below. All three circuits are used to solve cold junctions with a narrow temperature range (0 ° C to + 70 ° C and -40 ° C to + 85 ° C) Point temperature compensation, accuracy within a few degrees Celsius. The first circuit uses a temperature-sensing IC near the cold node to determine its temperature; the second circuit contains a far-junction diode temperature detector consisting of a diode-connected transistor (connected directly to the thermocouple connector) ) Provides a test signal for it; the analog-to-digital converter (ADC) in the third circuit has built-in cold junction compensation. All three circuits use K-type thermocouples (composed of Nichrome and Ni-based thermocouple alloys) for temperature measurement.
1. Typical application-In the circuit shown in Figure 2, a 16-bit ADC converts a low-level thermocouple voltage into a 16-bit serial data output. An integrated programmable gain amplifier helps improve the resolution of the A / D conversion, which is necessary to handle the small signal output of the thermocouple. The temperature detection IC is installed near the thermocouple connector to measure the temperature near the cold junction. This method assumes that the IC temperature is approximately equal to the cold junction temperature. The cold junction temperature sensor output is digitally converted by channel 2 of the ADC. The 2.56V reference inside the temperature sensor saves an external voltage reference IC.
When working in bipolar mode, the ADC can convert the positive and negative signals of the thermocouple and output on channel 1. Channel 2 of the ADC converts the single-node output voltage of the MAX6610 into a digital signal and provides it to the microcontroller. The output voltage of the temperature detection IC is directly proportional to the cold junction temperature. In order to determine the thermal junction temperature, the cold junction temperature needs to be determined first, and then the cold junction temperature is converted into the corresponding thermoelectric voltage through a K-type thermocouple lookup table provided by NBS. Add this voltage to the PGA gain calibrated thermocouple reading, and finally convert the summed result to temperature through a lookup table. The result is the thermal junction temperature.
Table 2 lists the temperature measurement results. The temperature range of the cold junction is -40 ° C to + 85 ° C, and the hot junction is maintained at + 100 ° C. The accuracy of the actual measurement results largely depends on the accuracy of the local temperature detection IC and the oven temperature.
2. Typical application 2. In the circuit shown in Figure 3, the remote junction temperature detection IC measures the cold junction temperature of the circuit. Unlike the local temperature detection IC, the IC does not need to be installed near the cold junction, but is connected externally to form a diode. The transistor measures the cold junction temperature. The transistor is mounted directly at the thermocouple connector. The temperature detection IC converts the measured temperature of the transistor into a digital output. ADC channel 1 converts the thermocouple voltage into a digital output. Channel 2 is not used and the input is directly connected to ground. An external 2.5V reference IC provides the reference voltage for the ADC.
Tables 2 and 3 list the temperature measurement results. The temperature range of the cold junction is -40 ° C to + 85 ° C, and the hot junction is maintained at + 100 ° C. The accuracy of the actual measurement results largely depends on the accuracy of the remote junction diode temperature detection IC and the oven temperature.
3. Typical application The 12-bit ADC in the circuit of Figure 3 has a temperature detection diode. The temperature detection diode converts the ambient temperature into a voltage. The IC calculates the thermal junction after compensation by processing the thermocouple voltage and the detection voltage of the diode. temperature. The digital output is the result of compensating the test temperature of the thermocouple. In the temperature range of 0 ° C to + 700 ° C, the temperature error of the device is kept within ± 9LSB. Although the device has a wide temperature measurement range, it cannot measure temperatures below 0 ° C.
Table 4 shows the measurement results of the circuit shown in Figure 4. The temperature range of the cold junction is 0 ° C to + 70 ° C, and the temperature of the hot junction is maintained at + 100 ° C.
(This article comes from Jiangsu Dongxiang Instrument: http://zzyfwh.com )