Introduction The thermocouple is a simple, widely used component for measuring temperature. This article provides a basic overview of thermocouples, describes common challenges encountered when designing with them, and suggests two signal conditioning solutions. The first solution combines both reference-junction compensation and signal conditioning in a single analog IC for convenience and ease of use; the second solution separates the reference-junction compensation from the signal conditioning to provide digital-output temperature sensing with greater flexibility and accuracy.
Thermocouple Theory
A thermocouple, shown in Figure 1, consists of two wires of dissimilar metals joined together at one end, called the measurement (“hot”) junction. The other end, where the wires are not joined, is connected to the signal conditioning circuitry traces, typically made of copper. This junction between the thermocouple metals and the copper traces is called the reference (“cold”) junction.* *We use the terms “measurement junction” and “reference junction” rather than the more traditional “hot junction” and “cold junction.” The traditional naming system can be confusing because in many applications the measurement junction can be colder than the reference junction. The voltage produced at the reference junction depends on the temperatures at both the measurement junction and the reference junction. Since the thermocouple is a differential device rather than an absolute temperature measurement device, the reference junction temperature must be known to get an accurate absolute temperature reading. This process is known as reference junction compensation (cold junction compensation.) Thermocouples have become the industry-standard method for cost-effective measurement of a wide range of temperatures with reasonable accuracy. They are used in a variety of applications up to approximately +2500°C in boilers, water heaters, ovens, and aircraft engines—to name just a few. The most popular thermocouple is the type K, consisting of Chromel® and Alumel® (trademarked nickel alloys containing chromium, and aluminum, manganese, and silicon, respectively), with a measurement range of –200°C to +1250°C.Why Use a Thermocouple?
Advantages
- Temperature range: Most practical temperature ranges, from cryogenics to jet-engine exhaust, can be served using thermocouples. Depending on the metal wires used, a thermocouple is capable of measuring temperature in the range –200°C to +2500°C.
- Robust: Thermocouples are rugged devices that are immune to shock and vibration and are suitable for use in hazardous environments.
- Rapid response: Because they are small and have low thermal capacity, thermocouples respond rapidly to temperature changes, especially if the sensing junction is exposed. They can respond to rapidly changing temperatures within a few hundred milliseconds.
- No self heating: Because thermocouples require no excitation power, they are not prone to self heating and are intrinsically safe.
Disadvantages
- Complex signal conditioning: Substantial signal conditioning is necessary to convert the thermocouple voltage into a usable temperature reading. Traditionally, signal conditioning has required a large investment in design time to avoid introducing errors that degrade accuracy.
- Accuracy: In addition to the inherent inaccuracies in thermocouples due to their metallurgical properties, a thermocouple measurement is only as accurate as the reference junction temperature can be measured, traditionally within 1°C to 2°C.
- Susceptibility to corrosion: Because thermocouples consist of two dissimilar metals, in some environments corrosion over time may result in deteriorating accuracy. Hence, they may need protection; and care and maintenance are essential.
- Susceptibility to noise: When measuring microvolt-level signal changes, noise from stray electrical and magnetic fields can be a problem. Twisting the thermocouple wire pair can greatly reduce magnetic field pickup. Using a shielded cable or running wires in metal conduit and guarding can reduce electric field pickup. The measuring device should provide signal filtering, either in hardware or by software, with strong rejection of the line frequency (50 Hz/60 Hz) and its harmonics.
Difficulties Measuring with Thermocouples
It is not easy to transform the voltage generated by a thermocouple into an accurate temperature reading for many reasons: the voltage signal is small, the temperature-voltage relationship is nonlinear, reference junction compensation is required, and thermocouples may pose grounding problems. Let’s consider these issues one by one. Voltage signal is small: The most common thermocouple types are J, K, and T. At room temperature, their voltage varies at 52 µV/°C, 41 µV/°C, and 41 µV/°C, respectively. Other less-common types have an even smaller voltage change with temperature. This small signal requires a high gain stage before analog-to-digital conversion. Table 1 compares sensitivities of various thermocouple types. Table 1. Voltage Change vs. Temperature Rise (Seebeck Coefficient) for Various Thermocouple Types at 25°C.Thermocouple Type | Seebeck Coefficient (µV/°C) |
E | 61 |
J | 52 |
K | 41 |
N | 27 |
R | 9 |
S | 6 |
T | 41 |
- Thermistors: They have fast response and a small package; but they require linearization and have limited accuracy, especially over a wide temperature range. They also require current for excitation, which can produce self-heating, leading to drift. Overall system accuracy, when combined with signal conditioning, can be poor.
- Resistance temperature-detectors (RTDs): RTDs are accurate, stable, and reasonably linear, however, package size and cost restrict their use to process-control applications.
- Remote thermal diodes: A diode is used to sense the temperature near the thermocouple connector. A conditioning chip converts the diode voltage, which is proportional to temperature, to an analog or digital output. Its accuracy is limited to about ±1°C.
- Integrated temperature sensor: An integrated temperature sensor, a standalone IC that senses the temperature locally, should be carefully mounted close to the reference junction, and can combine reference junction compensation and signal conditioning. Accuracies to within small fractions of 1°C can be achieved.
Measurement Solution 1: Optimized for Simplicity
Figure 6 shows a schematic for measuring a K-type thermocouple. It is based on using the AD8495 thermocouple amplifier, which is designed specifically to measure K-type thermocouples. This analog solution is optimized for minimum design time: It has a straightforward signal chain and requires no software coding. How does this simple signal chain address the signal conditioning requirements for K-type thermocouples? Gain and output scale factor: The small thermocouple signal is amplified by the AD8495’s gain of 122, resulting in a 5-mV/°C output signal sensitivity (200°C/V). Noise reduction: High-frequency common-mode and differential noise are removed by the external RFI filter. Low frequency common-mode noise is rejected by the AD8495’s instrumentation amplifier. Any remaining noise is addressed by the external post filter. Reference junction compensation: The AD8495, which includes a temperature sensor to compensate for changes in ambient temperature, must be placed near the reference junction to maintain both at the same temperature for accurate reference-junction compensation. Nonlinearity correction: The AD8495 is calibrated to give a 5 mV/°C output on the linear portion of the K-type thermocouple curve, with less than 2°C of linearity error in the –25°C to +400°C temperature range. If temperatures beyond this range are needed, Analog Devices Application Note AN-1087 describes how a lookup table or equation could be used in a microprocessor to extend the temperature range. Handling insulated, grounded, and exposed thermocouples: Figure 5 shows a 1-MΩ resistor connected to ground, which allows for all thermocouple tip types. The AD8495 was specifically designed to be able to measure a few hundred millivolts below ground when used with a single supply as shown. If a larger ground differential is expected, the AD8495 can also be operated with dual supplies. More about the AD8495: Figure 7 shows a block diagram of the AD8495 thermocouple amplifier. Amplifiers A1, A2, and A3—and the resistors shown—form an instrumentation amplifier that amplifies the K-type thermocouple output with a gain appropriate to produce an output voltage of 5 mV/°C. Inside the box labeled “Ref junction compensation” is an ambient temperature sensor. With the measurement junction temperature held constant, the differential voltage from the thermocouple will decrease if the reference junction temperature rises for any reason. If the tiny (3.2 mm × 3.2 mm × 1.2 mm) AD8495 is in close thermal proximity to the reference junction, the reference-junction compensation circuitry injects additional voltage into the amplifier, so that the output voltage stays constant, thus compensating for the reference temperature change. Table 2 summarizes the performance of the integrated hardware solution using the AD8495: Table 2. Solution 1 (Figure 6) Performance SummaryThermocouple Type | Measurement Junction Temperature Range | Reference Junction Temperature Range | Accuracy at 25°C | Power Consumption |
K | –25°C to +400°C |
0°C to 50°C
|
±3°C (A grade)
±1°C (C grade)
|
1.25 mW
|
Measurement Solution 2: Optimized for Accuracy and Flexibility
Figure 8 shows a schematic for measuring a J-, K-, or T-type thermocouple with a high degree of accuracy. This circuit includes a high-precision ADC to measure the small-signal thermocouple voltage and a high-accuracy temperature sensor to measure the reference junction temperature. Both devices are controlled using an SPI interface from an external microcontroller. How does this configuration address the signal conditioning requirements mentioned earlier? Remove noise and amplify voltage: The AD7793, shown in detail in Figure 9—a high-precision, low-power analog front end—is used to measure the thermocouple voltage. The thermocouple output is filtered externally and connects to a set of differential inputs, AIN1(+) and AIN1(–). The signal is then routed through a multiplexer, a buffer, and an instrumentation amplifier—which amplifies the small thermocouple signal—and to an ADC, which converts the signal to digital. Compensate for reference junction temperature: The ADT7320 (detailed in Figure 10), if placed close enough to the reference junction, can measure the reference-junction temperature accurately, to ±0.2°C, from –10°C to +85°C. An on-chip temperature sensor generates a voltage proportional to absolute temperature, which is compared to an internal voltage reference and applied to a precision digital modulator. The digitized result from the modulator updates a 16-bit temperature value register. The temperature value register can then be read back from a microcontroller, using an SPI interface, and combined with the temperature reading from the ADC to effect the compensation. Correct nonlinearity: The ADT7320 provides excellent linearity over its entire rated temperature range (–40°C to +125°C), requiring no correction or calibration by the user. Its digital output can thus be considered an accurate representation of the reference-junction state. To determine the actual thermocouple temperature, this reference temperature measurement must be converted into an equivalent thermoelectric voltage using equations provided by the National Institute of Standards and Technology (NIST). This voltage then gets added to the thermocouple voltage measured by the AD7793; and the summation is then translated back into a thermocouple temperature, again using NIST equations. Handle insulated and grounded thermocouples: Figure 8 shows a thermocouple with an exposed tip. This provides the best response time, but the same configuration could also be used with an insulated-tip thermocouple. Table 3 summarizes the performance of the software-based reference-junction measurement solution, using NIST data: Table 3. Solution 2 (Figure 8) Performance SummaryThermocouple Type | Measurement Junction Temperature Range | Reference Junction Temperature Range | Accuracy | Power Consumption |
J, K, T | Full Range |
–10°C to +85°C
–20°C to +105°C
|
±0.2°C
±0.25°C
|
3 mW
3 mW
|
Conclusion
Thermocouples offer robust temperature measurement over a quite wide temperature range, but they are often not a first choice for temperature measurement because of the required trade-offs between design time and accuracy. This article proposes cost-effective ways of resolving these concerns. The first solution concentrates on reducing the complexity of the measurement by means of a hardware-based analog reference junction compensation technique. It results in a straightforward signal chain with no software programming required, relying on the integration provided by the AD8495 thermocouple amplifier, which produces a 5-mV/°C output signal that can be fed into the analog input of a wide variety of microcontrollers. The second solution provides the highest accuracy measurement and also enables the use of various thermocouple types. A software-based reference junction compensation technique, it relies on the high-accuracy ADT7320 digital temperature sensor to provide a much more accurate reference junction compensation measurement than had been achievable until now. The ADT7320 comes fully calibrated and specified over the –40°C to +125°C temperature range. Completely transparent, unlike a traditional thermistor or RTD sensor measurement, it neither requires a costly calibration step after board assembly, nor does it consume processor or memory resources with calibration coefficients or linearization routines. Consuming only microwatts of power, it avoids self-heating issues that undermine the accuracy of traditional resistive sensor solutions.Appendix
Use of NIST Equation to Convert ADT7320 Temperature to Voltage The thermocouple reference junction compensation is based on the relationship:(1) |
(2) |