Introduction
The output of the thermocouple also can change when its insulation resistance decreases as the temperature increases. The change is exponential and can produce a leakage resistance so low that it bypasses an open-thermocouple wire detector circuit. In high-temperature applications using thin thermocouple wire, the insulation can degrade to the point of forming a virtual junction. The data acquisition system will then measure the output voltage of the virtual junction instead of the true junction.
The most common devices used for sensing temperature include thermocouples, resistance temperature detectors (RTDs), and thermistors. Each has unique characteristics and properties that make one more suitable than another for a certain application.
Thermocouples are the most widely used device for sensing temperature, and probably the least understood. They are simple and efficient, and provide a small voltage signal proportional to the temperature difference between two junctions in a closed thermoelectric circuit. In its most basic configuration, one junction is held at a constant reference temperature while the other is placed in contact with the medium to be measured.
This medium can be gas, liquid, or solid, but in all cases, the medium shall not be allowed to chemically, electrically, or physically contaminate or alter the thermocouple junction. For special applications or to protect them from the environment, thermocouples are available with protective coatings and shields or sheaths. RTDs are composed of metals with a high positive temperature coefficient of resistance. Most RTDs are simply wire-wound or thin-film resistors made of wire with a known resistance vs. temperature relationship. Platinum is one of the most widely used materials for RTDs. They come in a broad range of accuracies, and the most accurate are also used as NIST (National Institute of Standards and Technology) temperature standards.
Thermistors are similar to RTDs in that they also change resistance between their terminals with a change in temperature. However, they can be made with either a positive or negative temperature coefficient. In addition, they have a much higher ratio of resistance change per degree C (several %) than RTDs, which makes them more sensitive.
Thermocouples are the most widely used device for sensing temperature, and probably the least understood. They are simple and efficient, and provide a small voltage signal proportional to the temperature difference between two junctions in a closed thermoelectric circuit. In its most basic configuration, one junction is held at a constant reference temperature while the other is placed in contact with the medium to be measured.
This medium can be gas, liquid, or solid, but in all cases, the medium shall not be allowed to chemically, electrically, or physically contaminate or alter the thermocouple junction. For special applications or to protect them from the environment, thermocouples are available with protective coatings and shields or sheaths. RTDs are composed of metals with a high positive temperature coefficient of resistance. Most RTDs are simply wire-wound or thin-film resistors made of wire with a known resistance vs. temperature relationship. Platinum is one of the most widely used materials for RTDs. They come in a broad range of accuracies, and the most accurate are also used as NIST (National Institute of Standards and Technology) temperature standards.
Thermistors are similar to RTDs in that they also change resistance between their terminals with a change in temperature. However, they can be made with either a positive or negative temperature coefficient. In addition, they have a much higher ratio of resistance change per degree C (several %) than RTDs, which makes them more sensitive.
The Gradient Nature of Thermocouples
Thermocouple junctions alone do not generate voltages. The output or potential difference that develops at the open end is a function of both the closed junction and the open end temperatures. The principle of operation depends on the unique value of thermal emf generated between the open ends of the leads and the junction of two dissimilar metals held at a specific temperature. The principle is called the Seebeck Effect, named after the discoverer. The amount of voltage generated at the open ends of the sensor and the range of temperatures the device can measure depend on the Seebeck coefficient, which in turn depends upon the chemical composition of the materials comprising the thermocouple wire.
Thermocouple junctions alone do not generate voltages. The output or potential difference that develops at the open end is a function of both the closed junction and the open end temperatures. The principle of operation depends on the unique value of thermal emf generated between the open ends of the leads and the junction of two dissimilar metals held at a specific temperature. The principle is called the Seebeck Effect, named after the discoverer. The amount of voltage generated at the open ends of the sensor and the range of temperatures the device can measure depend on the Seebeck coefficient, which in turn depends upon the chemical composition of the materials comprising the thermocouple wire.
In principle, a TC can be made from any two dissimilar metals such as nickel and iron. In practice, however, only a few TC types have become standard because their temperature coefficients are highly repeatable, they are rugged, and they generate relatively large output voltages. The most common thermocouple types are called J, K, T, and E, followed by N28, N14, S, R, and B. The junction temperature could be inferred from the Seebeck voltage by consulting standard tables. However, this voltage cannot be used directly because the thermocouple wire connection to the copper terminal at the measurement device itself constitutes a thermocouple junction (unless the TC lead is also copper) and generates another emf that must be compensated.
Cold Junction Compensation
The classical method used to compensate the emf at the instrument terminals is a thermocouple immersed in an actual ice-water bath which in turn connects in series with the measuring thermocouple. The ice and water combination holds the temperature bath to a constant and accurate 0°C (32°F). NIST’s thermocouple emf tables list the emf output of a thermocouple based on a corresponding reference thermocouple junction held at 0°C.
The classical method used to compensate the emf at the instrument terminals is a thermocouple immersed in an actual ice-water bath which in turn connects in series with the measuring thermocouple. The ice and water combination holds the temperature bath to a constant and accurate 0°C (32°F). NIST’s thermocouple emf tables list the emf output of a thermocouple based on a corresponding reference thermocouple junction held at 0°C.
Software Compensation
Ice baths and multiple reference junctions in large test fixtures are nuisances to set up and maintain, and fortunately they all can be eliminated. The ice bath can be ignored when the temperature of the lead wires and the reference junction points (isothermal terminal block at the instrument) are the same. The emf correction needed at the terminals can be referenced and compensated to the NIST standards through computer software. When ice baths are eliminated, cold junction compensation (CJC) is still necessary in order to obtain accurate thermocouple measurements. The software has to read the isothermal block temperature. One common technique uses a thermistor, mounted close to the isothermal terminal block that connects to the external thermocouple leads. No temperature gradients are allowed in the region containing the thermistor and terminals.
The type of thermocouple employed is pre-programmed for its respective channel, and the dynamic input data for the software includes the isothermal block temperature and the measured environmental temperature. The software uses the isothermal block temperature and type of thermocouple to look up the value of the measured temperature corresponding to its voltage in a table, or it calculates the temperature with a polynomial equation. The latter method allows numerous channels of thermocouples of various types to be connected simultaneously while the computer handles all the conversions automatically.
Ice baths and multiple reference junctions in large test fixtures are nuisances to set up and maintain, and fortunately they all can be eliminated. The ice bath can be ignored when the temperature of the lead wires and the reference junction points (isothermal terminal block at the instrument) are the same. The emf correction needed at the terminals can be referenced and compensated to the NIST standards through computer software. When ice baths are eliminated, cold junction compensation (CJC) is still necessary in order to obtain accurate thermocouple measurements. The software has to read the isothermal block temperature. One common technique uses a thermistor, mounted close to the isothermal terminal block that connects to the external thermocouple leads. No temperature gradients are allowed in the region containing the thermistor and terminals.
The type of thermocouple employed is pre-programmed for its respective channel, and the dynamic input data for the software includes the isothermal block temperature and the measured environmental temperature. The software uses the isothermal block temperature and type of thermocouple to look up the value of the measured temperature corresponding to its voltage in a table, or it calculates the temperature with a polynomial equation. The latter method allows numerous channels of thermocouples of various types to be connected simultaneously while the computer handles all the conversions automatically.
Hardware Compensation
Although a polynomial approach is faster than a look-up table, a hardware method is even faster, because the correct voltage is immediately available to be scanned. One method uses a battery in the circuit to null the offset voltage from the reference junction so the net effect equals a 0°C junction. A more practical approach is an "electronic ice point reference," which generates a compensating voltage as a function of the temperature sensing circuit powered by a battery or similar voltage source. The voltage then corresponds to an equivalent reference junction at 0°C.
Although a polynomial approach is faster than a look-up table, a hardware method is even faster, because the correct voltage is immediately available to be scanned. One method uses a battery in the circuit to null the offset voltage from the reference junction so the net effect equals a 0°C junction. A more practical approach is an "electronic ice point reference," which generates a compensating voltage as a function of the temperature sensing circuit powered by a battery or similar voltage source. The voltage then corresponds to an equivalent reference junction at 0°C.
Type Mixing
Thermocouple test systems often measure tens to hundreds of points simultaneously. In order to conveniently handle such large numbers of channels without the complication of separate, unique compensation TCs for each, thermocouple-scanning modules come with multiple input channels and can accept any of the various types of thermocouples on any channel, simultaneously. They contain special copper-based input terminal blocks with numerous cold junction compensation sensors to ensure accurate readings, regardless of the sensor type used. Moreover, the module contains a built-in automatic zeroing channel as well as the cold-junction compensation channel. Although measurement speed is relatively slower than most other types of scanning modules, the readings are captured in ms, they contain less noise, and they are more accurate and stable. For example, one TC channel can be measured in 3 ms, 14 channels in 16 ms, and 56 channels in 61 ms. Typical measurement accuracies are better than 0.7°C, with channel-to-channel variation typically less than 0.5°C.
Thermocouple test systems often measure tens to hundreds of points simultaneously. In order to conveniently handle such large numbers of channels without the complication of separate, unique compensation TCs for each, thermocouple-scanning modules come with multiple input channels and can accept any of the various types of thermocouples on any channel, simultaneously. They contain special copper-based input terminal blocks with numerous cold junction compensation sensors to ensure accurate readings, regardless of the sensor type used. Moreover, the module contains a built-in automatic zeroing channel as well as the cold-junction compensation channel. Although measurement speed is relatively slower than most other types of scanning modules, the readings are captured in ms, they contain less noise, and they are more accurate and stable. For example, one TC channel can be measured in 3 ms, 14 channels in 16 ms, and 56 channels in 61 ms. Typical measurement accuracies are better than 0.7°C, with channel-to-channel variation typically less than 0.5°C.
Linearization
After setting up the equivalent ice point reference emf in either hardware or software, the measured thermocouple voltage must be converted to a temperature reading. Thermocouple output voltage is proportional to the temperature of the TC junction, but it is not perfectly linear over a very wide range.
The standard method for obtaining high conversion accuracy for any temperature uses the value of the measured thermocouple voltage plugged into a characteristic equation for that particular type thermocouple. The equation is a polynomial with an order of six to ten. The computer automatically handles the calculation, but high-order polynomials take considerable time to process. In order to accelerate the calculation, the thermocouple characteristic curve is divided into several segments. Each segment is then approximated by a lower order polynomial.
Analogue circuits are employed occasionally to linearize the curves, but when the polynomial method is not used, the thermocouple output voltage frequently connects to the input of an analogue to digital converter (ADC) where the correct voltage to temperature match is obtained from a table stored in the computer’s memory. For example, one data acquisition system TC card includes a software driver that contains a temperature conversion library that changes raw binary TC channels and CJC information into temperature readings. Some software packages supply CJC information and automatically linearize the thermocouples connected to the system.
After setting up the equivalent ice point reference emf in either hardware or software, the measured thermocouple voltage must be converted to a temperature reading. Thermocouple output voltage is proportional to the temperature of the TC junction, but it is not perfectly linear over a very wide range.
The standard method for obtaining high conversion accuracy for any temperature uses the value of the measured thermocouple voltage plugged into a characteristic equation for that particular type thermocouple. The equation is a polynomial with an order of six to ten. The computer automatically handles the calculation, but high-order polynomials take considerable time to process. In order to accelerate the calculation, the thermocouple characteristic curve is divided into several segments. Each segment is then approximated by a lower order polynomial.
Analogue circuits are employed occasionally to linearize the curves, but when the polynomial method is not used, the thermocouple output voltage frequently connects to the input of an analogue to digital converter (ADC) where the correct voltage to temperature match is obtained from a table stored in the computer’s memory. For example, one data acquisition system TC card includes a software driver that contains a temperature conversion library that changes raw binary TC channels and CJC information into temperature readings. Some software packages supply CJC information and automatically linearize the thermocouples connected to the system.
Thermocouple Measurement Pitfalls
Noisy Environments
Because thermocouples generate a relatively small voltage, noise is always an issue. The most common source of noise is the utility power lines (50 or 60 Hz). Thermocouple bandwidth is lower than 50 Hz, so a simple filter in each channel can reduce the interfering ac line noise. Common filters include resistors and capacitors and active filters built around op amps. Although a passive RC filter is inexpensive and works well for analogue circuits, it’s not recommended for a multiplexed front end because the multiplexer’s load can change the filter’s characteristics. On the other hand, an active filter composed of an op amp and a few passive components works well, but it’s more expensive and complex. Moreover, each channel must be calibrated to compensate for gain and offset errors.
Noisy Environments
Because thermocouples generate a relatively small voltage, noise is always an issue. The most common source of noise is the utility power lines (50 or 60 Hz). Thermocouple bandwidth is lower than 50 Hz, so a simple filter in each channel can reduce the interfering ac line noise. Common filters include resistors and capacitors and active filters built around op amps. Although a passive RC filter is inexpensive and works well for analogue circuits, it’s not recommended for a multiplexed front end because the multiplexer’s load can change the filter’s characteristics. On the other hand, an active filter composed of an op amp and a few passive components works well, but it’s more expensive and complex. Moreover, each channel must be calibrated to compensate for gain and offset errors.
Additional Concerns
Thermocouple Assembly
Thermocouples are twisted pairs of dissimilar wires and soldered or welded together at the junction. When not assembled properly, they can produce a variety of errors. For example, wires should not be twisted together to form a junction; they should be soldered or welded. But solder is sufficient only at relatively low temperatures, usually less than 200°C. And although soldering also introduces a third metal, such as a lead/tin alloy, it will not likely introduce errors if both sides of the junction are at the same temperature. Welding the junction is preferred, but it must be done without changing the wires’ characteristics. Commercially manufactured thermocouple junctions are typically joined with capacitive discharge welders that ensure uniformity and prevent contamination. Thermocouples can become un-calibrated and indicate thewrong temperature when the physical makeup of the wire is altered. Then it cannot meet the NIST standards. The change can come from a variety of sources, including exposure to temperature extremes, cold working the metal, stress placed on the cable when installed, vibration, or temperature gradients.
Thermocouple Assembly
Thermocouples are twisted pairs of dissimilar wires and soldered or welded together at the junction. When not assembled properly, they can produce a variety of errors. For example, wires should not be twisted together to form a junction; they should be soldered or welded. But solder is sufficient only at relatively low temperatures, usually less than 200°C. And although soldering also introduces a third metal, such as a lead/tin alloy, it will not likely introduce errors if both sides of the junction are at the same temperature. Welding the junction is preferred, but it must be done without changing the wires’ characteristics. Commercially manufactured thermocouple junctions are typically joined with capacitive discharge welders that ensure uniformity and prevent contamination. Thermocouples can become un-calibrated and indicate thewrong temperature when the physical makeup of the wire is altered. Then it cannot meet the NIST standards. The change can come from a variety of sources, including exposure to temperature extremes, cold working the metal, stress placed on the cable when installed, vibration, or temperature gradients.
The output of the thermocouple also can change when its insulation resistance decreases as the temperature increases. The change is exponential and can produce a leakage resistance so low that it bypasses an open-thermocouple wire detector circuit. In high-temperature applications using thin thermocouple wire, the insulation can degrade to the point of forming a virtual junction. The data acquisition system will then measure the output voltage of the virtual junction instead of the true junction.
In addition, high temperatures can release impurities and chemicals within the thermocouple wire insulation that diffuse into the thermocouple metal and change its characteristics. Then, the temperature vs. voltage relationship deviates from the published values. Choose protective insulation intended for high-temperature operation to minimize these problems.
Isolation
Thermocouple isolation reduces noise and errors typically introduced by ground loops. This is especially troublesome where numerous thermocouples with long leads fasten directly between an engine block (or another large metal object) and the thermocouple-measurement instrument. They may reference different grounds, and without isolation, the ground loop can introduce relatively large errors in the readings.
Thermocouple isolation reduces noise and errors typically introduced by ground loops. This is especially troublesome where numerous thermocouples with long leads fasten directly between an engine block (or another large metal object) and the thermocouple-measurement instrument. They may reference different grounds, and without isolation, the ground loop can introduce relatively large errors in the readings.
Auto-Zero Correction
Subtracting the output of a shorted channel from the measurement channel’s readings can minimize the effects of time and temperature drift on the system’s analogue circuitry. Although extremely small, this drift can become a significant part of the low-level voltage supplied by a thermocouple. One effective method of subtracting the offset due to drift is done in two steps. First, the internal channel sequencer switches to a reference node and stores the offset error voltage on a capacitor. Next, as the thermocouple channel switches onto the analogue path, the stored error voltage is applied to the offset correction input of a differential amplifier and automatically nulls out the offset. See Figure 9.
Subtracting the output of a shorted channel from the measurement channel’s readings can minimize the effects of time and temperature drift on the system’s analogue circuitry. Although extremely small, this drift can become a significant part of the low-level voltage supplied by a thermocouple. One effective method of subtracting the offset due to drift is done in two steps. First, the internal channel sequencer switches to a reference node and stores the offset error voltage on a capacitor. Next, as the thermocouple channel switches onto the analogue path, the stored error voltage is applied to the offset correction input of a differential amplifier and automatically nulls out the offset. See Figure 9.
Open Thermocouple Detection
Detecting open thermocouples easily and quickly is especially critical in systems with numerous channels. Thermocouples tend to break or increase in resistance when exposed to vibration, poor handling, and long service time. A simple open thermocouple detection circuit comprises a small capacitor placed across the thermocouple leads and driven with a low level current. The low impedance of the intact thermocouple presents a virtual short circuit across the capacitor so it cannot charge. But when a thermocouple opens or significantly changes resistance, the capacitor charges and drives the input to one of the voltage rails, which positively indicates a defective thermocouple.
Detecting open thermocouples easily and quickly is especially critical in systems with numerous channels. Thermocouples tend to break or increase in resistance when exposed to vibration, poor handling, and long service time. A simple open thermocouple detection circuit comprises a small capacitor placed across the thermocouple leads and driven with a low level current. The low impedance of the intact thermocouple presents a virtual short circuit across the capacitor so it cannot charge. But when a thermocouple opens or significantly changes resistance, the capacitor charges and drives the input to one of the voltage rails, which positively indicates a defective thermocouple.
Galvanic Action
Some thermocouple insulating materials contain dyes that form an electrolyte in the presence of water. The electrolyte generates a galvanic voltage between the leads, which in turn, produces output signals hundreds of times greater than the net open circuit voltage. Thus, good installation practice calls for shielding the thermocouple wires from high humidity and all liquids to avoid such problems.
Some thermocouple insulating materials contain dyes that form an electrolyte in the presence of water. The electrolyte generates a galvanic voltage between the leads, which in turn, produces output signals hundreds of times greater than the net open circuit voltage. Thus, good installation practice calls for shielding the thermocouple wires from high humidity and all liquids to avoid such problems.
Thermal Shunting
An ideal thermocouple does not affect the temperature of the device being measured, but a real thermocouple comprises a mass that when added to the device under test can alter the temperature measurement. Thermocouple mass can be minimized with small diameter wires, but smaller wire is more susceptible to contamination, annealing, strain, and shunt
impedance. One solution to help ease this problem is to use the small thermocouple wire at the junction but add special, heavier thermocouple extension wire to cover long distances. The material used in these extension wires has net open-circuit voltage coefficients similar to specific thermocouple types. Its series resistance is relatively low over long distances, and it can be pulled through conduit more easily than premium grade
thermocouple wire. In addition to its practical size advantage, extension wire is less expensive than standard thermocouple wire, especially platinum.
Despite these advantages, extension wire generally operates over a much narrower temperature range and is more likely to receive mechanical stress. For these reasons, temperature gradients across the extension wire should be kept to a minimum to ensure accurate temperature measurements.
An ideal thermocouple does not affect the temperature of the device being measured, but a real thermocouple comprises a mass that when added to the device under test can alter the temperature measurement. Thermocouple mass can be minimized with small diameter wires, but smaller wire is more susceptible to contamination, annealing, strain, and shunt
impedance. One solution to help ease this problem is to use the small thermocouple wire at the junction but add special, heavier thermocouple extension wire to cover long distances. The material used in these extension wires has net open-circuit voltage coefficients similar to specific thermocouple types. Its series resistance is relatively low over long distances, and it can be pulled through conduit more easily than premium grade
thermocouple wire. In addition to its practical size advantage, extension wire is less expensive than standard thermocouple wire, especially platinum.
Despite these advantages, extension wire generally operates over a much narrower temperature range and is more likely to receive mechanical stress. For these reasons, temperature gradients across the extension wire should be kept to a minimum to ensure accurate temperature measurements.
Improving Wire Calibration Accuracy
Thermocouple wire is manufactured to NIST specifications. Often, these specifications can be more closely met when the wire is calibrated on site against a known temperature standard.
Thermocouple wire is manufactured to NIST specifications. Often, these specifications can be more closely met when the wire is calibrated on site against a known temperature standard.
Thermocouples are available either as bare wire 'bead' thermocouples which offer low cost and fast response times, or built into probes. A wide variety of probes are available, suitable for different measuring applications (industrial, scientific, food temperature, medical research etc). One word of warning: when selecting probes take care to ensure they have the correct type of connector. The two common types of connector are 'standard' with round pins and 'miniature' with flat pins, this causes some confusion as 'miniature' connectors are more popular than 'standard' types.
When choosing a thermocouple consideration should be given to both the thermocouple type, insulation and probe construction. All of these will have an effect on the measurable temperature range, accuracy and reliability of the readings. Listed below is our (somewhat subjective) guide to thermocouple types.
Type K (Chromel / Alumel)
Type K is the ‘general purpose’ thermocouple. It is low cost and, owing to its popularity, it is available in a wide variety of probes. Thermocouples are available in the -200 °C to +1200 °C range. Sensitivity is approx 41 µV/°C. Use type K unless you have a good reason not to.
Type K is the ‘general purpose’ thermocouple. It is low cost and, owing to its popularity, it is available in a wide variety of probes. Thermocouples are available in the -200 °C to +1200 °C range. Sensitivity is approx 41 µV/°C. Use type K unless you have a good reason not to.
Type E (Chromel / Constantan)
Type E has a high output (68 µV/°C) which makes it well suited to low temperature (cryogenic) use. Another property is that it is non-magnetic.
Type E has a high output (68 µV/°C) which makes it well suited to low temperature (cryogenic) use. Another property is that it is non-magnetic.
Type J (Iron / Constantan)
Limited range (-40 to +750 °C) makes type J less popular than type K. The main application is with old equipment that can not accept ‘modern’ thermocouples. J types should not be used above 760 °C as an abrupt magnetic transformation will cause permanent decalibration.
Limited range (-40 to +750 °C) makes type J less popular than type K. The main application is with old equipment that can not accept ‘modern’ thermocouples. J types should not be used above 760 °C as an abrupt magnetic transformation will cause permanent decalibration.
Type N (Nicrosil / Nisil)
High stability and resistance to high temperature oxidation makes type N suitable for high temperature measurements without the cost of platinum (B,R,S) types. Designed to be an ‘improved’ type K, it is becoming more popular.
High stability and resistance to high temperature oxidation makes type N suitable for high temperature measurements without the cost of platinum (B,R,S) types. Designed to be an ‘improved’ type K, it is becoming more popular.
Thermocouple types B, R and S are all 'noble' metal thermocouples and exhibit similar characteristics. They are the most stable of all thermocouples, but due to their low sensitivity (approx 10 µV/°C) they are usually only used for high temperature measurement (>300 °C).
Type B (Platinum / Rhodium)
Suited for high temperature measurements up to 1800 °C. Unusually type B thermocouples (due to the shape of their temperature / voltage curve) give the same output at 0 °C and 42 °C. This makes them useless below 50 °C.
Suited for high temperature measurements up to 1800 °C. Unusually type B thermocouples (due to the shape of their temperature / voltage curve) give the same output at 0 °C and 42 °C. This makes them useless below 50 °C.
Type R (Platinum / Rhodium)
Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) and high cost makes them unsuitable for general purpose use.
Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) and high cost makes them unsuitable for general purpose use.
Type S (Platinum / Rhodium)
Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) and high cost makes them unsuitable for general purpose use. Due to its high stability type S is used as the standard of calibration for the melting point of gold (1064.43°C).
Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) and high cost makes them unsuitable for general purpose use. Due to its high stability type S is used as the standard of calibration for the melting point of gold (1064.43°C).
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