Thermocouples (TCs) are probably the most widely used and least understood of all temperature measurement devices. When connected in pairs, TCs are simple and efficient sensors that output an extremely small DC voltage proportional to the temperature difference between the two junctions in a closed thermoelectric circuit (see Figure 1).
Figure 1. A basic thermocouple measurement system requires two sensors, one for the environment under measurement and the other, a reference junction, normally held to 0°C (32°F). Type T is one of the dozen or more common thermocouples used in general-purpose applications. Made of copper and constantan metals, it typically operates from –270°C to 400°C (–454°F to 752°F).
One junction is normally held at a constant reference temperature, while the other is immersed in the environment to be measured. The operating principle, known as the Seebeck effect, depends on the unique value of thermal electromotive force (EMF) measured between the open ends of the leads and the junction of two dissimilar metals held at a specific temperature. The amount of voltage at the open ends of the sensor and the temperature range the device can measure depend on the Seebeck coefficient, which in turn depends on the chemical composition of the materials constituting the thermocouple wire. The Seebeck voltage is calculated from:
eAB = Seebeck voltage
ΔeAB=small change in Seebeck voltage
ΔT = small change in temperature at
α= Seebeck coefficient
Thermocouple junctions alone do not generate voltages. The voltage or potential difference that develops at the output (open) end is a function of both the temperature of the junction T1 and the temperature of the open end T19. T19 must be held at a constant temperature, e.g., 0°C, to ensure that the open-end voltage changes in proportion to the temperature change in T1. 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 output relatively large voltages. The most common thermocouple types are J, K, T, and E, followed by N28, N14, S, R, and B (see Figure 2). In theory, the junction temperature can be inferred from the Seebeck voltage by consulting standard tables. In practice, 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 outputs another EMF that must be compensated.
Figure 2. Common Thermocouple Types
Cold-Junction Compensation (CJC)
A cold-reference-junction thermocouple immersed in an actual ice water bath and connected in series with the measuring thermocouple is the classical method used to compensate the EMF at the instrument terminals (see Figure 3). In this example, both copper leads connect to the instrument's input terminals. An alternative method uses a single thermocouple with the copper-constantan connection immersed in the reference ice water bath, also shown in Figure 3. The constantan-copper thermocouple junction J2 in the ice bath contributes a small EMF that subtracts from thermocouple J1's EMF so that the voltage measured at the instrument or DA system input terminals corresponds accurately to the NIST tables. Likewise, the copper wires connected to the copper terminals on the instrument's isothermal block do not need compensation because they are all copper and at the same temperature. The voltage reading comes entirely from the NIST-adjusted constantan-copper thermocouple wire.
Figure 3. Whether J2 is a purchased thermocouple or not, the junction formed by the constantan and copper lead wires must be placed in the ice bath for temperature compensation.
The above example is a special case, however, because one lead of the type-T thermocouple is copper. A constantan-iron thermocouple needs further consideration (see Figure 4). Here, J2 in the ice bath is held constant, and J1 measures the environment. Although J3 and J4 are effectively thermocouple junctions, they are at the same temperature on the isothermal block, so they output equal and opposite voltages and thus cancel. The net voltage is then the thermocouple J1 output representing T1, calibrated to the NIST standard table. If the I/O block were not isothermal, copper wire leads would be added between the input terminal and the copper-iron leads, and the copper-iron junctions (J3 and J4) would be held in an ice bath as well (see Figure 5).
Figure 4. One lead of a Type T thermocouple is made of copper, so it does not require temperature compensation when connecting to copper terminals. A Type J constantan-iron thermocouple, on the other hand, needs a closer look. J2 remains constant in the ice bath, and J1 measures the environment. Although J3 and J4 are effectively thermocouple junctions, they are at the same temperature on the isothermal block, so they generate equal and opposite voltages and cancel. Without an isothermal block, copper wire leads would be added between the input terminal and the copper-iron leads, and the copper-iron junctions (J3 and J4) would be held in an ice bath as well.
Software Compensation. Ice baths and multiple reference junctions in large test fixtures are nuisances to set up and maintain. 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.
Figure 5. The chromel and alumel wire connections at the copper leads constitute additional thermocouple junctions that must be held at the same, constant temperature. They generate equal and opposite potentials that prevent them from contributing to the voltage output from the chromel-alumel thermocouple.
When the ice baths are eliminated, CJC is still necessary to obtain accurate thermocouple measurements. The software has to read the isothermal block temperature. One popular solution is 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 (see Figure 6). The type of thermocouple used is preprogrammed for its respective channel, and the dynamic input data for the software include 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 more quickly with a polynomial equation. The method allows numerous channels of thermocouples of various types to be connected simultaneously, while the computer handles all the conversions automatically.
Figure 6. A thermistor placed near the lead wire connections is an alternative method of replacing the ice bath. The measured temperature is the difference between the thermocouple temperature and the reference thermistor temperature.
Hardware Compensation. A polynomial approach is faster than a look-up table and a hardware method is even faster because the correct voltage is immediately available for scanning. One technique is to add a battery to the circuit to null the offset voltage from the reference junction so the net effect equals a 0°C junction. A more practical approach based on this principle 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 (see Figure 7A). The voltage then corresponds to an equivalent reference junction at 0°C.
Figure 7 (A). Alternatively, a number of electronic circuits or modules can replace the ice bath. The temperature-sensitive resistor changes the calibrated value of voltage e in proportion to the amount of temperature compensation required
Thermocouple test systems often measure tens to hundreds of points simultaneously. 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 CJC sensors to ensure accurate readings, regardless of the type of sensor. Moreover, the module contains a built-in automatic zeroing channel as well as the CJC channel. Although measurement speed is relatively slower than most other types of scanning modules, the readings are accurate, low noise, stable, and captured in only milliseconds. For example, one TC channel can be measured in 3 ms, 14 channels in 16 ms, and up to 56 in 61 ms. Typical measurement accuracies are better than 0.7°C, with channel-to-channel variation typically <0.5°C (see Figure 7B).
Figure 7 (B). A typical input scanning module can accommodate up to 56 thermocouples of any type, and up to 896 channels can be connected to one A/D mainframe.
Linearization. After setting up the equivalent ice point reference EMF in either hardware or software, the measured thermocouple output must be converted to a temperature reading. The output is proportional to the temperature of the TC junction, but is not perfectly linear over a very wide range.
The standard way to obtain high conversion accuracy for any temperature uses the value of the measured thermocouple voltage plugged into a characteristic equation for that particular type of thermocouple. The equation is a polynomial with an order of six to ten. (Thermocouple polynomial coefficients are available from NIST at http://srdata.nist.gov/its90/main.) The computer automatically handles the calculation, but high-order polynomials take significant time to process. To accelerate the calculation, the thermocouple characteristic curve is divided into several segments. Each segment is then approximated by a lower-order polynomial.
Analog circuits are occasionally used to linearize the curves, but when the polynomial method is not used, the thermocouple output frequently connects to the input of an ADC, where the correct voltage-to-temperature match is obtained from a table stored in the computer's memory. For example, one DA system's TC card includes a software driver with a temperature conversion library that changes raw binary TC channels and CJC information into temperature readings, and automatically linearizes the thermocouples connected to the system.
Potential Problems and Their Solutions
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). Because thermocouple bandwidth is lower than 50 Hz, 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 analog 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 (see Figure 8).
Figure 8. Passive filters (A) come in a variety of configurations to suit the application. They are built in single or multiple sections to provide increasingly steeper slopes for faster rolloff. An active filter (B) easily eliminates the most common sources of electrical noise that competes with the thermocouple signal such as the interference from 50/60 Hz supply lines.
Thermocouple Assembly. Thermocouples are twisted pairs of dissimilar wires that are 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. Solder, however, is sufficient only at relatively low temperatures, usually <200°C. And although soldering also adds a third metal, such as a lead-tin alloy, it is unlikely to 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 lose their calibration and indicate the wrong temperature when the physical makeup of the wire is altered. Then they cannot meet the NIST standards. The alteration 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.
A thermocouple's output can change also when its insulation resistance decreases with increasing temperature. 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 (see Figure 9). The DA system will then measure the output voltage of the virtual junction at T1 instead of the true junction at T2.
Figure 9. A short circuit or an insulation failure between the leads of a thermocouple can form an unwanted, inadvertent thermocouple junction called a virtual 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. The temperature vs. voltage relationship then deviates from the published values. Choosing protective insulation intended for high-temperature operation can minimize these problems.
Thermocouple 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 other large metal object) and the thermocouple-measurement instrument. The thermocouples 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 analog 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 analog path, the stored error voltage is applied to the offset correction input of a differential amplifier and automatically nulls out the offset (see Figure 10).
Figure 10. Auto-zero correction compensates for analog circuitry drift over time and temperature. Although small, the offset could approach the magnitude of the thermocouple signal.
Open Thermocouple Detection. Detect-ing open thermocouples easily and quickly is especially critical in systems with numerous channels. Thermocouples tend to break or increase resistance when exposed to vibration, poor handling, and long service time. A simple open-thermocouple detection circuit consists of 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 to the capacitor so that it cannot charge. When a thermocouple opens or significantly changes resistance, the capacitor charges and drives the input to one of the voltage rails, which indicates a defective thermocouple (see Figure 11).
Figure 11. The thermocouple provides a short-circuit path for DC around the capacitor, preventing it from charging through the resistors. When the thermocouple opens due to rough handling or vibration, the capacitor charges and drives the input amplifier to the power supply rails, signaling a failure.
Galvanic Action. Some thermocouple insulation 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. Good installation practice therefore 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 has 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 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, the temperature gradient across the extension wire should be kept to a minimum to ensure accurate temperature measurements. Thermocouple wire is manufactured to NIST specifications, which can be better satisfied when the wire is calibrated on site against a known temperature standard.
This article was excerpted from the Signal Conditioning & PC-Based Data Acquisition Handbook, 3rd Ed., 2004, ed. John R. Gyorki, $29.95, available from IOtech.