There are many thermocouple types and proper selection is essential to obtain the most accurate temperature measurement and control from thermocouple sensors. Use these guidelines to ensure the right sensor for the application.
Thermocouples are widely used as temperature sensors for data acquisition, instrumentation, process control, and hand-held digital thermometers. They are inexpensive, rugged, and can measure temperatures of several thousand degrees C. In fact, thermocouples and noncontact infrared pyrometers are the only devices available that can measure temperatures above 650°C. By comparison, thermocouples are significantly less expensive than infrared pyrometers and can be permanently installed.
Although thermocouples have been used for many years, their principal of operation is frequently misunderstood. The most appropriate and accurate thermocouple type that you can select for an application depends upon its basic construction and the method of installation.
How thermocouple types work
When two wires of different metals are connected at two junctions and the measuring junction is exposed to a higher temperature than the colder reference junction, current flows in the series circuit. This phenomenon is called the Seebeck effect, discovered by T. Seebeck in 1821. When the reference junction is opened, the voltage that appears across the open ends is a function of the sensing junction temperature. Its polarity and magnitude depend on the materials used to form the thermocouple.
Thermocouples cannot be connected to just any type of instrument terminal. For example, when a thermocouple is connected to copper input terminals of a meter, two new thermocouple junctions are created. Additional cold or reference junctions produce additional voltages in series that can add to or subtract from the voltage produced by the measuring junction and significantly reduce accuracy.
To achieve reasonable accuracy, an ice bath can be used to keep the temperature of the two additional junctions stable at 0ºC regardless of the ambient temperature. Because the two junction temperatures are constant and the same, the voltages produced by the additional junctions are also constant and can be easily compensated in the meter. However, handling ice baths are a bother and modern instruments use an electronic circuit to replace the ice bath for cold junction compensation.
Selecting thermocouple types
Different types of thermocouples use various combinations of several metals resulting in a wide range of sensitivies, linearities, temperature ranges, and corrosion resistances.
The temperature extremes and environment to which the bare thermocouple (TC) wires will be exposed are the starting points in the type selection. For example, the most commonly used J-type is recommended for reducing atmospheres and operates between -270 and 760ºC.
Another widely used general-purpose TC, type, K, has an operating temperature range of -270 to 1,372ºC, but is recommended only for clean oxidizing atmospheres. Many digital thermometers and multimeters with built-in thermometers are supplied with K-type thermocouples.
Recommended for sub-zero temperature measurements due to its moisture resistance is the T-type thermocouple that spans -270 to 400ºC. It is compatible with oxidizing and reducing atmospheres. The E-type has a range of -270 to 1,000ºC and the highest EMF output compared to other standard thermocouples. It has a high resistance to corrosion at low temperatures and can be used in oxidizing, reducing, and all inert atmospheres.
These thermocouple types belong to what is called the base-metal group. They are inexpensive and widely available. A second major group of thermocouples is called the noble-metal group (platinum alloy). Noble-metal thermocouples are more expensive than base-metal types and have the highest corrosion, and oxidation resistance, and temperature limits.
Accuracy varies among the different thermocouple types. For example, the standard tolerance for J and K types is ± 0.75% or ±2.2°C. R and S types have a tolerance of ± 0.25% or ±1.5°C. The tolerance is defined as the deviation from an ideal thermocouple, not the degree of nonlinearity.
All thermocouple types have different sensitivity. For example, the most sensitive E-type generates 58.5 µV/°C voltage change at 0°C. The least sensitive B-type generates only 6 µV/°C at 600°C. All thermocouples have nonlinear voltage vs. temperature curves. Thermocouples of the same type are completely interchangeable and produce measurements within the given tolerances.
Response time and protection
The construction and type of insulating material surrounding the thermocouple must match the temperature range and atmospheric environment to prevent deterioration. The simplest and least protected type is the wire thermocouple, which has an exposed junction and no protective sheath. Advantages of this type are fast response, low cost, light-weight, and flexibility of use. A major disadvantage, however, is that it is susceptible to environmental and mechanical damage.
Sheath covered probes are used where the thermocouple wire must be protected. Here, the wires are embedded in ceramic insulation, with a stainless steel or nickel alloy sheath enclosing the assembly. The selection of sheath material depends on the thermocouple’s operating temperature and atmospheric environment. In addition, the ceramic insulator must survive the upper temperature limit.
Thermowells and tube assemblies made of carbon steel, stainless steel, and brass sheaths are frequently used for heavy-duty industrial and corrosive environment applications.
Sheath covered thermocouples can have an exposed, ungrounded, or grounded measuring junction. The junction selection depends on the environment and mechanical impact the junction will be subjected to. The exposed junction has the fastest response of all types, but is the least protected from the environment. Another advantage is its relatively small mass in contact with the measured object. This minimizes the heat sinking effect that could temporarily lower the temperature of a small object during the test.
Thermocouple types with exposed junctions are not usually isolated electrically from the metering circuit. If the thermometer shares a common lead or electrical potential with the circuit being measured, touching a live circuit with the exposed thermocouple can create a short.
The grounded thermocouple junction also can be a potential safety hazard. The thermocouple and insulator are completely sealed and protected from the atmosphere, but the junction is welded to the sheath from the inside. It provides a path for a ground return circuit through the sheath, which can be a shock hazard. This type provides a faster thermal response than the ungrounded type, but it is much slower than the exposed type with the same sheath diameter.
The ungrounded junction has the best thermocouple protection of all types, including electrical isolation. However, it is also the slowest.
Scientists and engineers are developing new thermocouple sensors especially for applications such as aerospace, metal processing, petrochemical, cryogenics, and scientific research where sensors are subjected to extreme temperatures and aggressive environments. The challenge is to increase the thermocouple protection from the environment without the penalty of increasing the response time.
Another area of development is in the design of nonstandard thermocouples for very specific types of applications. Metal alloys are optimized to increase the measurement accuracy and reduce drift over time. Such sensors maintain required accuracy over longer periods of time reducing maintenance cost and equipment down time.
Thin-film thermocouples are now being used in many environmentally troublesome applications. These sensors are formed by depositing thin film strips of thermocouple metals on various base materials. Their small size, low profile, and fast response time are ideal for a broad range of surface temperature measurements. For example, thin film thermocouples are formed on metal parts inside jet engines to monitor engine temperature without causing flow perturbation. Other applications include thermopiles, temperature measurements on silicon wafers, and thermal converters.
Make your own thermocouple type
Simple thermocouple types can be constructed in-house by combining either bare or insulated thermocouple wires and bonding them at the measuring end. The wires can be welded, soldered, silver soldered, or twisted to make a measurement junction.
Twisting thermocouple wires is the easiest method and does not require any special tools, but it produces the least reliable connection. Since the wires are not actually bonded together, corrosion or vibration can interrupt the electrical contact over time. Twisting can also lead to acquiring erroneous temperature readings. Several turns are required to get a secure junction, so the first point where the two wires touch is a certain distance away from the point of measurement. As a result, a surface measurement reading can be different from the actual temperature because the measuring junction is not in contact with the surface. A twisted thermocouple is usually satisfactory for liquid or air measurements where the whole twisted area is submerged.
Soldering or brazing techniques limit the operating temperature range to the melting temperature of the solder. Care should be taken to make a firm mechanical connection between the thermocouple wires. If the solder holding the wires together separates the thermocouple wires from each other the junction will not be formed even though electrically the circuit will be complete.
Welding is the preferred way of assembly, but producing a good welded thermocouple tip without proper equipment is not easy. A poor weld can produce an open circuit connection. Overheating the welded connection can change the operating characteristics of the thermocouple metals. For these reasons, commercially available thermocouple junctions are bonded on special capacitive discharge welders where energy and weld temperature are closely controlled.