We manufacture all thermocouples (Type K, J, T, N, E, R etc) to your specification, typically shipped within five days
What is a Thermocouple Sensor?
A thermocouple sensor is a simple type of temperature sensor used for a wide variety of temperature measurement applications. Thermocouple sensors consist of two different types of metal wire joined (welded) together to form a measuring junction (also known as a hot junction). When this junction is heated or cooled, a voltage is created from which the temperature can be calculated.
There are many types of thermocouple sensors available in a wide range of designs and constructions making them a practical choice for almost all temperature measurement applications and temperature ranges in industry, science and beyond.
Practical Thermocouple Sensors / Probes
Popularity of the Thermocouple Sensor
The reasons for the popularity of thermocouples are not just the existence of a range of types designed to cover almost all temperature, environmental and accuracy requirements, or the fact that they are small. Others include the ease with which they can be made and applied, and the availability of a vast assortment of housings and special packages to match almost every imaginable application.
For simple applications, thermocouples can easily be made from lengths of bare thermocouple wire or insulated cable, the insulation material being selected for compatibility with the application, and likewise the cable itself. As for wire diameter, 0.004” to 0.118” OD or larger for industrial use is common.
The measuring junction is best constructed by welding the two wires together. Soldering or twisting are less satisfactory, although with the aid of a clamping screw in a connecting block, greater security can be obtained. The key to success is a good electrical connection which does not disrupt the composition of the thermocouple wires themselves. Bear in mind the expected operating conditions for the measuring junction.
Base Metal Thermocouple Sensors
Base metal thermocouples are usually welded electrically in an argon atmosphere, while platinum thermocouples can be welded using a small oxy-hydrogen flame. Beyond this, base metal thermocouple wires are normally supplied ready annealed and are thus prepared for use directly after welding. The same is not usually the case with the platinum equivalents which therefore have to be annealed after inserting the wires into the insulators and making up the junction.
At the other end of the cables, each thermocouple wire can be joined to a copper wire to form the reference junction. Again, welding is the best bet, but silver soldering using a very small quantity of solder in paste form together with a miniature flame is a reasonable alternative - as long as all traces of corrosive flux are removed. The junctions can then be fitted into closed end tubes or potted for immersion in an ice-water mixture.
This method of thermocouple construction is simple, versatile, and fine for experimentation in the laboratory. Reasonable accuracies can be expected.
Thermocouple Sensor Construction
Figures 1.1: A Typical Industrial Thermocouple
Figures 1.2: Enclosed Thermocouple Probe and Head
Figures 1.3: Hand Held Probe and Pipe Probe
Figures 1.4: Heavy Duty Industrial metal Sheathed and High Temperature Ceramic Sheathed thermocouples with terminal Head Assemblies
Figures 1.5: Typical Mineral Insulated Thermocouple Sensor Construction
Figures 1.6: Typical thermocouple Connectors
Style of Thermocouples
However, DIY thermocouples are not for everyone, nor are they necessarily ideal for industrial applications. For industrial use, quantities, installation methods, maintenance, and replacement must be considered. It is more common to select a custom manufactured thermocouple from the wide variety described.
Thermocouples may be insulated with anything from PVC to ceramics depending on the application (see Part 2, Section 2). Frequently, the thermocouple conductors will be fitted into a closed end probe of some kind, outer sheath (protection tube), or thermowell made from a suitable heat-resistant alloy or refractory material (Figure 1.2).
There are almost as many sizes and styles as there are applications. Standard sizes range from 0.010” to 0.250” with larger diameters of 1” and larger available to cover all of the possible requirements. As for styles, there are general purpose welded sheath probes, bolt style probes, hand-held probes, surface probes, moving surface probes and these can all be encased in a metal sheath. There are also several junction types and connector systems available.
Additionally, there are sheathed probes for autoclave temperature measurement (incorporating flexible stainless steel conduit and pressure entry glands), bayonet and compression fitting style thermocouples for the plastics and other industries) and heavy duty and high temperature industrial sheathed thermocouples (Figure 1.4) with options like head-mounting terminal assemblies and thermowell extension pieces.
In all cases, great care is taken by the suppliers to ensure that the conductors are correctly manufactured, and installed into the sensor housing under closely controlled conditions. Thus, the amount of change the heated region of the conductors may experience during service (which affects uniformity) is minimized. This is important, since it is this unit that will almost certainly be sitting in the area of greatest temperature gradient, and therefore contributing to most of the output voltage (see Part 1, Section 2).
Mineral insulated Thermocouples
An alternative form of construction uses mineral insulated (MI) cable, where the thermocouple conductors are embedded in a closely compacted, inert mineral powder, and surrounded by a metal sheath (like stainless steel or nickel alloy) to form a hermetically sealed assembly. The sheath functions as a useful protective cover. This type of device is available with sheath diameters ranging from 0.010” to 0.5”. Lengths can be from a few inches to hundreds of feet.
For rather special applications, where high speed response is needed, it can be advantageous for an MI thermocouple to be manufactured with the junction itself exposed. However, this style of junction requires the expertise and skill of an experienced technician.
Thermocouple sensors are often provided with a connection or terminal box which allows convenient linking to the rest of the circuit. Alternatively, a special plug and female connector can be supplied with connecting pins made from the appropriate thermoelectric material. (Figure 1.6).
Thermocouple Sensor Insulation
for Thermocouple Probes
Although there are applications where thermocouples can be used without protection, in most they must be protected from the environment and media in which they are being asked to measure by the use of insulation materials often with protective sheaths. These latter, provided in the form of tubes or whatever, also serve to protect the thermocouple from mechanical damage.
As a general rule of thumb, engineering practice has it that an exposed thermocouple junction is only recommended for the measurement of static or flowing non-corrosive gas temperatures where fast response is a key issue.
Beyond this, insulated thermocouple junctions are more suitable, certainly for corrosive gases and liquids, accepting that thermal response is slower whether an outer sheath is involved or not. Incidentally, grounded thermocouple junctions, where the thermocouple elements are welded to the sheath tip, are preferred for corrosive gases, liquids and high pressure applications where faster thermal response is required.
Insulating Materials for Thermocouple Wires
Popularity of the Thermocouple Sensor
There is a wide choice of insulating materials available for thermocouples. When practical, the insulation will be color coded in accordance with the related standard. Although there is no international standard for materials, engineering practice dictates the use of six main materials.
PVC can be used over the range -30 to +105°C, and is available in many different types of construction. PFA offers a greater temperature range, covering from -273 to +250°C, or 300°C for short periods.
For higher temperatures, we find varnish impregnated glass fiber, which handles from -50 to +400°C, while unvarnished glass fiber takes this up to 500°C and in some cases, 800°C. Throughout the above, all of the standard thermocouple types can be accommodated.
High Temperature Thermocouple Sensors
For higher temperature realms of industrial applications, ceramic insulators are available in various forms. Porcelain ceramic, 2-hole insulators can be used on base metal thermocouple wires of approximately 0.040” and larger. Meanwhile, aluminum silicate (Mullite) insulators are frequently used with Type K thermocouple wires, particularly in furnace type applications - whether the sensor is unprotected or housed in metal or ceramic outer sheaths/tubes. As for platinum based thermocouples, high purity alumina 2-hole insulators are generally preferred to reduce the risk of contamination. Click here for more details on ceramic insulated thermocouples.
Recommended Thermocouple Sensors for General Use
Mineral insulated Thermocouples
The most popular insulation and thermocouple style for industry today is the mineral insulated (MI) style. These are comprised of a seamless metal sheath enclosing highly compacted mineral insulant powder (typically, magnesium oxide) which supports and electrically insulates the thermocouple wires held inside. Click here for details of available sensors.
Temperature ranges covered are from -200°C to +1250°C. These assemblies provide a high integrity, compact, hermetically sealed, self armored construction. They can be bent or formed and are suitable for the most difficult operating conditions. MI cable is generally available with two to six conductors, and with diameters from 0.010” to 0.5”.
Advantages of the Mineral Insulated Thermocouple
There are many advantages with this construction. They include small size, ease of installation (they can be bent, twisted and flattened without impairing performance), good mechanical strength, excellent isolation of the junction from hostile environments, high long term accuracy and stability, fast response and good insulation resistance. They are also readily available off the shelf and are reasonably priced. They are thus ideal for accurate measurement in a very wide range of applications, including extreme environments, like high vibration and high pressure/vacuum.
Additionally, they allow the use of a wide range of outer sheaths and seal termination styles to suit tremendously diverse operating conditions. Sheath materials include stainless steels, Inconel, and the Nicrobell alloys. The selection of the sheath material is based on the temperature and environment (click here for more details). Finally, platinum-rhodium alloy sheaths are often used with platinum thermocouples. The finished assembly length can be from a few inches to hundreds of feet. Beyond this, all of the usual thermocouple alloy combinations are available as MI thermocouples - both rare and base metal types. Also, the measuring junction can be exposed or insulated from the sheath, or grounded to the sheath. The insulated version has an insulation resistance of over 100M Ohms. By being insulated, ground loops on associated instrumentation are prevented. Although, with the grounded junction, the response time is faster.
Limitations of the Mineral Insulated Thermocouple
On the down side, limitations can include problems due to the different thermal coefficients of expansion of the stainless steel sheath variants, for example, as compared particularly to the Type K and N thermocouple materials - sometimes leading to premature mechanical fatigue failure. Also ironically, with both the stainless steels and Inconel 600 sheaths, there are possibilities of material contamination problems due to vapor diffusion of the elements, leading to actual contamination of the thermocouple wires by the sheath material itself.
There can also be problems relating to the ingress of water vapor resulting in reduced insulation resistance, causing calibration instability and possibly premature failure. This latter phenomenon, however, is really a matter of care in manufacture and repair.
Higher Temperature Applications
Special sheathing alloys have been developed to deal with these limitations, particularly for higher temperature applications with Type K and N thermocouples. These include the Nicrotherm DTM alloy which take on the inherent advantages of Type N thermocouple materials.
This means that MI thermocouples using Nicrotherm DTM can last four to six times longer than their stainless steel based alternatives. And, remembering the transmutation reductions achieved using Nicrotherm DTM, sheaths constructed from this material also out-perform Inconel 600 in terms of long term drift due to thermocouple wire contamination.
In general, it is recommended that the smallest diameter mineral insulated metal sheathed thermocouples should be avoided if possible for very high temperature or corrosive environment measurements. There does seem to be a correlation between MI cable diameter and its survival and long term performance. For details of sheath materials and available configurations for non mineral insulated thermocouples click here.
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