The most common sensor in a thermal system is a thermocouple (TC), which is sensitive to a change in process conditions, reasonably accurate and offers the required precision at a modest cost. Thermocouples are rugged and can be used over a wide range of process temperatures-from subzero to over 4000F (2205C). However, a combination of many factors influence the successful application of a thermocouple, including thermocouple material purity and manufacturing quality, temperature exposure, thermal cycling, chemical exposure, the protection applied and physical abuse. This article provides a perspective on the selection and application of thermocouples.
All thermocouples are not equal
One would think that the process of fastening a couple of wires together to form a thermocouple would result in the same useful device every time. However, there are several important factors to consider when selecting a thermocouple, from the type of material to quality of the supplier. For example, manufacturing-derived material inconsistencies are a source of loss in thermocouple accuracy, which, in this case, is defined as the amount of error that exists in a temperature measurement. Material inconsistencies can be introduced during the melting of the metals (either when the alloys are produced or when the TC tips are welded), during cold forming such as swaging, drawing and bending and during heat treatment and cooling. Most thermocouple manufacturers provide information on the initial calibration tolerances for their thermocouples (Table 1). For example, standard tolerance for a Type K thermocouple at a temperature of 2000F (1095C) is about 15F (8C). In critical high-temperature applications, special tolerances (e.g., 8F, or 4C) may be required to ensure a favorable process outcome.
Selecting the right thermocouple
Selecting the proper thermocouple requires that needs are defined and matched with the appropriate kind and type of thermocouple. For example:
- What are the maximum and minimum temperatures the thermocouple will see?
- What is the furnace atmosphere?
- What are the process specifications and allowable tolerances for error?
- How long is the measurement process expected to run?
- What is the expected thermocouple life?
- Are there any cost limits?
- Are the users experienced in proper thermocouple management?
For example, the kind of thermocouple used to provide long-term process monitoring and control might have to be a lower gage (thicker) wire having better mechanical and corrosion protection than a periodic load thermocouple, which might be less expensive and flexible enough for proper insertion, but can have a shorter life. If the users of the thermocouples have only limited experience, or if the devices can't be serviced frequently, a more robust thermocouple system that has additional engineered protection might be a better choice.
Two most common styles of thermocouples in the metals processing industry are the bare-wire standard industrial thermocouple (also called beaded thermocouple) and the mineral-insulated, metal-sheath (also called MI, MIMS and MgO) thermocouple.
Bare wire thermocouples, the oldest style of thermocouple used, generally are constructed using the two dissimilar metal wires separated by nonconducting ceramic insulators. They are relatively easy to make and generally have the lowest first cost. They also require the most attention, both in frequency of service and attention to consistent detail. Two primary junction styles used are twisted and butt-welded. Twisted vs. butt-welded junction
Although a mechanical joint such as a twist can provide the necessary contact between the two dissimilar wires for the thermocouple to operate, a more robust connection is generally needed for extended service life. The Law of Intermediate Temperatures states if two dissimilar homogeneous metals produce a thermal emf value, it will remain constant if a third material is introduced into the circuit, provided both ends of that material are at the same temperature. This permits welding of the hot junction.
The two most common methods of joining are twisted and welded or butt-welded. Twisting the junction may slow down the system response due to the added mass of the wire. However, a twisted and welded junction is more durable mechanically. Note that bright, clean wires form a junction wherever they contact. This means that as the thermocouple ages, the actual location of the junction may change from two bright wires in contact at one location on the twist to the fused portion of the junction that may be farther down the thermocouple. Butt-welded TCs offer higher sensitivity, are quicker to respond to temperature changes, and may be considered more accurate, as there is only one contact point. There must be consistency in the length of the twist in beaded industrial thermocouples. Longer or shorter twisting can move the hot junction by an inch or more. Consistent fusing techniques also are required. Insulator selection
Insulators are used to separate the two conductors from one another, thus preventing an undesirable secondary measuring junction at a different location in the thermocouple. Insulator material can affect the overall life and performance of the TC system. In high temperature applications, alumina insulators are recommended when using noble-metal wire, such as platinum, because silicon in the insulator can attack noble metals. Following are some tips for using and maintaining standard industrial beaded thermocouples:
- Use protection tubes when measuring temperatures in corrosive atmospheres.
- Always inspect the protection tube when changing thermocouples. Things to look for include pinholes at the refractory line, cracks in the tip of the tube and carbon build-up or contamination on the outer surface of the tube. Tube alloy selection is important, and the material may not be correct for the application if excessive deterioration is occurring.
- Inspect the thermocouple when removed from the protection tube. Contamination on the surface of the wire can dramatically affect TC performance because the generated emf concentrates on the wire surface and the signal will be dampened if the wire is not clean and bright. Green coloring on Type K thermocouples indicates the occurrence of aging and a change in alloy chemical composition (green rot). Orange coloring (iron oxide) on Types J and K TCs indicates a leaking protection tube or moisture contamination. A soft, fuzzy black appearance can be from carbon contamination. Hard black scale on the tip of a Type K TC indicates the thermocouple has been overheated. The tip may even appear swollen. Silver discoloration on the insulators indicates a leaking protection tube.
- When using a protection tube, insert the thermocouple so it does not touch the bottom or walls of the protection tube. Insertion depth should be a minimum of 4 to 10 times the outside diameter of the protection tube.
- Do not allow excessive bending or flexing of thermocouples, because cold working causes recrystallization of the microstructure, which affects accuracy.
- In applications where temperature ranges permit the use of beaded Type K thermocouples, such as repeated thermal cycling from 1800 to 800F (980 to 430C) in a car-bottom furnace, thermocouple life can be improved by selecting lower gage wire or by using a mineral-insulated, metal-sheathed thermocouple.
- Always replace the protection tube cap. Protection tubes that are filled up with debris result in erroneous temperature readings.
- Avoid changing the thermocouple immersion depth after initial installation. This especially is true for process temperatures above 1000F (540C).
- Take care not to locate the sensor too close to any radiant heating element.
- Always use the largest practical wire size for optimum life, response, stability and performance.
- Establish a preventative maintenance program and record the life and cause of failure for each TC.
- Do not use platinum wire to support anything except its own weight in noble-metal TCs, and use a collar to support the weight of the insulators. Platinum wire has practically no strength at high temperature. The use of 26 gage platinum wire is acceptable at temperatures below 2400F (1315C). However, 24 gage or larger wire is recommended at temperatures above 2400F. In all cases, platinum wire should only be terminated with a butt weld, as the material may yield and fail when twisted.
The most common insulation used for mineral-insulated, metal-sheath TCs is magnesium oxide (therefore, the reference to MgOs). Metal-sheath thermocouples are made of a pair of wires packed in a refractory material and encapsulated in a metallic sheath. They typically can withstand adverse conditions for a significantly longer time than bare thermocouples. Although their first cost often is more than double that of a standard bare-wire thermocouple assembly, they have become increasingly popular due to ease of replacement and extended life. MgOs frequently are specified to contain plug in-type terminations, which removes the possibility of polarity reversal. They are available with three types of thermocouple exposures (Fig. 1). The most common is ungrounded, where the welded bead is isolated from the sheath. Although this type has somewhat slower response to changes in process temperature, it also has less risk of electrical noise interfering with the signal from the source. Grounded thermocouples have a faster response and can be successfully used where electrical interference is not a concern. Exposed-tip MgOs provide the fastest response, but at the expense of TC life.
Various grades of purity of the refractory filler are available. The so-called standard grade contains 96% magnesium oxide with the balance mostly SiO2, and provides a higher insulation resistance. Typically, the higher the process temperature, the more important is refractory purity. A 96% purity level is good for load sensors at temperatures below 1600F (870C). The higher purity MgOs (minimum 99.4%) are better for use in applications that use platinum because impurities in the refractory may dramatically affect the life of the platinum. High purity insulation also is better for long-term drift protection, even though insulation resistance is lower. High-purity MgO should be used in vacuum applications to ensure long life and consistent accuracy.
The moisture content of the refractory in the sheath must be kept to an absolute minimum to prevent internal corrosion of the measuring element. Chemically bound moisture in an MgO is released at higher temperatures and condenses in the cold end of the sleeve. Moisture also can significantly affect the insulation resistance of the ceramic within the sensor to the point where a secondary junction is created. Although most specifications discuss resistance checks at room temperature, the real impact of moisture does not take place until the sensor reaches high temperatures. A high-temperature insulation resistance specification is sometimes used to control moisture concerns. Certain epoxies used to seal out moisture can attack thermocouple wire, shortening the life of the MgO.
The fine ceramic material in an MgO thermocouple needs to be packed very tightly to minimize voids so wire-to-wire or sheath-to-wire contact is prevented. When the wire and refractory packed sheath is swaged or rolled to size, it also needs to be properly heat treated and uniformly cooled to remove the effects of any cold working on the thermocouple. The thermocouple should be packaged in an airtight plastic wrap to prevent tramp moisture from causing contamination if long storage is expected. Also, MgOs should not be stored in humid environments for an extended period of time.
Two primary methods of manufacturing MgOs are the powder-fill and crushed-insulator methods. Power fill produces a less expensive but inferior product having a higher likelihood of voids (lower compaction density) and nonuniform wire spacing. Crushed insulator ensures uniform compaction and wire separation, but usually costs more. The higher quality process might be a better choice if the application has vibrations or thermal cycling. Following are some tips for using MgOs:
- MgO sensors are ideal to monitor process load applications. However, it is good practice to keep the bends to a minimum and keep the bend radiuses as large as possible. Bending an MgO lowers the insulation resistance at the bend and causes unwanted cold working of the wire. Try to prevent right angle bends and kinking. If the insulation resistance becomes too low, an undesirable secondary measuring junction develops at the bend. A rule of thumb is you can bend twice the sensor diameter over a mandrel, but you can't do it by hand. The bend radius should be 1 in. (25 mm) or greater.
- Use a 10 times the sheath diameter rule for good practice when inserting the MgO. A minimum immersion of 10 times the outside diameter of the protection tube or MI cable should be used.
- In high temperature furnace applications, an unsupported horizontal MgO control couple may not be able to support itself and may droop.
- In applications where the shell of the furnace is hot, use a conservative length MgO. The epoxy on the end seal of the MgO will crack if the thermocouple is too short and the epoxy gets too hot, which allows moisture to get into the end of the MgO causing reading errors and premature failure.
Buying load thermocouples in large diameters might be impractical because they need to be bendable (flexible), and are used only a few times and then discarded. A 0.125 in. (3 mm) diameter wire has a resistance of approximately 20 _ and a 0.062 in. (1.5 mm) diameter wire has a resistance of approximately 80 _. As the temperature increases, the electrical resistance of insulation decreases and the resistance in the wire increases. Long thermocouples in very hot environments can result in the creation of an unwanted secondary junction and result in an erroneous low reading.
Thermocouple extension wire
The wire connecting the thermocouple to the instrument may be made of the same material as the thermocouple with the same specifications. However, this is not always the most cost-effective means of delivering the signal to the instrument, so a less expensive substitute material frequently is selected. Three general grades of thermocouple wire are:
- Thermocouple Grade wire - the same material as that used to manufacture sensors. This wire has emf characteristics matching established temperature versus emf tables.
- Extension Grade wire - typically used to connect a thermocouple sensor to instrumentation. This wire has a similar chemical composition and emf characteristics similar to thermocouple grade materials over a limited temperature range.
- Compensating Grade - contains alloys that have emf characteristics similar to the thermocouple alloy. Compensating materials usually are low-cost alternatives for extension lead-wire types. An example would be copper-nickel alloy used as a compensating lead wire for a high-temperature platinum alloy thermocouple.
- The key to long life for extension wire is the quality of the insulation wrapping the conductors and the outer casing. Although this can significantly affect the price of the wire, a high PIC count (referring to the number of fiber strands per inch of length) results in a tightly wrapped and durable wire.
Extension wire-related problems frequently encountered in the field include the use of copper wire to connect thermocouple to instrument, using connectors without compensating contacts in areas of temperature gradients, reversal of "+" and "-" orientation not maintained in system, not matching calibration type to instrumentation capability, having power and sensor wires in same conduit or in close proximity to one another and electrical interference induced into thermocouple circuit.