Temperature Measurement Using Thermocouples:
The Theory BehindMotto:"You don't know what a thermocouple is ?In that case, it'll cost you a little more !"
TC Technical Refences
Temperature can be measured via a diverse array of sensors. All of them infer temperature by sensing some change in a physical characteristic. Six types with which the engineer is likely to come into contact are: thermocouples, resistive temperature devices (RTDs and thermistors), infrared radiators, bimetallic devices, liquid expansion devices, and change-of-state devices. This paper intend to present the theory behind using thermocouples in temperature measurements.
A thermocouple is a device primarily used for the measurement of temperature. It is based upon the findings of Seebeck (1821) who showed that a small electric current will flow in a circuit composed of two dissimilar conductors when their junctions are kept at different temperatures. The electromotive force (emf) produced under these conditions is known as the "Seebeck emf". The pair of conductors that constitute the thermoelectric circuit is called thermocouple.
The Thermocouple Effect
Conductivity and Band Theory
Figure 1 shows a thermocouple junction formed by joining two metallic alloys, A and B. The voltage across the thermocouple junction depends on the type of metals used and the temperature of the junction. The mechanism responsible for this voltage is quite complicated, however, there are certain phenomenological results which make the effect useful for measuring temperature. One of these results is that the voltage is approximately linear with temperature.
Fig. 1 Thermocouple Junction
In order to understant the thermocouple (Seebeck) effect, first one has to study the microstructure of a metal and of the component atoms from the crystal lattice.
The atomic structure, postulated by Bohr and modified by Schrodinger and Heisenberg, states that the electrons must revolve around the nucleus, with the electrostatic attraction toward the nucleus being exactly balanced by the centrifugal force of the motion of the electrons in their orbits. The solution of Schrodinger wave equation defined discrete energy levels which an electron may occupy without loss of energy. Figure 2(a) depicts energy level scheme for an atom at room temperature. The energy scale shows negative values for the electron energy; they increase numerically as the electron nears the nucleus. The dotted lines represent discrete stable configurations where the electrostatic attractions (forces) of the nucleus for an electron are balanced by centrifugal forces. In Figure 2(b) are schematically indicated the first five energy levels for a sodium atom with 11 electrons in the orbital structure. The electrons in the first three levels, being closer to the nucleus, have lower (more negative) potential energies as a result of greater electrostatic attraction to the nucleus. The single electron in the fourth level, being much farther from the nucleus and therefore least tightly held, has the higest energy and is most easily removed. This single electron in the upper level is known as the valence electron.
Fig. 2 Electron levels for single atoms
The allowed energy levels for a single atom broaden into bands when many atoms forms a periodic array. In Figure 3 is shown what happens when a large number of sodium atoms are combined to form a crystal of sodium metal. The inner, completted levels are very tightly bound to their nuclei and are virtually unaffected by the presence of the other atoms in the lattice. The outtermost shells, whether they overlap or whether they are completely filled or not, play a significant role in electron transport phenomena. Consequently, a valence electron may easily leave its parent atom and wander freely through the crystal.
In the preceding paragraphs was presented the existence of levels of discrete energy at varying distances from the nucleus. The spaces between these levels which never contain electrons (conduction bands) are known as forbidden gaps.It is possible for an electron of lower energy to move up into an unoccupied state at a higher level, but this process requires the absorption of energy by the electron in a quantity equivalent to the difference in energy between the two levels. This energy absorption may take the form of thermal excitation. Consider the sodium metal whose atoms have completed inner bands and the valence (outter) bands are incompletely filled. The application of normal thermal energy can excite the electrons in the valence band to jump to the next outter, completely vacant (conduction) band and enter in the physical process of conduction.
Much depend upon the extent of the energy gap between valence and conduction bands. If the gap is very large, the element will be an insullator. This results from the fact that low energies cannot cause the valence electrons to jump accros the energy gap and occupy states in the bottom of the unfilled conduction bands and to engage in the conduction process. If the gap is very small or the bands are overlaped, the element is a conductor. The more electrons that enter into this proces, the better the conduction becomes. The resulting "holes" (absence of electrons) in the valence band also can enter in a "conduction" process.
If a conductor is heated at one end, the mechanism giving rise to a thermoelectric current is that the electrons at the "hot" end (hot junction) will acquire increased thermal energy relative to those at the "cold" end (cold or reference junction). The electrons from the hot end will diffuse to the cold end where their energy may be lowered. This is essentially the manner in which heat is conducted in a metal and is accompanied by the accumulation of negative charge at the cold end, thus setting up an electric field or a potential difference between the ends of the material. This electric field will biuld up until a state of dynamic equilibrium is established between the electrons urged down the temperature gradient and electrostatic repulsion due to the excess of charge at the cold end. The number of electrons passing through a cross section normal to the direction of flow per second in both directions will be equal, but the velocities of electrons proceeding from the hot end will be higher than those velocities of electrons passing through the section from the cold end. This difference ensures that there is a continuous transfer of heat (i.e. thermal conduction) down the temperature gradient without actual charge transfer once dynamic equilibrium is set up. This phenomenon may be considered to be a basic thermoelectric effect.
If the potential difference so created is to be measured, electrical connections must be made to either end of the metal, thus setting up a similar temperature gradient in the complete detecting system, and this will contribute its own thermoelectric emf to the circuit. If the entire detecting circuit is made of the same material as that under test, a symmetrical circuit will result and no net emf will be detected. Therefore, in order to measure thermoelectric emf unsymmetrical circuits of at least two different materials must be constructed, as stated earlier about thermocouples.
A problem arises when measuring the voltage across a dissimilar metal junction - two additional thermocouple junctions form where the wires connect to the voltmeter (Figure 4). If the wire leads which connect to the voltmeter are made of alloy "C", then there exist thermal emf's at the A-C and B-C junctions. There are two approaches to solve this problem: use a reference junction at a known temperature, or make corrections for the thermocouples formed by the connection to the voltmeter.
Figure 5 shows the use of a "reference" or "compensating" junction. With this arrangement, there are still two additional thermocouple junctions formed where the compensated thermocouple is connected to the voltmeter. However, the junctions are identical (they are both junctions between alloys A and C). If the junctions are at the same temperature, then the voltages across each junction will be equal and opposite, and will not affect the measurement. Typically, the reference junction is held at 0 °C (by an ice bath, for example) so that the voltmeter readings may be used to look up the temperature.
Fig. 4 Additional Junctions
Fig. 5 Reference Junction Compensation
Compensation Without Reference Junctions
The second approach to the problem relies on the fact that the voltage across the junction A-C plus the voltage across the junction C-B is the same as the voltage across a junction of A-B. As long as all the junctions are at the same temperature, the presence of an intermediate metal (C) has no effect. This allows us to correct for the voltage seen by the voltmeter in Figure 4 by measuring the temperature at the A-C and B-C junctions and subtracting the voltage which we would expect for an A-B junction (at the measured temperature). In the SR630 the temperature of the A-C and C-B junctions are measured with a low cost, high resolution semiconductor detector, and the subtracted voltage is the tabulated voltage of the A-B thermocouple at the measured temperature of the A-C and C-B junctions. The advantage of this method is that any type thermocouple may be used without having to change compensation junctions or maintain ice baths.
Characteristics of Thermocouple Types
Any two dissimilar metals may be used to make a thermocouple. Of the infinite number of thermocouple combinations which can be made, the world has standardized seven types which exhibit a range of desirable features. These thermocouple types are known by a single letter designation: J, K, T, E, R, S or B. While the composition of these thermocouples are international standards, the color codes of the wires are not. For example, in the USA, the negative lead is always red, while the rest of the world uses red to designate the positive lead. Often, the standard thermocouple types are referred to by their trade names. For example, K type is sometimes called Chromel-Alumel, which is the trade names of the Ni-Cr and Ni-Al wire alloys.
It is important for a good thermocouple to have a large, stable Seebeck coefficient, wide temperature range, corrosion resistance, etc. Generally, each wire of the thermocouple is an alloy. Variations in the alloy composition and the condition of the junction between the wires are sources of error in temperature measurements. The standard error of thermocouple wire varies from ±0.8 °C to ±4.4 °C, depending on the type of thermocouple used.
Voltage vs. temperature measurements have been tabulated by NIST for each of the seven standard thermocouple types. These tables are stored in the read-only memory of the SR630. The instrument operates by converting a voltage measurement to a temperature, with the internal microprocessor interpolating to achieve 0.1 °C resolution.
The K type thermocouple is recommended for most general purpose applications. It offers a wide temperature range, low standard error, and has good corrosion resistance. The K type thermocouples provided by SRS have a standard error of ±1.1 °C, half the standard error designated for this type.
Thermocouple Reference Data
Type B E J K R S T
Positive Material Pt/Rh(30%) Ni/Cr Fe Ni/Cr Pt/Rh(13%) Pt/Rh(10%) Cu Negative Material Pt/Rh(6%) Ni/Cr Cu/Ni Ni/Al Pt Pt Cu/Ni Positive Color(USA) Grey Purple White Yellow Black Black Blue Negative Color(USA) Red Red Red Red Red Red Red Lowest Temperature 50C -200C 0C -200C 0C 0C -200C Highest Temperature 1700C 900C 750C 1250C 1450C 1450C 350C Minimum Std Error ±4.4C ±1.7C ±2.2C ±2.2C ±1.4C ±1.4C ±0.8C
What is a thermocouple?
A thermocouple is a sensor for measuring temperature. It consists of two dissimilar metals, joined together at one end, which produce a small unique voltage at a given temperature. This voltage is measured and interpreted by a thermocouple thermometer.
What are the different thermocouple types?
Thermocouples are available in different combinations of metals or calibrations.' The four most common calibrations are J, K, T and E. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple.
How do I choose a thermocouple type?
Because thermocouples measure in wide temperature ranges and can be relatively rugged, they are very often used in industry. The following criteria are used in selecting a thermocouple:
How do I know which junction type to choose?
- Temperature range
- Chemical resistance of the thermocouple or sheath material
- Abrasion and vibration resistance
- Installation requirements (may need to be compatible with existing equipment; existing holes may determine probe diameter).
Sheathed thermocouple probes are available with one of three junction types: grounded, ungrounded or exposed. At the tip of a grounded junction probe, the thermocouple wires are physically attached to the inside of the probe wall. This results in good heat transfer from the outside, through the probe wall to the thermocouple junction. In an underground probe, the thermocouple junction is detached from the probe wall. Response time is slowed down from the grounded style, but the ungrounded offers electrical isolation of 1.5 M1/2 at 500 Vdc in all diameters. The thermocouple in the exposed junction style protrudes out of the tip of the sheath and is exposed to the surrounding environment. This type offers the best response time, but is limited in use to noncorrosive and nonpressurized applications. See the illustrations at the right for a full discussion of junction types.
Grounded Ungrounded Exposed
What is response time?
A time constant has been defined as the time required by a sensor to reach 63.2% of a step change in temperature under a specified set of conditions. Five time constants are required for the sensor to stabilize at 600 of the step change value. Exposed junction thermocouples are the fastest responding. Also, the smaller the probe sheath diameter, the faster the response, but the maximum temperature may be lower. Be aware, however, that sometimes the probe sheath cannot withstand the full temperature range of the thermocouple type.
Standard diameters: 0.010", 0.020", 0.032", 0.040", 1/16", 1/8", 3/16", and 1/4" with two wires.
Standard OMEGA thermocouples have 12 inch immersion lengths. Other lengths available.
304 stainless steel and Inconel are standard. Other sheath materials available; call for price and availability.
Magnesium Oxide is standard. Minimum insulation resistance wire to wire or wire to sheath is 1.5 megohms at 500 volts dc in all diameters.
Iron-Constantan (J), Chromega®-Alomega® (K),
Copper-Constantan (T), and Chromega-Constantan (E) are standard calibrations.
Easily bent and formed. Bend radius should be not less than twice the diameter of the sheath.
Off-the-Shelf, other sheaths available; call for price and delivery. Dual Elements: Thermocouples with a sheath diameter of 0.040" (1.0 mm) thru 1/4" (6.3mm) are available in dual element.
The wires used in OMEGA thermocouples are selected and matched to meet ANSI Limits of Error. Special limits of error thermocouples can be made from all 1/16" (1.5 mm) O.D. or larger OMEGACLAD®. Thermocouple wire.
Extension Wire:Thermocouples: Consider first the thermocouple, probably the most-often-used and least-understood of the three. Essentially, a thermocouple consists of two alloys joined together at one end and open at the other. The emf at the output end (the open end; V1 in Figure 1a) is a function of the temperature T1 at the closed end. As the temperature rises, the emf goes up.
Thermocouple alloy wire must always be used to connect a thermocouple sensor to the instrumentation to assure accurate measurements.
The grounded junction is recommended for the measurement of static or flowing corrosive gas and liquid temperatures and for high pressure applications. The junction of a grounded thermocouple is welded to the protective sheath giving faster response than the ungrounded junction type.
An ungrounded junction is recommended for measurements in corrosive environments where it is desirable to have the thermocouple electronically isolated from and shielded by the sheath. The welded wire thermocouple is physically insulated from the thermocouple sheath by MgO powder (soft).
An exposed junction is recommended for the measurement of static or flowing non-corrosive gas temperatures where fast response time is required. The junction extends beyond the protective metallic sheath to give accurate fast response. The sheath insulation is sealed where the junction extends to prevent penetration of moisture or gas which could cause errors.
Often the thermocouple is located inside a metal or ceramic shield that protects it from a variety of environments. Metal-sheathed thermocouples are also available with many types of outer coatings, such as polytetrafluoroethylene, for trouble-free use in corrosive solutions.
The open-end emf is a function of not only the closed-end temperature (i.e., the temperature at the point of measurement) but also the temperature at the open end (T2 in Figure 1a). Only by holding T2 at a standard temperature can the measured emf be considered a direct function of the change in T1. The industrially accepted standard for T2 is 0°C; therefore, most tables and charts make the assumption that T2 is at that level. In industrial instrumentation, the difference between the actual temperature at T2 and 0°C is usually corrected for electronically, within the instrumentation. This emf adjustment is referred to as the cold-junction, or CJ, correction.
Temperature changes in the wiring between the input and output ends do not affect the output voltage, provided that the wiring is of thermocouple alloy or a thermoelectric equivalent (Figure 1a). For example, if a thermocouple is measuring temperature in a furnace and the instrument that shows the reading is some distance away, the wiring between the two could pass near another furnace and not be affected by its temperature, unless it becomes hot enough to melt the wire or permanently change its electrothermal behavior.
The composition of the junction itself does not affect the thermocouple action in any way, so long as the temperature, T1, is kept constant throughout the junction and the junction material is electrically conductive (Figure 1b). Similarly, the reading is not affected by insertion of non-thermocouple alloys in either or both leads, provided that the temperature at the ends of the "spurious" material is the same (Figure 1c).
This ability of the thermocouple to work with a spurious metal in the transmission path enables the use of a number of specialized devices, such as thermocouple switches. Whereas the transmission wiring itself is normally the thermoelectrical equivalent of the thermocouple alloy, properly operating thermocouple switches must be made of gold-plated or silver-plated copper alloy elements with appropriate steel springs to ensure good contact. So long as the temperature at the input and output junctions of the switch are equal, this change in composition makes no difference.
It is important to be aware of what might be called the Law of Successive Thermocouples. Of the two elements that are shown in the upper portion of Figure 1d, one thermocouple has T1 at the hot end and T2 at the open end. The second thermocouple has its hot end at T2 and its open end at T3. The emf level for the thermocouple that is measuring T1 is V1; that for the other thermocouple is V2. The sum of the two emfs, V1 plus V2, equals the emf V3 that would be generated by the combined thermocouple operating between T1 and T3. By virtue of this law, a thermocouple designated for one open-end reference temperature can be used with a different open-end temperature.
This is a device that consists of two unlike metal pairs at different temperatures. This produces electricity due to the Seebeck effect. Just as in a battery, when two unlike metals are in contact, their outer electrons are at two different energy levels. This energy difference depends on the temperature at which these two metals reside. The thermocouple uses a second pair of the exact two metals at a different temperature to create a potential difference between the two.
If electrons come to position (1) they transform some of their kinetic energy into potential energy to get a position in a higher energy band. (All the lower positions are already occupied.) At position (2), they get their old kinetic energy.
So I suppose, that position (1) will be colder and (2) will be warmer. Is this correct? If so, this would be a novel way to produce heat and coldness similar to the seebeck-effect, but with only one material. This effect should be measurable in thin semiconductor layers, e.g. like MOS systems.
This heat is converted to electricity by a thermoelectric converter which uses the Seebeck effect, a basic principle of thermoelectricity discovered in 1822. An electromotive force, or voltage, is produced from the diffusion of electrons across the joining of two different materials (like metals or semiconductors) that then form a circuit when the ends of the converter are at different temperatures.
The most basic temperature sensor is the thermocouple. A thermocouple is simply a junction of two dissimilar metals. This junction creates a very low electro-magnetic field (EMF) which increases in a basically linearly manner with respect to absolute temperature. This EMF field induces a very low current in the circuit. This effect was discovered in 1821 by Thomas Seebeck, and the voltage induced is known as the Seebeck voltage. This voltage is then read by a specialized thermocouple input module and converted to a standard temperature based on published scales and algorithms from the NIST (Jacob 1988) . Because any junction of two dissimilar metals is by definition a thermocouple, and induces a Seebeck voltage, the connections between the thermocouple and the thermocouple input module introduces two additional thermocouples, and their associated Seebeck voltages, into the circuit. The voltages induced by these cold junctions will oppose that of the main thermocouple, thus lowering the voltage read by the input module. This error can be corrected if the temperature at the connections to the input module is known. This process is known as cold junction compensation. Industrial input modules place all of the cold junctions onto a large heat sink which is heated to a specific temperature. This allows for the module to correct for the Seebeck voltage for the cold junction at that known temperature (Jacob 1988) . The sensitivity and temperature ranges of thermocouples is dependent upon the two metals used in their construction. Several types of thermocouples have been defined by the NIST. Type K thermocouples are the most common used in industrial applications due to their large temperature range. Food processing typically uses Type T thermocouples for their sensitivity in the food operating temperature range and their resistance to moisture. Please refer to Table 1 for a complete list of thermocouple types and their ranges and sensitivities (The Temperature Handbook, 1991) . Thermocouples are the most robust temperature sensor available and can be readily packaged in a small profile sensor, which lessens the chance of sinking heat into the probe assembly, causing low readings. Thermocouples also have a short reaction time due to their low mass and are the least costly temperature sensor available (Trietley, 1992, March) . CRC Handbook of Thermoelectrics
Editor - D.M. Rowe
University of Wales College of Cardiff, U.K.
Description | Features | Contents | Audience | Publication Information and Price
Thermoelectrics is the science and technology associated with thermoelectric converters, that is, the generation of electrical power by the Seebeck effect and refrigeration by the Peltier effect. Thermoelectric generators are being used in increasing numbers to provide electrical power in medical, military, and deep space applications where combinations of their desirable properties outweigh their relatively high cost and low generating efficiency. In recent years there also has been an increase in the requirement for thermoelectric coolers (Peltier devices) for use in infrared detectors and in optical communications. Information on thermoelectrics is not readily available as it is widely scattered throughout the literature. The Handbook centralizes this information in a convenient format under a single cover.
Sixty of the world's foremost authorities on thermoelectrics have contributed to this Handbook. It is comprised of fifty-five chapters, a number of which contain previously unpublished material. The contents are arranged in eight sections: general principles and theoretical considerations, material preparation, measurement of thermoelectric properties, thermoelectric materials, thermoelectric generation, generator applications, thermoelectric refrigeration, and applications of thermoelectric cooling.
The CRC Handbook of Thermoelectrics has a broad-based scope. It will interest researchers, technologists, and manufacturers, as well as students and the well-informed, non-specialist reader.
Catalog number 146WGBA July 1995, 720 pp., ISBN: 0-8493-0146-7 U.S. $160.00 / Outside U.S. $192.00
A thermocouple is formed any time two dissimilar metals touch each other. When the temperature of this junction is different to the temperature of other parts of the metals an EMF is generated. Fortunately for a lot of people working with metals this EMF is very small (usually measured in çV/¯C) so there is not much chance of receiving an electric shock but it is sufficient to cause corrosion problems (which we won't tackle here).
This property is used in industry to measure temperatures, especially if these temperatures cannot be measured by other techniques.
The two metals are usually formed into wires and welded together, however crimping, soldering or even just twisting the wires together gives results. There is no theoretical limit to the wire diameter and thermocouples can be made in just about any size required.
Any two metals can be used in theory but in practice we need something which is robust, stable, has a large enough signal to use, and is affordable. The most popular thermocouples are listed later.
Over the years the EMF output of each standard thermocouples has been plotted and tables are available for calibration purposes. To calibrate a thermocouple instrument it is only necessary to inject a millivolt signal to match the tables.
WHAT'S THE DIFFERENCE?
A thermocouple is a differential device and the output is related to the DIFFERENCE in temperature between the hot junction and the cold junction.
Thermocouple tables are based on a cold junction at 0¯C. If the cold junction is not at 0¯C (which is usually the case) a correction has to be applied.
To determine the output of a thermocouple using tables, use the formula:-
Vo = Vh - Vc
Where Vo is the output voltage we are looking for, Vh is the voltage shown in the tables for the hot junction, and Vc the voltage shown for the cold junction.
For example: Assume a type J thermocouple has its hot junction in boiling water at 100¯C and the cold junction is at an instrument which is at a room temperature of 25¯C.
The output at 100¯C according to tables is 5.268 mV. The output at 25¯C from the same tables is 1.019mV.
The output of the thermocouple measured at the instrument is therefore :-
5.268mV - 1.019mV = 4.249mV.
The accuracy of a thermocouple (actual output vs. value in tables) depends on the composition of the alloys making up the wires. This is typically of the order of Ý3¯C at 100¯C for a standard type J thermocouple and Ý1.5¯C for specially selected types.
This is not all that great if we require extremely high accuracy. So why do we use them?
Easy. They are easy and cheap to make and can measure temperatures from around -200¯C to +1800¯C depending on the wires used.
The simple construction with no moving parts makes them suitable for even the heaviest of industrial applications and they can remain stable over long periods. Most industrial processes don't require better accuracy than given by a thermocouple and if they do, it is a fairly straightforward task to plot an error table for each individual device.
about FireRight THERMOCOUPLE TEMPERATURE SENSORS ISA Type K (Chromel vs Alumel)
Thermocouples are made simply by connecting two dissimilar types of wire. The junction thus formed is temperature- sensitive, and produces a very small voltage which varies with its temperature. Since this physical phenomenon depends mostly on (1) the type of wire used and (2) the temperature of the junction, this very simple device is a very accurate and reliable temperature sensor.
When used at temperatures exceeding 1800¯:F, type K thermocouple elements are subject to a corrosion process known as "green rot", which eventually consumes the wire. The higher the temperature and the longer the exposure, the shorter the life of the element. To inspect the condition of the element, use any sharp metal tool to scrape away the greenish, carbonaceous scale from the wire, until bright metal is exposed. If this corrosion process has reduced the wire to less than one-third of its original diameter, its accuracy and reliability are doubtful, and it should be replaced.
It is impossible to connect the thermocouple element to any temperature measuring device without forming a second junction, normally referred to as the cold junction. The free ends of the thermocouple element must inevitably be connected to brass screws on the back of a pyrometer or to a copper circuit board, forming temperature-sensitive second and third junctions. These become the algebraic equivalent of a single cold junction having the same temperature-sensitive characteristics as the hot junction. Because of this, it is important that this cold junction be formed at a known location, and properly compensated for. THE USE OF THERMOCOUPLE EXTENSION WIRE BETWEEN THE SENSOR AND THE MEASURING INSTRUMENT IS THEREFORE MANDATORY. Thermocouple extension wire is made of the same type of wire as the sensor, in this case ISA type K, also called "Chromel vs Alumel".
The voltage produced by the sensor has a definite polarity, and POLARITY MUST THEREFORE BE CONSIDERED WHEN CONNECTING THE SENSOR AND EXTENSION WIRE TO THE MEASURING INSTRUMENT. To facilitate this, the positive wire is usually marked with a (+) symbol, or the pair may be or color-coded:
YELLOW: (+) (nonmagnetic wire) RED: (-) (magnetic wire)
If the colors are missing or worn off, the magnetic property of the (-) wire is the key to determining the polarity, and can be checked using any small magnet (even the "bug" from the refrigerator door). FireRight replacement elements have one bright-colored wire (+), and one dark-colored wire (-).
TO REPLACE THE OLD THERMOCOUPLE ELEMENT, loosen the two screws which hold it, then pull it out of the sensor assembly. If you wish to re-use the existing mounting hardware, bend the ends of the new element to fit, then slip the base plate and insulator onto the element, insulator first. To complete the assembly, secure the two terminal ferrules to the ends of the element, being careful to observe polarity. Make sure that all screws are tightened securely.
Standard thermocouple replacement elements are 7-1/2" long, and are formed to fit the FireRight ceramic mounting block (p/n 179012). Bending the element to fit other mounting blocks will not affect the accuracy of the sensor in any way. The wire is pliable, and bends rather easily. The ceramic insulators are quite brittle, however, and will break under very little stress. Cracked or broken insulators will not affect the accuracy of the element as long as the wires do not touch at any point along its length.
Thermoelectric voltages are the most common source of error in low-level dc voltage measurements. Thermoelectric voltages are generated when you make circuit connections using dissimilar metals at different temperatures. Each metal-to-metal junction forms a thermocouple, which generates a voltage proportional to the junction temperature. You should take the necessary precautions to minimize thermocouple voltages and temperature variations in low-level voltage measurements. The best connections are formed using copper-to-copper crimped connections. The table below shows common thermoelectric voltages for connections between dissimilar metals.
Copper-to- Approx. microV/Co Copper Gold Sliver Brass Beryliium Copper Aluminum Kovar or Alloy 42 Silicon Copper-Oxide Cadmium-Tin Solder Tin-Lead Solder <0.3 0.5 0.5 3 5 5 40 500 1000 0.2 5