Non-contact temperature sensors (Thermal Imaging)

Thermal imaging

A particularly useful application of non-contact sensor technology is thermal imaging, where a dense array of infrared radiation sensors provides a graphic display of objects in its view according to their temperatures. Each object shown on the digital display of a thermal imager is artificially colored in the display on a chromatic scale that varies with temperature, hot objects typically registering as red tones and cold objects typically registering as blue tones. Thermal imaging is very useful in the electric power distribution industry. Thermal imaging is also useful in performing “energy audits” of buildings and other heated structures, providing a means of revealing points of heat escape through walls, windows, and roofs.

In such applications, relative differences in temperature are often more important to detect than specific temperature values. “Hot spots” readily appear on a thermal imager display and may give useful data on the test subject even in the absence of accurate temperature measurement at any one spot.

A digital image taken with a thermal imaging instrument by maintenance personnel at a municipal water treatment facility shows “hot spots” on an electric motor. A color scale on the right-hand side of the image serves as a “legend” for interpreting color as temperature. In this below image, dark blue is 68.1 degree F and white is 152 degree F:

This electric motor is in a vertical orientation, with the electrical connection box in the upper-left corner and two prominent hot spots on both the near and the left-facing sides of the case. A manual valve handle appears in the foreground, silhouetted in dark blue against a lighter blue (warmer) background. A lifting “eye” on the motor case appears as a green (cooler) shape in the middle of a white (warmer) area. The two “hot spots” correspond directly to stator windings and iron pole faces inside the motor.

Thermal imaging is particularly useful for detecting hot spots on equipment unsafe to directly touch, as is the case with many “live” electrical components. This next thermal image shows an operating three-phase motor starter (contactor and overload block):

The bright spot in the center of the contactor is the higher temperature of the electromagnetic coil, providing magnetic force to actuate the contactor mechanism. The middle heater’s screws register slightly higher temperatures than the screws on either of the other two heater elements.

Large temperature differences may indicate poor electrical connections (i.e. greater resistance) at the hot spots. It is possible that the elevated temperature of this overload heater is simply due to it having a less open surface area for it to radiate heat since the two overload heaters flanking it enjoys the advantage of having more air cooling. If three people pack themselves into a narrow bench seat, the middle person is going to be warmer than either of the outer two!

Another noteworthy detail in this image is the “Spot Difference” measurement provided by the thermal imager. A cross-hair cursor on the display serves to locate a particular spot in the image, which in this case is contrasted against a reference spot chosen in an earlier step.

A thermal image of a three-phase circuit breaker shows a much more even distribution of temperature:\

The hottest objects in this image are the three load screw terminals, appearing as white/red against a blue/green background. Note the range of the temperature scale on the right of the image: This image only spans a temperature range of 61.3 degree F to 68.6 degree F. This narrow temperature range tells us the differences in temperature shown by colors in this image are nothing to worry about.

Emissivity

The main disadvantage of non-contact temperature sensors is their inaccuracy. The emissivity factor varies with the composition of a substance, but beyond that, there are several other factors (surface finish, shape, etc.) that affect the amount of radiation a sensor will receive from an object. For this reason, emissivity is not a very practical way to gauge the effectiveness of a non-contact pyrometer. Instead, a more comprehensive measure of an object’s “thermal-optical measurability” is emittance.

A perfect emitter of thermal radiation is known as a blackbody. Emittance for a blackbody is unity (1), while emittance figures for any real object is a value between 1 and 0. The only certain way to know the emittance of an object is to test that object’s thermal radiation at a known temperature.

This assumes we have the ability to measure that object’s temperature by direct contact, which of course renders void one of the major purposes of non-contact thermometry: to be able to measure an object’s temperature without having to touch it. Not all hope is lost, though: all we have to do is obtain an emittance value for that object one time, and then we may calibrate any non-contact pyrometer for that object’s particular emittance so as to measure its temperature in the future without contact.

Nevertheless, non-contact pyrometers have been and will continue to be useful in specific applications where other, contact-based temperature measurement techniques are impractical.

List of Prominent Manufacturers: Advanced Energy, Ametek, Capital Instrument, Chauvin Arnoux, Chino, Dahua, Dewalt, Dias, Durag Group, Dwyer, Ecom, Flir, Fluke, Guide Sensmart, Jenoptik, Keysight, Laserliner, Metrel, MSA, Optris, Orlaco, PCE, Pepperl+Fuchs, Rigid, Savox, Scott Bafety, Tempsens, Testo, Testboy, Thermoray, Trotec, Workswell, Xenics

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Non-contact temperature sensors (Pyrometers)

Virtually any mass above absolute zero temperature emits electromagnetic radiation (photons or light) as a function of that temperature. This basic fact makes possible the measurement of temperature by analyzing the light emitted by an object.

The primary advantage of non-contact thermometry (pyrometry as the high-temperature measurement is often referred to) is rather obvious: with no need to place a sensor in direct contact with the process, a wide variety of temperature measurements may be made that are either impractical or impossible to make using any other technology.

Concentrating pyrometers

A time-honored design for non-contact pyrometers is to concentrate incident light from a heated object onto a small temperature-sensing element. A rise in temperature at the sensor reveals the intensity of the infrared optical energy falling upon it, which as discussed previously is a function of the target object’s temperature (absolute temperature to the fourth power):

A doubling of absolute temperature at the hot object results in sixteen times as much radiant energy falling on the sensor, and therefore a sixteen-fold increase in the sensor’s temperature rise over ambient. A tripling of target temperature (absolute) yields eighty-one times as much radiant energy, and therefore an 81- fold increase in sensor temperature rise. This extreme nonlinearity limits the practical application of non-contact pyrometry to relatively narrow ranges of target temperature wherever good accuracy is required.

Thermocouples were the first type of sensor used in non-contact pyrometers, and they still find application in modern versions of the same technology. Since the sensor does not become nearly as hot as the target object, the output of any single thermocouple junction at the sensor area will be quite small. For this reason, instrument manufacturers often employ a series-connected array of thermocouples called a thermopile to generate a stronger electrical signal.

The basic concept of a thermopile is to connect multiple thermocouple junctions in series so their voltages will add:

Examining the polarity marks of each junction (type E thermocouple wires are assumed in this example: chromel and constantan), we see that all the “hot” junctions’ voltages aid each other, as do all the “cold” junctions’ voltages. Like all thermocouple circuits, though, each “cold” junction voltage opposes the “hot” junction voltage. The example thermopile shown in this diagram, with four hot junctions and four cold junctions will generate four times the potential difference that a single type E thermocouple hot/cold junction pair would generate, assuming all the hot junctions are at the same temperature and all the cold junctions are at the same temperature.

When used as the detector for a non-contact pyrometer, the thermopile is oriented, so all the concentrated light falls on the hot junctions (the “focal point” where the light focuses on a small spot), while the cold junctions face away from the focal point to a region of ambient temperature. Thus, the thermopile acts like a multiplied thermocouple, generating more voltage than a single thermocouple junction could under the same temperature conditions.

Distance considerations

A counter-intuitive characteristic of non-contact pyrometers is that their calibration does not depend on the distance separating the sensor from the target object. This is counter-intuitive to anyone who has ever stood to an intense radiative heat source: standing near a bonfire, for example, results in much hotter skin temperature than standing far away from it.

Placing a sensor at three integer distances (x, 2x, and 3x) from a radiation point-source results in relative power levels of 100%, 25% (one-quarter), and 11.1% (one-ninth) falling upon the sensor at those locations, respectively:

This is a basic physical principle for all kinds of radiation, grounded in simple geometry. If we examine the radiation flux emanating from a point source, we find that it must spread out as it travels in straight lines and that the spreading-out happens at a rate defined by the square of the distance. An analogy for this phenomenon is to imagine a spherical latex balloon expanding as air is blown into it. The surface area of the balloon is proportional to the square of its radius. Likewise, the radiation flux emanating from a point-source spreads out in straight lines, in all directions, reaching a total area proportional to the square of the distance from the point (center). The total flux measured as a sphere will be the same no matter what the distance from the point-source, but the area it is divided over increases with the square of the distance, and so any object of fixed area backing away from a point-source of radiation encounters a smaller and smaller fraction of that flux.

If non-contact pyrometers really were “looking” at a point-source of infrared radiation, their signals would indeed decrease with distance. The saving grace here is that non-contact pyrometers are focused-optic devices, with a definite field of view and that field of view should always be filled by the target object (assumed to be at a uniform temperature). As the distance between the pyrometer and the target object changes, the cone-shaped field of view covers a surface area on that object proportional to the square of the distance21 Backing the pyrometer away to twice the distance increases the viewing area on the target object by a factor of four; backing away to three times the distance increases the viewing area nine times:

So, even though the inverse square law correctly declares that radiation emanating from the hot wall (which may be thought of as a collection of point sources) decreases in intensity with the square of the distance, this attenuation is perfectly balanced by an increased viewing area of the pyrometer. Doubling the separation distance does result in the flux from any given point on the wall spreading out by a factor of four, but the pyrometer now sees four times as many similar points on the wall as it did previously. So long as all the points within the field of view are uniform in temperature, the result is a perfect cancellation with the pyrometer providing the exact same temperature measurement at any distance from the target.

If the sensor’s field of view expands far enough to capture objects other than the one whose temperature we intend to measure, measurement errors will result. The sensor will now yield a weighted average of all objects within its field of view, and so it is important to ensure that field is limited to cover just the object we intend to measure.

Non-contact sensor fields of view are typically specified either as an angle, as a distance ratio, or both. For example, the following illustration shows a non-contact temperature sensor with a 5:1field of view:

List of Prominent Manufacturers: Advanced Energy, Ametek, Baltech, Bartec, Calex, Chino, Fluke, Jiebo, Keller, Mahlo, Optris, Optron, Tempsens

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Software Compensation and Burnout Detection of Thermocouples

What is software compensation in thermocouple?

Automatic compensation could be accomplished by intentionally inserting the temperature-dependent voltage source in series with the circuit, oriented in such a way as to oppose the reference junction’s voltage:

Vmeter = VJ1 − VJ2 + Vrjc

If the series voltage source Vrjc is exactly equal in magnitude to the reference junction’s voltage (VJ2), those two terms cancel out of the equation and lead to the voltmeter measuring only the voltage of the measurement junction J1:

Vmeter = VJ1 + 0

Vmeter = VJ1

This technique is known as hardware compensation and is employed in analog thermocouple temperature transmitter designs. A stand-alone circuit called an ice point, the purpose of which was to electrically counter the reference junction voltage as if that junction were immersed in a bath of ice water.

A modern technique for reference junction compensation more suitable to digital transmitter designs are called software compensation:

Instead of canceling the effect of the reference junction electrically, we cancel the effect arithmetically inside the microprocessor-based transmitter. In other words, we let the receiving analog-digital converter circuit see the difference in voltage between the measurement and reference junctions (V input = VJ1 − VJ2), but then after digitizing this voltage measurement we have the microprocessor add the equivalent voltage value corresponding to the ambient temperature sensed by the RTD or thermistor (Vrjc):

Compensated total = Vinput + Vrjc

Compensated total = (VJ1 − VJ2) + Vrjc

Since we know the calculated value of Vrjc should be equal to the real reference junction voltage (VJ2), the result of this digital addition should be a compensated total equal only to the measurement junction voltage VJ1:

Compensated total = VJ1 − VJ2 + Vrjc

Compensated total = VJ1 + 0

Compensated total = VJ1

The greatest advantage of software compensation is the flexibility to easily switch between different thermocouple types with no hardware modification. So long as the microprocessor memory is programmed with look-up tables relating voltage values to temperature values, it may accurately measure any thermocouple type. Hardware-based compensation schemes (e.g., an analog “ice point” circuit) require re-wiring or replacement to accommodate different thermocouple types since each ice-point circuit is built to generate a compensating voltage for a specific type of thermocouple.

Extension wire

In every thermocouple circuit, there must be both a measurement junction and a reference junction: this is an inevitable consequence of forming a complete circuit (loop) using dissimilar-metal wires. As we already know, the voltage received by the measuring instrument from a thermocouple will be the difference between the voltages produced by the measurement and reference junctions. Since the purpose of most temperature instruments is to accurately measure the temperature at a specific location, the effects of the reference junction’s voltage must be “compensated” for by some means, either a special circuit designed to add an additional canceling voltage or by a software algorithm to digitally cancel the reference junction’s effect.

For reference junction compensation to be effective, the compensation mechanism must “know” the temperature of the reference junction. This fact is so obvious, it hardly requires mentioning. However, what is not so obvious is how easily this compensation may be unintentionally defeated simply by installing a different type of wire in a thermocouple circuit.

Like all modern thermocouple instruments, the panel-mounted indicator contains its own reference junction compensation, so that it is able to compensate for the temperature of the reference junction formed at its connection terminals, where the internal (copper) wires of the indicator join to the chromel and alumel wires of the thermocouple. The indicator senses this junction temperature using a small thermistor thermally bonded to the connection terminals.

A more economical alternative, however, is to use something called extension-grade wire to make the connection between the thermocouple and the receiving instrument. “Extension-grade” thermocouple wire is made less expensive than full “thermocouple-grade” wire by choosing metal alloys similar in thermo-electrical characteristics to the real thermocouple wires within the modest temperature ranges. So long as the temperatures at the thermocouple head and receiving instrument terminals don’t get too hot or too cold, the extension wire metals joining to the thermocouple wires and joining to the instrument’s copper wires need not be precisely identical to the true thermocouple wire alloys. This allows for a wider selection of metal types, some of which are substantially less expensive than the measurement-grade thermocouple alloys. Also, the extension-grade wire may use insulation with a narrower temperature rating than thermocouple-grade wire, reducing the cost even further.

Extension-grade cable is denoted by a letter “X” following the thermocouple letter. For our hypothetical type K thermocouple system, this would mean type “KX” extension cable. Thermocouple extension cable also differs from thermocouple-grade (measurement) cable in the coloring of its outer jacket. Whereas thermocouple-grade cable is typically brown in exterior color, the extension-grade cable is usually colored to match the thermocouple plug (yellow for type K, black for type J, blue for type T, etc.).

What is burnout detection?

Another consideration for thermocouples is burnout detection. The most common failure mode for thermocouples is to fail open, otherwise known as “burning out.” An open thermocouple is problematic for any voltage-measuring instrument with high input impedance because the lack of a complete circuit on the input makes it possible for electrical noise from surrounding sources to be detected by the instrument and falsely interpreted as a wildly varying temperature.

For this reason, it is prudent to design into the thermocouple instrument some provision for generating a consistent state in the absence of a complete circuit. This is called the burnout mode of a thermocouple instrument. Only when the thermocouple fails open will the minuscule current through the resistor have any substantial effect on the voltmeter’s indication. The SPDT switch provides a selectable burnout mode: in the event of a burnt-out thermocouple, we can configure the meter to either read high temperature or low temperature (grounded), depending on what failure mode we deem safest for the application.

List of Prominent Manufacturers: ABB, Ametek, Arthur Grillo, Ashcroft, Ascon Tecnologic, Azbil, Carel, Chauvin Arnoux, Chino, Comeco, Comet, Conax, Crouzet, Danfoss, Durex, Elco, Enda, FineTek, Fisher, Focusens, Fluke, Galvatek, Gefran, Greenlee, Hascco, Herz, Hotset, Jumo, Krohne, Prisma, Meggitt, Minnco, Misumi, Moore Industries, Namac, New-Flow, Noris, Pace Scientific, Pixsys, Radionode, Riels, Sauermann, Sensirion, Simex, Sterling Sensors, Supmea, Taisuo, Tecnosoft, Tecsystem, Tempsens, Termya, Testo, Tewa, Thermokon, Thermal Detection, United Electric Controls, Viltrus, Watlow, Wika

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Thermocouples (Law of Intermediate Metals)

What is the law of intermediate metals?

It is critical to realize that the phenomenon of a “reference junction” is an inevitable effect of having to close the electric circuit loop in a circuit made of dissimilar metals. This is true regardless of the number of metals involved. In the last example, only two metals were involved: iron and copper.

This formed one iron-copper junction (J1) at the measurement end and one iron-copper junction (J2) at the indicator end. Recall that the copper-copper junction J3 was of no consequence because its identical metallic composition generates no thermal voltage:

But what about more complex thermocouple circuits, involving more than two-wire types? How do we define what a “reference junction” is, or how it behaves when we have more than two dissimilar-metal junctions in the same circuit? Take for instance this example of a type J thermocouple:

Here we have three voltage-generating junctions: J1 of iron and constantan, J2 of iron and copper, and J3 of copper and constantan. Upon first inspection, it would seem we have a much more complex situation than we did with just two metals (iron and copper), but fortunately, the situation is just as simple as it was before provided the temperatures of J2 and J3 are equal, which will be true if those two junctions are located very near each other.

A principle of thermo-electric circuits called the Law of Intermediate Metals helps us see this clearly. According to this law, intermediate metals in a series of junctions are of no consequence to the overall (net) voltage so long as those intermediate junctions are all at the same temperature. Representing this pictorially, the net effect of having four different metals (A, B, C, and D) joined together in a series is the same as just having the first and last metal in that series (A and D) joined with one junction if all intermediate junctions are at the same temperature:

In our Type J thermocouple circuit where iron and constantan both join to copper, we see copper as an intermediate metal between junctions J2 and J3 so long as those two junctions are at the same temperature. Being located next to each other on the indicating instrument, the identical temperature is a reasonable assumption for J1 and J2, so we may invoke the Law of Intermediate Metals and simply treat junctions J2 and J3 as a single iron-constantan reference junction. In other words, the Law of Intermediate Metals tells us we can treat these two circuits identically:

The practical importance of this Law is that we can always treat the reference junction(s) as a single junction made from the same two metal types as the measurement junction, so long as all dissimilar metal junctions at the reference location are at the same temperature.

This fact is extremely important in the age of semiconductor circuitry, where the connection of a thermocouple to an electronic amplifier involves many different junctions, from the thermocouple wires to the amplifier’s silicon. Here we see a multitude of reference junctions, inevitably formed by the necessary connections from thermocouple wire to the silicon substrate inside the amplifier chip:

Here we see the metals of the thermocouple wire (type J: iron and constantan) joining to a pair of brass terminal screws, which in turn join to copper traces on a printed circuit board, which join to lead/tin solder, which join to thin wires made of Kovar, which terminate at gold traces on the silicon chip, which are bonded to the silicon itself.

It should be obvious that each complementary junction pair in this series loop cancel each other if each pair is at the same temperature (e.g. gold-silicon junction J12 cancels with silicon-gold junction J13 because they generate the exact same amount of voltage with opposing polarities; Kovar-gold junction J10 cancels with gold-Kovar junction J11 for the same reason; etc.). The Law of Intermediate Metals goes one step further by telling us junctions J2 through J13 taken together in series are of the same effect as a single reference junction of iron and constantan. Automatic reference junction compensation is as simple as counter-acting the voltage produced by this equivalent iron-constantan junction at whatever temperature junctions J2 through J13 happen to be at.

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Thermocouple Reference junction compensation

What is reference junction compensation?

One technique is to physically fix the temperature of that junction at some constant value, so it is always stable. This way, any changes in measured voltage must be due to changes in temperature at the measurement junction since the reference junction has been rendered incapable of changing temperature. This may be accomplished by immersing the reference junction in a bath of ice and water:

In fact, this is how thermocouple temperature/voltage tables are referenced: describing the amount of voltage produced for given temperatures at the measurement junction with the reference junction held at the freezing point of water. With the reference junction maintained at the freezing point of water and thermocouple tables referenced to that specific cold junction temperature, the voltmeter’s indication will simply and directly always correspond to the temperature of measurement junction J1.

However, fixing the reference junction at the temperature of freezing water is impractical for any real thermocouple application outside of a laboratory. Instead, we need to find some other way to compensate for changes in reference junction temperature, so that we may accurately interpret the temperature of the measurement junction despite random changes in reference junction temperature.

A practical way to compensate for the reference junction voltage is to include an additional voltage source within the thermocouple circuit equal in magnitude and opposite in polarity to the reference junction voltage. If this additional voltage is made continually equal to the reference junction’s potential, it will precisely counter the reference junction voltage, resulting in the full (measurement junction) voltage appearing at the measuring instrument terminals. This is called a reference junction compensation or cold junction compensation circuit:

For such a compensation strategy to work, the compensating voltage must continuously track the voltage produced by the reference junction. To do this, the compensating voltage source (Vrjc in the above schematic) uses some other temperature-sensing device such as a thermistor or RTD to sense the local temperature at the terminal block where junction J2 is formed and produce a counter-voltage that is precisely equal and opposite to J2’s voltage (Vrjc = VJ2) at all times. Having canceled the effect of the reference junction, the voltmeter now only registers the voltage produced by the measurement junction J1:

Vmeter = VJ1 − VJ2 + Vrjc

Vmeter = VJ1 + 0 (If Vrjc = VJ2)

Vmeter = VJ1

This compensating voltage is maintained at the proper value according to the terminal temperature where the thermocouple wires connect to the ice point module, sensed by a thermistor or RTD:

Thermocouples are extremely rugged and have far greater temperature measurement ranges than thermistors, RTDs, and other primary sensing elements. However, if the application does not demand extreme ruggedness or large measurement ranges, a thermistor or RTD is probably the better choice!

What are the side effects of reference junction compensation?

The presence of reference junction compensation in every precision thermocouple instrument results in an interesting phenomenon: if you directly short-circuit the thermocouple input terminals of such an instrument, it will always register ambient temperature, regardless of the thermocouple type the instrument is built or configured for.

The transmitter’s internal reference junction compensation feature compensates for the ambient temperature of 68 degrees Fahrenheit. If the ambient temperature rises or falls, the compensation will automatically adjust for the change in reference junction potential, such that the output will still register the process (measurement junction) temperature. This is what the reference junction compensation is designed to do.

With the input short-circuited, the transmitter “sees” no voltage at all from the thermocouple circuit. There is no measurement junction nor a reference junction to compensate for, just a piece of wire making both input terminals electrically common. This means the reference junction compensation inside the transmitter no longer performs a useful function. However, the transmitter does not “know” it is no longer connected to the thermocouple, so the compensation keeps on working even though it has nothing to compensate for.

Therefore, the instrument registers the equivalent temperature created by the reference junction compensation feature: this is the only signal it “sees” with its input short-circuited. This phenomenon is true regardless of which thermocouple type the instrument is configured for, which makes it a convenient “quick test” of the instrument function in the field. If a technician short-circuits the input terminals of any thermocouple instrument, it should respond as though it is sensing ambient temperature.

While this interesting trait is a somewhat useful side-effect of the reference junction compensation in thermocouple instruments, other effects are not quite so useful. The presence of reference junction compensation becomes quite troublesome, for example, if one tries to simulate a thermocouple using a precision millivoltage source. Simply setting the millivoltage source to the value corresponding to the desired (simulation) temperature given in a thermocouple table will yield an incorrect result for any ambient temperature other than the freezing point of water!

The only suitable piece of test equipment available for generating the precise millivoltage signals necessary to calibrate thermocouple instruments was a precision potentiometer device. These “potentiometers” used a stable mercury cell battery (sometimes called a standard cell) as a voltage reference and a potentiometer with a calibrated knob to output low voltage signals.

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Thermocouple Connector and tip styles

What are the different connector and tip styles of thermocouples?

In its simplest form, a thermocouple is nothing more than a pair of dissimilar metal wires joined together. However, in industrial practice, we often need to package thermocouples in a way that optimizes their ruggedness and reliability. For instance, most industrial thermocouples are manufactured in such a way that the dissimilar-metal wires are protected from physical damage by stainless steel or ceramic sheath, and they are often equipped with molded-plastic plugs for quick connection to and disconnection from a thermocouple-based instrument.

A photograph of a type K industrial thermocouple (approximately 20 inches in length) reveals this “sheathed” and “connectorized” construction:

The stainless-steel sheath of this thermocouple shows signs of discoloration from the previous service in a hot process. Note the different diameters of the plug terminals. This “polarized” design makes it difficult to insert backward into a matching socket.

A miniature version of this same plug (designed to attach to thermocouple wire by screw terminals, rather than be molded onto the end of a sheathed assembly) is shown here, situated next to a ballpoint pen for size comparison:

Industrial-grade thermocouples are available with this miniature style of molded plug end as an alternative to the larger (standard) plug. Miniature plug-ends are often the preferred choice for laboratory applications, while standard-sized plugs are often the preferred choice for field applications.

Some industrial thermocouples have no molded plug at all but terminate simply in a pair of open-wire ends. The following photograph shows a type J thermocouple of this construction:

If the electronic measuring instrument (e.g., temperature transmitter) is located near enough for the thermocouple’s wires to reach the connection terminals, no plug or socket is needed at all in the circuit. If, however, the distance between the thermocouple and measuring instrument is too far to span with the thermocouple’s own wires, a common termination technique is to attach a special terminal block and connection “head” to the top of the thermocouple allowing a pair of thermocouple extension wires to join and carry the millivoltage signal to the measuring instrument.

This next photograph shows a close-up view of such a thermocouple “head”:

As shown in this photograph, the screws directly press against the solid metal thermocouple wires to make a firm connection between each wire and the brass terminal block.

Since the “head” attaches directly to one end of the thermocouple, the thermocouple’s wires will be trimmed just long enough to engage with the terminal screws inside the head. Both brass terminal blocks are mounted on a ceramic base, the purpose of the ceramic being to help equalize the temperatures between the two brass blocks while still maintaining electrical isolation. This assembly is sometimes referred to as an isothermal terminal block because it acts to keep all connection points at the same temperature (“isothermal”). A threaded cover on the head provides easy access to these connection points for installation and maintenance while ensuring the connections are covered and protected from ambient weather conditions the rest of the time.

Thermocouple wires are most often manufactured in solid form rather than stranded form.

A common mistake made with thermocouple wires is for technicians to crimp compression-style terminals (“lugs”) onto the solid wires. While this may form a usable connection at first, compression-style terminals are simply unable to maintain adequate compression when applied to the solid wire of any type, thermocouple wire included. Over time, solid wires will loosen inside compression terminals leading to circuit problems. In the case of a thermocouple circuit, bad wire connections lead to a situation where the receiving instrument “thinks” the thermocouple has failed open. This situation is commonly called burnout, referring to the phenomenon where a thermocouple junction fails open from being “burned out” by excessive temperature.

You will most often find compression terminals applied to solid thermocouple wire tips where those wires must terminate under the head of a screw. Compression terminals are correct to use in applications where stranded wire terminates at a screw head, but not solid wire. The proper termination technique for solid wire under a screw head is to wrap the solid wire in a semi-circle and directly clamp it under the screw head.

At the other end of the thermocouple, we have a choice of tip styles. For maximum sensitivity and the fastest response, the dissimilar-metal junction may be unsheathed (bare). This design, however, makes the thermocouple more fragile. Sheathed tips are typical for industrial applications, available in either grounded or ungrounded forms:

Grounded-tip thermocouples exhibit faster response time and greater sensitivity than ungrounded-tip thermocouples, but they are vulnerable to ground loops: circuitous paths for the electric current between the conductive sheath of the thermocouple and some other point in the thermocouple circuit. In order to avoid this potentially troublesome effect, most industrial thermocouples are of the ungrounded design.

Manually interpreting thermocouple voltages

Recall that the amount of voltage indicated by a voltmeter connected to a thermocouple is the difference between the voltage produced by the measurement junction (the point where the two dissimilar metals joined at the location, we desire to sense the temperature) and the voltage produced by the reference junction (the point where the thermocouple wires join to the voltmeter wires):

Vmeter = VJ1 − VJ2

This makes thermocouples inherently differential sensing devices: they generate a measurable voltage in proportion to the difference in temperature between two locations. This inescapable fact of thermocouple circuits complicates the task of interpreting any voltage measurement obtained from a thermocouple.

In order to translate a voltage measurement produced by a voltmeter connected to a thermocouple, we must add the voltage produced by the measurement junction (VJ2) to the voltage indicated by the voltmeter to find the voltage being produced by the measurement junction (VJ1).

In other words, we manipulate the previous equation into the following form:

VJ1 = VJ2 + Vmeter

We may ascertain the reference junction voltage by placing a thermometer near that junction and referencing a thermocouple table showing temperatures and corresponding voltages for that thermocouple type. Then, we may take the voltage sum for VJ1 and re-reference that same table, finding the temperature value corresponding to the calculated measurement junction voltage. While it is possible to mathematically model a thermocouple junction’s voltage, in the same way, we may model an RTD’s resistance, the functions for thermocouples are less linear than for RTDs, and so tables are greatly preferred for practical use.

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