BASICS OF DESIGN ENGINEERING

T

ELECTRICAL/ELECTRONIC

the wire. It does so because the hot atoms impart some of their kinetic energy to their colder neighbors. The more kinetic energy an atom has, the faster it vibrates. Atoms with the highest kinetic energy are at the hot end of the wire, so they vibrate the most. While the nucleus of the atom remains trapped within the solid structure of the wire, some outer electrons are free to move. The vibrating atoms force free electrons toward the cold end and spread the kinetic energy in the form of heat con-

is created by the thermocouple junction, like some sort of min-iature battery. A thermocouple voltage is actually generated along an entire length of wire that has a temperature gradi-ent. A useful concept to keep in mind concerns the relationship between heat and electrical energy within electrical con-ductors — including thermo-couple wires. As a demonstra-tion, hold a single piece of wire on one end and then heat the opposite end. You’ll quickly dis-cover that the heat moves up

What may be the most common method of measuring industrial temperatures is mostly misunderstood by engineers and technicians.Mike NagerPHOENIX CONTACT AMERICAS R.B.U.HARRISBURG, PA.

Thermocouples aren’t exactly the latest fad in automation and control. They’ve been in use for a long time, work well, and seem pretty simple in op-eration. More than 60% of all temperature measurements in the U.S. use thermocouples. There are good reasons to use thermocouples over other temperature sensors. Your facil-ity may already use them. The application may need a sensor that withstands a lot of physical stress or one that is physically small. The expected high, low, or span of temperature may exceed the limitations of other sensor types. Finally, it might be diffi cult to justify a higher price for more sophisticated devices. Perhaps the most common misconception about ther-mocouples is that the signal

Edited by Robert Repas

What may be the moost th d f

Hot tips on THERMOCOUPLES

Thermocouple transmitters typically convert weak millivolt signals from the thermocouple into robust 4-to-20-mA process control signals. Thermowell-mount circular (hockey puck) and DIN-rail cabinet mount transmitter styles perform identical functions.

Connectors and terminal blocks specifically designed for thermocouple installations have contacts made of the same material as the thermocouple to prevent the formation of cold junctions.

Electrons move away from the hot end

Hot end Cold endAs the wire end is heated, the thermally excited atoms force negative-charge electrons towards the cold end of the wire.

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BASICS OF DESIGN ENGINEERINGELECTRICAL/ELECTRONIC

Pur

ple

+

Cu

-Ni (

or

Co

nst

anta

n)

Recommended maximum ranges

for 14-awgthermocouples

Tem

per

atu

re

1,100°C (2,012°F)

1,000°C (1,832°F)

900°C (1,652°F)

800°C (1,472°F)

700°C (1,292°F)

600°C (1,112°F)

500°C (932°F)

400°C (752°F)

300°C (572°F)

200°C (392°F)

100°C (212°F)

0°C (32°F)

–100°C (–148°F)

–200°C (–328°F)

K

Type

T

TypeE

TypeJ

Type

+Y

ello

w

–Red

Ni-

Cr

(or

Ch

rom

el)

Ni-

Al (

or

Alu

mel

)

Source: ISA - MC96, 1-1982

+W

hit

e

–Red

Fe Cu

-Ni (

or

Co

nst

anta

n)

+B

lue

–Red

Cu

Cu

-Ni (

or

Co

nst

anta

n) +

Pu

rple

–Red

Ni-

Cr

(or

Ch

rom

el)

Four thermocouple types account for 99% of the applications involving thermocouples. Color codes identify the types and approximate temperature range best suited for each type.

tion. Thermocouple circuits can have more than one cold junc-tion, but that is strongly dis-couraged and easily avoided. The raw voltage signal that represents temperature is the sum of all the hot and cold junc-tions. The temperature of the hot junction is determined by subtracting the off sets gener-ated by all of the cold junctions. Thus the output voltage of the thermocouple depends only on the temperature of the hot junction. The materials used to create the hot junction determines the type of thermocouple. The material type to use for a given application depends on tem-perature range and environ-mental conditions. Four types of thermocouples, identifi ed by the letters K, J, T and E, ac-

the wires. Any other point those wires connect, including terminal blocks and measuring instru-ments, is called a “cold junc-tion.” One cold junction at the place thermocouples connect to the measuring instrument is unavoidable. This is often called the reference junction. Temperature readings made at the reference junction cor-rect any thermocouple voltage generated there in a process called cold junction compensa-

duction. Because electrons have a negative charge, their forced migration creates a positive potential at the hot end of the wire and a negative potential at the cold end. The magni-tude of the voltage depends on the composition of the wire and the temperature dif-ference between the hot and cold ends. The length or gage size of the wire has no eff ect on the voltage generated. Although there is electron movement in any single con-ductor, it is virtually impossible to get a voltage measurement from it. Readers will recall volt-age is the diff erence in elec-trical potential between two points. Trying to take a voltage reading from a single point on the wire just can’t be done. To get around this limitation, a second wire made from a dif-ferent metal or alloy attaches to the fi rst wire at the high-tem-perature point. The connection is called the “hot junction.” Dif-ferent metals produce diff erent quantities of electron motion when the wires are heated to the same temperature. The im-balance between the number of electrons creates a potential diff erence (voltage) between

Cold-junction temperature compensation is done here

(reference junction)

Hotjunction

Metal A

Metal B

+

–Electrons move away fromheat at different rates

Each metal wire moves a different quantity of electrons when heated to the same temperature. The difference between electron quantities is measured as a voltage.

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BASICS OF DESIGN ENGINEERINGELECTRICAL/ELECTRONIC automation applications, such

as those in sealing operations on packaging machines. The

length of wire needed is often a fraction of that found in pro-cess applications. So it is rea-sonable to run the thermo-couple wires back to a control cabinet or junction box for conversion. Also, thermowells found in the process indus-tries may not be well suited for physical installation on a machine. Embedded thermo-couples with tape or bolt-on connections are often used inside machine components. Electrical isolation is impor-tant to consider when decid-ing on a transmitter. Isolation between the thermocouple input and output circuit on loop-powered devices pre-vents ground loops from degrading the temperature

signal. Typical isolation values are in the 1 to 2-kV range. Trans-mitters that get their operat-ing power from an external dc power supply need additional isolation. Often called three-way isolation, it provides similar levels of protection between in-put/output, input/power sup-ply, and output/power supply. Isolation also provides a degree of protection for the measuring devices in the control cabinet in case the thermocouple some-how touches a high-voltage source.

FIELD CONNECTIONS If a transmitter is impractical, then it may be possible to ex-tend the length of the thermo-couple wires by using special terminals mounted in the ther-

the region where temperature measurements are made. The hockey-puck-shaped transmit-ter fi ts in the exposed end of the well while the thermocou-ple extends into the other end. Hockey-puck-style transmit-ters work well for hazard-clas-sifi ed areas. The transmitters mount in metal probes with sealed covers and connect se-curely to metal conduit that meets the various encapsula-tion or explosion-proof require-ments. Intrinsically safe circuits are used to power the device while conduit fi ttings protect the wires mechanically. Hockey-puck transmitters are generally output loop-powered, meaning they are powered from a supply back in the control cabinet. DIN-rail-mounted transmit-ters are most widely used in

count for more than 99% of all thermocouple applications.

PRACTICAL MEASUREMENTS The voltage thermocouples generate is small — often less than 30 μV/°F. Such low poten-tial is easily corrupted by noise. Typically, the microvolt signal feeds a transmitter that converts it into a robust process signal, such as a 4-to-20-mA instrumen-tation current loop. Transmitters have become economical with some priced in the $150 range. The low cost lets them serve in noncritical applications. It’s fairly common to use trans-mitters that fi t in a thermowell probe when connecting ther-mocouples with runs from sev-eral hundreds to thousands of feet. Thermowells are closed metal tubes that extend into

101112

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4 5 6

1 2 3

1 2 3

4 N.C . N.C.

+24V SW GNDPOWER

I U GNDOUT

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MC

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MODE

Temperature controller,PLC, or distributed-control system

Sensor headassembly

Protectivethermowell

Thermocouple junction

Small millivoltsignal fromthermocouple

Thermocoupletransmitter

Strong milliampsignal

Analoginput board

Input (4 to20 mA)CommonStatus LED

A complete thermo-couple temperature

loop consists of a ther-mocouple (shown in a

thermowell), a thermo-couple transmitter, and a temperature monitor

or controller, here shown as a PLC or dis-

tributed-control sys-tem. The loop is com-

pleted as the controller adjusts the amount of

heat added to the pro-cess monitored by the

thermocouple.

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BASICS OF DESIGN ENGINEERINGELECTRICAL/ELECTRONIC

“N” for the negative. Extension wires are also designated by the letter “X.” So a “JP” designa-tion refers to the positive leg of a Type J thermocouple. “KNX” refers to the negative leg of a Type K extension cable.

CABINET CONNECTIONS

thermocouples, while com-pensation wires have a diff er-ent composition. In practice, though, the extension cable term is most often used re-gardless of the wire composi-tion. Extension cables can cost up to 50% less than thermocouple wire. They also come in sizes up to 14 awg, which reduces loop resistance in long runs. Between 0 and 200°F, extension cables exhibit the same electrical prop-erties as the thermocouples themselves and are electrically indistinguishable from thermo-couple wire. To keep extension wire from inadvertently being used as thermocouple wire, the outer insulation jacket on the extension wires is color coded diff erently than the outer jacket on thermocouple wires. Howev-er, the individual wire insulation color matches that of thermo-couple wire. Convention for both thermo-couples and extension wires calls for a “P” to be used to in-dicate the positive leg and an

mowell or surface-mounted on the machine. The connection area should be away from ex-treme process temperatures. In fact, temperatures should be close to that of the control-ler. Thermocouples are wired to terminals specially constructed to minimize any temperature diff erence between their two ends and between each indi-vidual wire. These terminals are sometimes referred to as “iso-therm” blocks. Isotherm blocks allow the famous “Law of Intermediate Metals” to operate. This law states that if all connection points on the isotherm block are of equal temperature, add-ing a third metal in both wires of the circuit won’t degrade the signal. With the thermocouple connected to one side of the isotherm block, the other side connects the special thermo-couple extension or compen-sation wire. Technically, ex-tension wires have the same chemical composition as their

Power supplies and thermocouple loopsTemperature loops that use thermocou-ple transmitters require steady and reli-able 24-Vdc power for all components to work correctly. Most supplies are now switch-mode, giving users several advantages that include low purchase price, light weight, and long life. These modern supplies have efficiencies in the 90% range for more economical opera-tion and lower expenditures for cabinet cooling equipment and operation.

From a purely technical standpoint, selecting a power supply means find-ing one that accepts a convenient input voltage and provides enough amperage to drive all the loads. Supplies can come in a wide-range of input voltages with output currents up to 40 A.

Supply location also plays an im-portant role in the selection process. For example, supplies listed with UL1604 Class I, Div. 2 ratings limit the escape of electrical and heat energy for safe use in areas that may contain explosive vapors.

NEC Class 2 power supplies find use in or on machines because that class limits the amount of energy they can produce to 100 W or less, or about 4.2 A at 24 Vdc. Wires connected to that type of supply do not need conduit runs.

A recent innovation in control power is the 24-Vdc uninterruptible power supply, or UPS. The UPS system combines a power supply, battery, and a charging circuit in one box. In the event of power loss, the UPS maintains volt-age long enough to ride through the outage or provide an orderly shutdown if the outage lasts too long.

THERMOCOUPLE EXTENSIONS CABLESType Composition Conductor

insulationOuter jacket

Temperature range

KPX Nickel chromium Yellow Yellow 0 to 200°C (32 to 392°F)KNX Nickel aluminum silicon Red

JPX Iron White Black 0 to 200°C (32 to 392°F)JNX Copper nickel Red

TPX Copper Blue Blue 0 to 100°C (32 to 212°F)TNX Copper nickel Red

EPX Nickel chromium Purple Purple 0 to 200°C (32 to 392°F)ENX Copper nickel Red

This table describes the extension cables for the four main types of thermocouples.

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BASICS OF DESIGN ENGINEERINGELECTRICAL/ELECTRONIC

ed temperature transmitters convert low-level thermocou-ple signals to 4-to-20 mA and electrically isolate the circuit from ground loops. Some allow simple ON/OFF control with programmable dead bands us-ing a transistor or relay output while others may also incorpo-rate displays. Other devices measure sev-eral thermocouples at once using sophisticated control al-gorithms to do more advanced control than simple ON/OFF. Proportional, integral, and de-rivative (PID) control has been used for decades and is rapidly fi nding applications in the dis-crete world. In applications like heat seal packaging and extru-sion processes, temperatures must be controlled rapidly and precisely. Heat must be added or removed in fractions of a sec-ond. One of the latest devices for temperature monitoring uses a communication bus controlling and transmitting temperatures. Because they typically mount in a junction box on standard DIN-rail, they

ment, fans, and other devices that heat and cool the interior unevenly. Standard terminal blocks are not designed to min-imize temperature diff erentials like isotherm terminals. They make the measurement unpre-dictable whenever a heat-gen-erating device is switched on or off or the cabinet is opened. The solution is to use ther-mocouple blocks whose metal parts match the composition of the thermocouple extension wire. Because the materials match, there is no cold junction. To the thermocouple circuit, it looks like a continuous piece of extension wire. Changing tem-perature won’t introduce er-rors. Thermocouple blocks are typically more expensive than standard terminal blocks, but eliminate a potentially tedious source of errors.

TEMPERATURE CONTROL Main system controllers, whether PLC, PC, or DCS, no lon-ger need to perform tempera-ture control. It may be preferable to “outsource” or distribute that control to another device. For example, DIN-rail-mount-

Extension wires typically run into distributed control systems, PLCs, or temperature analyzers back at the control cabinet. For the very best re-sults, a user would run one con-tinuous piece of wire from the hot junction to the terminals on the measuring instrument. But often this isn’t practical. Cabi-nets built off -site and by third-parties can’t be wired directly. Therefore, terminal blocks are used to help make wiring easier. The panel shop wires from the controller to the terminal block, leaving the other side open for the fi eld connection of the ex-tension wire. There is an unfortunate widespread belief that thermo-couples can simply connect to standard terminal blocks with-out degrading the signal, using the Law of Intermediate Metals as the rationale. The contact area and current bar in most terminal blocks is made of steel, though high-quality blocks use corrosion-resistant alloys. Ei-ther material forms a cold junc-tion because it doesn’t match the chemical composition of the thermocouple wire. As mentioned in the last section, it’s possible to use terminal blocks without making a cold-junction correction. But that works only if the temperature on both sides of the block re-mains the same or changes at exactly the same rate. That isn’t likely in control cabinets packed with heat-producing equip-

More advanced temperature controllers can use digital communication protocols such as USB or Ethernet. Temperature signals can then feed directly to computer control systems for monitoring and data logging.

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Mike Nager's fi rst job out of college was to write an instruction manual for a thermo-couple input card. He has a BSEE from the Uni-versity of Scranton and manages the industry marketing for Phoenix Contact Americas.

BASICS OF DESIGN ENGINEERINGELECTRICAL/ELECTRONIC

from outside the U.S., verify the color coding is correct. Europe and Japan use diff erent con-ventions than the standards that originated in the U.S. The varying conventions have been used for more than 60 years in their respective countries, so there is little chance of further “harmonization” in the future. Many aff ordable products exist that can simplify and en-hance thermocouple measure-ments. Transmitter, meter, and switch prices have become low enough to avoid many tradi-tional diffi culties using ther-mocouples. This also allows consideration of new tempera-ture measurements that were either too expensive or too complicated in the past. Finally, conversion to a process control signal (4-to-20 mA) or a digi-tal signal allows long-distance transmission without having to worry about the quirks of a thermocouple circuit. MD

Circle 622

always positive. Uncompensated cold junc-tions cause many problems in areas where stable tempera-tures can’t be guaranteed, which is just about every-where. Only use thermocou-ple or extension wire in the circuit. Be suspicious of any standard nonisotherm or non-thermocouple terminal blocks in the loop. Make sure the loop resistance isn’t too high. The maximum value you should shoot for is usually 100 Ω. Remember that total length is twice the actual length because there is both a supply and return wire length. Use wire tables or test the cable to determine its resistance. Keep thermocouple wires away from other current-car-rying conductors, motors, and sources of radio frequency noise. If in doubt (that is, al-ways), convert the thermocou-ple voltage to a more robust signal like a 4-to-20-mA instru-mentation loop. When using thermocouples

can reside close to the ther-mocouples, reducing possible sources of error. The input fi lter-ing, control logic, and process output all take place in the de-vice. There is no need to stretch PLC resources trying to process the PID loops. And because the device is part of a bus network, the information can be sent out via standard industrial proto-cols such as Ethernet, Interbus, Profi bus, or DeviceNet. Other I/O signals can be added for an-alog, digital, or serial devices.

TROUBLESHOOTING HINTS A few problems show up regularly in thermocouple ap-plications. Watch out for wires that are damaged or broken by rough installation, vibration, or other stress. Take care not to reverse the polarity of the thermocouple loop. Remember that USA standard ASTM E 230 dictates the negative leg always has red insulation and its symbol is the thicker wire. The fi rst named element of a thermocouple is

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