October 2008

Process Automation

The versatile thermocouple

Thermocouples are small, convenient, flexible, cover wide ranges, reasonably stable, reproducible, accurate, fast

Fast Forward

  • Heating the junction of two dissimilar metals causes the generation of a continuous electromotive force (volts).
  • The weakest link in nearly all measurements is the temperature sensor.
  • The main disadvantage of a thermocouple is its weak output signal.
 
By H.M. Hashemian

Thermocouples are among the most simple temperature sensors for industrial applications. Basically, a thermocouple is made of two different metals (wires) joined at one end and open at the other end. The point where the two wires join up is the measuring junction, hot junction, or simply the junction.

The point at which the thermocouple wires link to the extension wires leading to a temperature indicator is the reference junction or cold junction. If the measuring junction and the reference junction are at two different temperatures, a voltage called electromotive force (EMF) occurs.

The magnitude of the EMF normally depends on the properties of the two-thermocouple wires and the temperature difference between the measuring junction and the reference junction.

For laboratory work and calibrating thermocouples, the reference junction is usually in an ice bath (at 0°C). However, in industrial applications, a circuit known as a cold junction compensation circuit automatically accounts for the temperature of the reference junction.

Thermocouple materials come as bare wires or flexible insulated pairs of wires. For uses at high temperatures or in hostile environments, thermocouples store away for protection in a metallic tube called a sheath, packed with dry insulation material to secure the thermocouple wires and to ensure electrical isolation.

We hermetically seal the assembly to keep the insulation material from contacting humid air. The insulation material in most thermocouples is often highly hygroscopic and can easily lose its insulation capability as moisture enters through the thermocouple seal. One of the consequences of moisture ingress is a noisy thermocouple signal.

For additional protection beyond what the sheath provides, especially when the thermocouple is in high-velocity flow fields or reactive environments, an additional metallic jacket called a thermowell is sometimes used.

In addition to protecting the sensor, a thermowell provides for easy replacement of the thermocouple and is sometimes in industrial processes only for this purpose, especially when the transient response of the sensor is not important.

Thermocouple junction styles

The measuring junction of a thermocouple may come together by any one of several methods. The three most common methods for sheathed thermocouple junctions are exposed junction, insulated junction, and grounded junction.

Exposed junction: In this method, the measuring junction comes in direct contact with the measured medium. The junction is a twist-and-weld procedure or it is butt-welded. There are other ways to form the junction, but these are among the most common methods.

Exposed junction thermocouples are usually for measuring the temperatures of gas or solid materials. The advantage of this construction is a fast response; the disadvantage is the wires are a part of the environment and are therefore subject to mechanical and chemical damage.

If the exposed junction thermocouple is in a liquid or moisture environment, one needs to cover its measuring junction with an insulating paint or epoxy. Furthermore, in these environments, it is important to seal the measuring tip of the thermocouple in such a way that no moisture enters into the thermocouple.

Insulated junction: An insulated junction thermocouple, also known as an ungrounded junction thermocouple, is usually made of a sheathed thermocouple stock cut to a desired length. The junction is made by removing some of the insulation from the tip of the assembly and forming the junction using a procedure similar to that used for the exposed junction. After forming the junction, recess it into the assembly and tightly pack in insulation material, then weld the tip closed with the same metal as the sheath material.

The advantages of insulated junction thermocouples are their circuit is isolated from the ground and their insulation resistance is readily measurable to diagnose insulation defects if they occur. Their disadvantage is a longer response time than exposed junction thermocouples provide and difficulty in fabricating them in small diameters.

Grounded junction: These thermocouples are in a sheath, but their junction style is much different than for exposed and insulated junctions. The thermocouple fabrication uses the same procedure as in insulated junction thermocouples.

Namely, cut sheathed thermocouple stock to length and then weld the tip closed, forming the junction with the sheath closure weld. The advantages of this thermocouple are fast response and ease of construction. The disadvantages are susceptibility to electrical ground loops, noise pickup, and the possibility that the thermo elements may alloy with the sheath. Grounded junction thermocouples are also more susceptible to open circuit failure with thermal cycling.

Another disadvantage of grounded junction thermocouples is their response times are not readily testable by the loop current step response method.

Grounded junction thermocouples sometimes have a slower response time than expected. They also are occasionally slower than insulated junction thermocouples of the same size and type. This happens when the hot junction is inadvertently formed somewhere other than in the inside wall of the sheath.

When we manufacture grounded junction thermocouples, we melt the sheath and the thermocouple wires together and allow them to solidify and form a junction at the tip of the assembly.

If the junction forms inside the thermocouple wire and away from the sheath instead of on the inside wall at the tip of the sheath, then the thermocouple can have a slow response time. In fact, to ensure a fast response time we make some grounded junction thermocouples by bending and welding the wires to the inside wall of the sheath rather than to the tip.

The three junction styles discussed apply mostly to sheathed thermocouples. For unsheathed thermocouples (or bare wire thermocouples), the hot junction is formed much like an exposed junction thermocouple. More specifically, the junction may be in the form of a bead or it may be butt-welded, lap-welded, twisted, and silver-soldered, and so on.

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Standardized thermocouples

There are 300 types of thermocouples that exist, are the subject of research, or that are in actual use. Of these, only eight are in common industrial service. These thermocouples classify as either base metal or noble metal depending on whether a noble or precious metal such as platinum is included in the thermocouple material.

Two of the eight thermocouples, type K and N, are identical in most characteristics. In fact, type N is a relatively new thermocouple developed to overcome some of the drawbacks of the type K thermocouple-such as atomic ordering, drift, and oxidation problems.

The type E and type J thermocouple have better relative output than type K, although type K is more widely used than type E or J. One reason for this is the better linearity offered by the type K thermocouple.

Prior to the early 1960s, thermocouples had proprietary names that the manufacturers assigned them. ISA introduced the letter designation we use today, and it became an American standard in 1964. The letter designations are part of the ANSI-MC 96.1 standard, from the American National Standard Institute (ANSI), and the ASTM 230 standard, issued by the American Society for Testing and Materials, or ASTM.

These standards specify if a thermocouple meets the nominal tolerances for its letter designation, then the tables given in Monograph 125, published by the National Bureau of Standards (now the National Institute of Standards and Technology) relate directly to their EMF to temperature.

Thermocouple extension wires

Thermocouple extension wires are necessary when one has to locate the reference junction away from the thermocouple. To avoid any non-homogeneity in the thermocouple circuit before it reaches the reference junction, the extension wires for base metal thermocouples are of the same material as the thermocouple wires.

However, noble metal thermocouples often use compensating extension wires fabricated from material different from the thermocouple but with similar thermoelectric properties within a limited temperature range.

Thermocouple assemblies for regular industrial use often have the extension wires and thermocouple joined together through a connector. In other designs, the thermocouple wires themselves are long enough to also serve as extension wires.

In this design, the extension wires penetrate out of the thermocouple assembly through a transition piece with no discontinuity in the thermocouple wires. The two different designs are the quick-disconnect and transition types.

In the quick-disconnect design, the metal contacts inside the connector are the same material as the thermocouple and the extension wires.

Thermocouples and their extension wires are usually color coded to aid in identification and to avoid inadvertent cross wiring. Unfortunately, the color coding of thermocouples has not become standard or universal. Different countries use different color coding for thermocouple extension wires.

In addition, the overall jacket material for thermocouple wires has different color coding in different countries.

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Reference junction compensation

The EMF output of a thermocouple can convert to the temperature of the measuring junction only if we know the reference junction temperature and compensation for its changes take place in the measuring circuitry.

A simple remedy is to keep the reference junction at a known and constant temperature medium such as an ice bath or an oven.

In measurement and control instrumentation, maintaining a constant reference junction temperature normally is inconvenient.

Consequently, some measuring instruments use a reference junction compensating resistor to automatically compensate for the changes in reference junction temperature. The reference junction resistor is at reference junction temperature and usually sized such that the EMF from the voltage divider is zero at a reference ambient temperature.

If the reference junction temperature increases, thermocouple EMF normally decreases. In the meantime, the reference junction resistor increases in resistance as the temperature increases. This adds an EMF in series with the thermocouple that is equal to the decrease in the thermocouple EMF. The measuring instrument consequently sees an EMF that relates directly to and only to the temperature of the measuring junction, regardless of a changing ambient temperature.

In digital instruments, the compensation for changes in reference junction temperature transpires differently. The incremental EMF caused by changes in the reference junction temperature adds directly to or subtracts directly from the thermocouple EMF.

A small constant current flows to the compensating resistor and the variations of the corresponding voltage digitize and combine with the thermocouple EMF to account for temperature changes at the reference junction.

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Seebeck theory makes it go

According to Estonian-German physicist Thomas Johann Seebeck, if the two ends of a wire are at different temperatures, a voltage is theoretically develops in the wire that manifests as:

V = S (T2 - T1)

where, S is the Seebeck Coefficient (thermoelectric power) in microvolts/ºC, and T2 and T1 are the tow different temperatures that drive a thermocouple, the reference, and hot measurements, the delta temperature.

ABOUT THE AUTHOR

H. M. Hashemian (hash@ams-corp.com) is an ISA Fellow. He has a degree in physics and a master's degree in nuclear engineering. He is president of Analysis and Measurement Services Corp., which specializes in I&C testing in process and power industries, test equipment, and technical training. His book is Sensor performance and reliability, ISA Press, 2005.

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