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 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.

In order 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.

To illustrate, let us examine a simple type K thermocouple installation, where the thermocouple connects directly to a panel-mounted temperature indicator by long wires:

Like all modern thermocouple instruments, the panel-mounted indicator contains its own internal 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.

Now let us consider the same thermocouple installation with a length of copper cable (two wires) joining the field-mounted thermocouple to the panel-mounted indicator:

Even though nothing has changed in the thermocouple circuit except for the type of wires joining the thermocouple to the indicator, the reference junction has completely shifted position. What used to be a reference junction (at the indicator’s terminals) is no longer, because now we have copper wires joining to copper wires. Where there is no dissimilarity of metals, there can be no thermoelectric potential. At the thermocouple’s connection “head,” however we now have a joining of chromel and alumel wires to copper wires, thus forming a reference junction in a new location at the thermocouple head. What is worse, this new location is likely to be at a different temperature than the panel-mounted indicator, which means the indicator’s reference junction compensation will be compensating for the wrong temperature.

The only practical way to avoid this problem is to keep the reference junction where it belongs: at the terminals of the panel-mounted instrument where the ambient temperature is measured and the reference junction’s effects accurately compensated. If we must install “extension” wire to join a thermocouple to a remotely-located instrument, that wire must be of a type that does not form another dissimilar-metal junction at the thermocouple head, but will form one at the receiving instrument.

An obvious approach is to simply use thermocouple wire of the same type as the installed thermocouple to join the thermocouple to the indicator. For our hypothetical type K thermocouple, this means a type K cable installed between the thermocouple head and the panel-mounted indicator:

With chromel joining to chromel and alumel joining to alumel at the head, no dissimilar-metal junctions are created at the thermocouple. However, with chromel and alumel joining to copper at the indicator (again), the reference junction has been relocated to its rightful place. This means the thermocouple head’s temperature will have no effect on the performance of this measurement system, and the indicator will be able to properly compensate for any ambient temperature changes at the panel as it was designed to do. The only problem with this approach is the potential expense of thermocouple-grade cable. This is especially true with some types of thermocouples, where the metals used are somewhat exotic (e.g. types R, S, and B).

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 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, extension-grade wire may use insulation with a narrower temperature rating than thermocouple-grade wire, reducing 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, extension-grade cable is usually colored to match the thermocouple plug (yellow for type K, black for type J, blue for type T, etc.)

For more information on thermocouple extension wire, contact Duro-Sense by visiting https://duro-sense.com or by calling 310-533-6877

Reprinted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.

Understanding Dissimilar Metal Junctions and the Need for Reference Junctions

When two dissimilar metal wires are joined together at one end, a voltage is produced at the other end that is approximately proportional to temperature. That is to say, the junction of two different metals behaves like a temperature-sensitive battery. This form of electrical temperature sensor is called a thermocouple:

This phenomenon provides us with a simple way to electrically infer temperature: simply measure the voltage produced by the junction, and you can tell the temperature of that junction. And it would be that simple, if it were not for an unavoidable consequence of electric circuits: when we connect any kind of electrical instrument to the thermocouple wires, we inevitably produce another junction of dissimilar metals. The following schematic shows this fact, where the iron-copper junction J₁ is necessarily complemented by a second iron-copper junction J₂ of opposing polarity:

Junction J₁ is a junction of iron and copper – two dissimilar metals – which will generate a voltage related to temperature. Note that junction J₂, which is necessary for the simple fact that we must somehow connect our copper-wired voltmeter to the iron wire, is also a dissimilar-metal junction which will also generate a voltage related to temperature. Further note how the polarity of junction J₂ stands opposed to the polarity of junction J₁ (iron = positive ; copper = negative). A third junction (J₃) also exists between wires, but it is of no consequence because it is a junction of two identical metals which does not generate a temperature-dependent voltage at all.

The presence of this second voltage-generating junction (J₂) helps explain why the voltmeter registers 0 volts when the entire system is at room temperature: any voltage generated by the iron- copper junctions will be equal in magnitude and opposite in polarity, resulting in a net (series-total) voltage of zero. Only when the two junctions J₁ and J₂ are at different temperatures will the voltmeter register any voltage at all.

We may express this relationship mathematically as follows:  Vmeter = VJ₁ − VJ₂

With the measurement (J₁) and reference (J₂) junction voltages opposed to each other, the voltmeter only “sees” the difference between these two voltages.

Thus, thermocouple systems are fundamentally differential temperature sensors. That is, they provide an electrical output proportional to the difference in temperature between two different points. For this reason, the wire junction we use to measure the temperature of interest is called the measurement junction while the other junction (which we cannot eliminate from the circuit) is called the reference junction (or the cold junction, because it is typically at a cooler temperature than the process measurement junction).

For more information on this subject, contact Duro-Sense, Inc. by visiting https://duro-sense.com or by calling 310-533-6877.

Reprinted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.

What is the Difference Between Thermocouple Wire and Thermocouple Extension Wire?

Thermocouple Wire

Insulated thermocouple wire is single pair wire that can be made into a thermocouple. More specifically, it is the wire from the sensing junction, to the point of cold junction compensation. Thermocouple wire is made from two dissimilar metals conductors that are joined (welded) together at the sensing end. Different metal wires are used for the various types of  thermocouple types (J, K, T, E, etc). These dissimilar metals produce a millivolt electrical signal at the cold junction that is proportional to the change in temperature at the sensing end.  Thermocouple wire has two grades; standard and Special Limits of Error (SLE). Special Limits of Error wire is made of the same metals as standard wire, but uses a higher grade with increased accuracy and higher expense than standard thermocouple wire.

Thermocouple extension wire.

Thermocouple wire is almost always insulated. Insulation varies from low temp (Nylon, FEP Teflon, TFE Teflon, PFA Teflon, Kapton) to high temp (fiberglass, Hi-Temp fiberglass, Refrasil, Ceramic Fiber).  Thermocouple wire is typically sold in 1000' spools, but can be sold in small quantities as well.

Thermocouple extension wire is a single pair wire that cannot be made into a thermocouple, but is used to carry the signal from a thermocouple to the recorder, controller, or instrument reading the signal. Extension grade wire is used to carry a signal representing the higher temperature seen by the sensing location, but extension wire itself cannot be generally exposed to those higher temperatures. Extension wire cannot be used to make a thermocouple, but thermocouple wire can be used as extension wire. Insulation is typically PVC, but other option are available.

Multi-pair extension wire is simply more than a single pair in the same jacket.  It is extension only and is usually available in 2,4,6,8,12,16,24 pairs. It is used primarily when a contractor has to run multiple runs of wire. It allows them to run one piece of wire rather than multiple individual runs.

For more information on all varieties of thermocouple wire, contact Duro-Sense by calling 310-533-6877 or visiting https://duro-sense.com.