A thermocouple is a device which is used to measure temperature. It is essentially one of the temperature measurement sensors.
It is used in all the industries where the control of temperature or heat flow is required for the processes.
Hence it is used in chemical, process, oil and gas, metal extraction, petroleum, petrochemical, pharmaceuticals, cement, glass, ceramics, power generation, paper and pulp and many more industries. They are also used in regular everyday appliances like stove, toasters, furnace etc.
The thermocouple consists of two metal wires, both the wires are dissimilar in nature. They are joined by a thermocouple thermometer at the other end.
Working of Thermocouple
The dissimilar metal wires in the thermocouple are joined to form a junction. Joining them completes a thermoelectric circuit.
When the temperature at the junction will increase then a current is produced. The junction at which temperature increases is called the hot junction and there is one more junction at the other end called cold junction.
The cold junction will also have a certain temperature.
Essentially there is a temperature difference in the circuit.
The heat flows from the hot side to the cold side, at an atomic scale the charge carriers also diffuse from hot end to cold end. It means the temperature difference has created a voltage in the circuit.
The relationship between voltage and temperature difference can be quantified and the thermocouple calibrated for use.
Theory
Thomas Seebeck discovered if metals of two different materials were joined at both ends and one end was at a different temperature than the other, a current was created. This phenomenon is known as the Seebeck effect and is the basis for all thermocouples.
A thermocouple is a type of temperature sensor, which is made by joining two dissimilar metals at one end. The joined end is referred to as the HOT JUNCTION.
The other end of these dissimilar metals is referred to as the COLD END or COLD JUNCTION. The cold junction is actually formed at the last point of thermocouple material
Certain combinations of metals must be used to make up the thermocouple pairs.
If there is a difference in temperature between the hot junction and cold junction, a small voltage is created. This voltage is referred to as an EMF (electro-motive force) and can be measured and in turn used to indicate temperature.
The voltage created by a thermocouple is extremely small and is measured in terms of millivolts (one millivolt is equal to one thousandth of a volt). In fact, the human body creates a larger millivolt signal than a thermocouple.
To establish a means to measure temperature with thermocouples, a standard scale of millivolt outputs was established. This scale was established using 32 deg. F (0°C) as the standard cold junction temperature (32 deg. F (0°C) = 0 millivolts output).
COLD JUNCTION COMPENSATION
As we mentioned earlier, the last point of thermocouple material is known as the cold junction. The amount of output the t/c produces is determined by the difference between the hot junction and the cold junction temperatures. The cold junction temperature must be known to accurately determine the temperature.
Lets look at the following examples;
If we had a thermocouple in a heat treat furnace and wanted to know what temperature it was in that furnace, we could attach a voltmeter to the cold junction and measure the voltage.
Let’s say that the furnace is operating at 1000 deg. F. and it is 100 deg. F at the cool end of the T/C. Since we said that a T/C measures the difference between the hot and cold junctions, our formula would be:
1000 (hot junction) - 100 (cold junction) = 900 deg. F.
There seems to be a problem since we said that the furnace was at 1000 deg. F. This brings us to COLD JUNCTION COMPENSATION.
COLD JUNCTION COMPENSATION is usually done automatically by the measuring instrument. The instrument measures the temperature at the cold junction and adds it back to the equation.
1000 (hot junction) - 100 (cold junction) = 900 deg. F + 100 deg. F (cold junction temp) = 1000 deg F
This way the instrument indicates the actual temperature of the hot junction.
This COLD JUNCTION compensator is usually located at the terminals on the back of the indicating instrument and you must maintain T/C material all the way to this point.
For a thermocouple to function properly, there must be no other metals used between the hot junction and the cold junction. If wire is needed to connect the T/C to the indicating instrument, the leadwire must be made of the same material as the T/C.
It is acceptable to use terminal blocks and lugs made of plain copper in a thermocouple circuit as long as the positive and negative terminals are at the same temperature. (Example: terminal blocks in heads or spade lugs on wire)
If you were to use plain copper wire instead of T/C extension wire to run to the instrument, your cold junction would be formed at the junction between the copper and the T/C wire.
This junction would most likely not be at the same temperature as the back of the instrument where the compensator is located. This would then create an error in the indicated temperature.
If a we were to use the wrong T/C extension wire, the same problem could appear. This is why we must use the correct T/C extension wire on our assemblies.
It is also acceptable to have a third metal in the hot junction as long as that metal is at the same temperature as the thermocouple material.
Types of Thermocouples
Thermocouples come in various forms and in numerous varieties.
The varieties are so large that appropriate thermocouples have to be selected for a particular application.
The most common type of thermocouple are type K, J, T and E. All theses common thermocouples have a standard accuracy and all are made of base metals.
Type K thermocouple:
The material of construction of such thermocouples are either Nickel-Chromium or Nickel-Alumel. It is the most common thermocouple used because it is inexpensive, reliable, accurate, also has a wide temperature range.
Type J thermocouple:
The material of construction of such thermocouples are Iron or Constantan. It is the next common thermocouple after type K. It is also inexpensive and reliable but it has a relatively shorter temperature range and life span.
Type T thermocouple:
The material of construction is Copper or Constantan. It is a very stable thermocouple, it is made to be used in low temperature applications.
Type E thermocouple:
The material of construction is Nickel-Chromium or Constantan. This thermocouple is preferred in applications which demand a better accuracy compared to type K or J at moderate conditions.
NOBLE METAL THERMOCOUPLE TYPES
Noble Metal Thermocouples are another category of thermocouples and are made of the expensive precious metals Platinum and Rhodium. There are three types of noble metal thermocouples:
Type B (Platinum/Platinum-30% Rhodium)
Type R (Platinum/Platinum-13% Rhodium)
Type S (Platinum/Platinum-10% Rhodium)
Types R and S have temperature ranges of 1000 to 2700F and Type B thermocouples have a temperature range of 32 to 3100F.
As can be seen above, the difference between these three thermocouples is the amount of Rhodium contained in the negative leg. Types R and S can exhibit excessive grain growth in the platinum when exposed to the higher end of its temperature range.
The increased amount of Rhodium in the Type B thermocouple helps to reduce the grain growth problem allowing for a slightly increased temperature range.
Noble metal thermocouples are intended for use in oxidizing or inert atmospheres. They must not be used in reducing atmospheres or in applications containing metallic or nonmetallic vapors. Noble metal thermocouples are soft and prone to being damaged if mishandled.
These thermocouple assemblies are usually assembled in ceramic insulators and supplied with ceramic protection tubes. Noble metals should never be supplied in metal protection tubes only. The color code for Types R and S is black and red, and the color code for Type B is gray and red.
[ads id="ads1"] Refractory Metal Thermocouples
Refractory Metal Thermocouples are the last category of thermocouple that Pyromation manufactures. These thermocouples are made of the exotic metals Tungsten and Rhenium, which are expensive, difficult to manufacture, brittle, and must be handled carefully. There are three types of refractory metal thermocouples:
Type G (Tungsten/Tungsten 26% Rhenium)
Type D (Tungsten 3% Rhenium/Tungsten 26% Rhenium)
Type C (Tungsten 5% Rhenium/Tungsten 26% Rhenium)
(Note: The Type designations listed for the refractory thermocouples are industry standard designations, not standardized thermocouple types)
All of these types have a temperature range of 32-4200 deg F. Refractory metal thermocouples are normally used in vacuum furnaces beyond the temperature capabilities of platinum.
They are seldom used below approximately 2500F since there are other thermocouple types more suited for the lower temperatures. Refractory metal thermocouples and their associated refractory metal protection tubes must not be used in the presence of oxygen at temperatures above 500°F.
The color code for refractory thermocouples are:
Type G: White with blue tracer and red Type D: White with yellow tracer and red Type C: White with red tracer and red
Advantages of Thermocouple
They are quite simple in construction, have low cost and are quite durable in nature.
They offer wide range of temperature measurement.
They have a fast response time of measurement.
Disadvantages of thermocouple
The underlying relationship between variables is not linear. It is non-linear in nature.
It requires reference and recalibration is difficult.
It is quite susceptible to electromagnetic interference and radio frequency interference.
Limits of Error
Tolerances now covered by ASTM E-230
T/C wire is manufactured, then tested and sorted by accuracy.
Refer to Pyromation catalog page GEN-5
TC Rules of the road
The most important rule to remember when manufacturing thermocouples is that red is always negative! (For domestic manufacturers)
The second most important rule to remember is that red is always negative! Regardless of the T/C type, the red leg is the negative leg. This is different than your car battery or a DC circuit.
Each thermocouple type has a different color code, which is shown on the handout. This will allow you to make sure that you are using the right calibration plug or jack with the right T/C.
It is very important to memorize the color codes for types K, J, T, E, and N.
Thermocouple types J and K have one leg, which is magnetic. This is one of the easiest ways to determine polarity if there is no color-coding to help (as with MgO T/C’s). If you were to refer to the THERMOCOUPLE TYPE COLOR CODE catalog handout, it has a column that indicates the magnetic leg.
On the type J thermocouple, the POSITIVE leg is strongly magnetic. According to the handout, the white leg would be the magnetic leg.
With the type K T/C, the RED OR NEGATIVE leg is the magnetic leg. It is not as strongly magnetic and can be hard to detect on smaller diameter wire.
These are the only two types of thermocouples that have a magnetic leg so if a magnet sticks to one of the wires, you know it is type K or J.
Color codes and magnetic legs are just tools to help assure that our connections are made properly.
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TC Myths
There should never be a third metal in the hot junction To create a thermocouple junction, all that is needed is to electrically short the ends together. Butting the wire ends against a metal surface will create a junction. Remember, that the thermocouple signal is generated over the entire length of wire.
You must use special limits of error extension wire if your thermocouple is special limits This is not necessary if the extension wire is outside the temperature gradient area. Although the signal is generated over the entire length of wire, the important area is the gradient between the hot and ambient areas.
Non-thermocouple materials cannot be used in the thermocouple circuit. It is permissible to use non-thermocouple materials as terminal blocks or splices as long as there is no temperature gradient across these devises.
I can get an average temperature just by wiring my thermocouples in parallel. To get a true average, all thermocouples in a parallel circuit must be the same length or have the same resistance.
The largest possible extension wire should be used to connect a thermocouple. This phrase used to be true 30 years ago before there was solid-state electronics. The old instruments were Voltage based circuits and resistance was critical. The newer solid-state electronics are current based so extension wire resistance is not important.