What is a thermocouple & how does thermocouple-works ?

A thermocouple is comprised of at least two metals joined together to form two junctions. One is connected to the body whose temperature is to be measured; this is the hot or measuring junction. The other junction is connected to a body of known temperature; this is the cold or reference junction. Therefore the thermocouple measures unknown temperature of the body with reference to the known temperature of the other body.

Reference

Book: Mechanical Measurements by Thomas G. Beckwith and N. Lewis Buck

Principle of Working of Thermocouple

The working principle of thermocouple is based on three effects, discovered by Seebeck, Peltier and Thomson. They are as follows:

1) Seebeck effect: The Seebeck effect states that when two different or unlike metals are joined together at two junctions, an electromotive force (emf) is generated at the two junctions. The amount of emf generated is different for different combinations of the metals.

2) Peltier effect: As per the Peltier effect, when two dissimilar metals are joined together to form two junctions, emf is generated within the circuit due to the different temperatures of the two junctions of the circuit.

3) Thomson effect: As per the Thomson effect, when two unlike metals are joined together forming two junctions, the potential exists within the circuit due to temperature gradient along the entire length of the conductors within the circuit.

In most of the cases the emf suggested by the Thomson effect is very small and it can be neglected by making proper selection of the metals. The Peltier effect plays a prominent role in the working principle of the thermocouple.

Diagrams

How it Works

The general circuit for the working of thermocouple is shown in the figure 1 above. It comprises of two dissimilar metals, A and B. These are joined together to form two junctions, p and q, which are maintained at the temperatures T₁ and T₂respectively. Remember that the thermocouple cannot be formed if there are not two junctions. Since the two junctions are maintained at different temperatures the Peltier emf is generated within the circuit and it is the function of the temperatures of two junctions.

If the temperature of both the junctions is same, equal and opposite emf will be generated at both junctions and the net current flowing through the junction is zero. If the junctions are maintained at different temperatures, the emf’s will not become zero and there will be a net current flowing through the circuit. The total emf flowing through this circuit depends on the metals used within the circuit as well as the temperature of the two junctions. The total emf or the current flowing through the circuit can be measured easily by the suitable device.

The device for measuring the current or emf is connected within the circuit of the thermocouple. It measures the amount of emf flowing through the circuit due to the two junctions of the two dissimilar metals maintained at different temperatures. In figure 2 the two junctions of the thermocouple and the device used for measurement of emf (potentiometer) are shown.

Now, the temperature of the reference junctions is already known, while the temperature of measuring junction is unknown. The output obtained from the thermocouple circuit is calibrated directly against the unknown temperature. Thus the voltage or current output obtained from thermocouple circuit gives the value of unknown temperature directly.

Types of Thermocouples:

Before discussing the various types of thermocouples, it should be noted that a thermocouple is often enclosed in a protective sheath to isolate it from the local atmosphere. This protective sheath drastically reduces the effects of corrosion.

1 Type K Thermocouple : (Nickel-Chromium / Nickel-Alumel): The type K is the most common type of thermocouple. It’s inexpensive, accurate, reliable, and has a wide temperature range.

Temperature Range:

Thermocouple grade wire, –454 to 2,300F (–270 to 1260C)
Extension wire, 32 to 392F (0 to 200C)

Accuracy (whichever is greater):

Standard: +/- 2.2C or +/- .75%
Special Limits of Error: +/- 1.1C or 0.4%

2 Type J Thermocouple : (Iron/Constantan): The type J is also very common. It has a smaller temperature range and a shorter lifespan at higher temperatures than the Type K. It is equivalent to the Type K in terms of expense and reliability.

Temperature Range:

Thermocouple grade wire, -346 to 1,400F (-210 to 760C)
Extension wire, 32 to 392F (0 to 200C)

Accuracy (whichever is greater):

Standard: +/- 2.2C or +/- .75%
Special Limits of Error: +/- 1.1C or 0.4%

3 Type T Thermocouple : (Copper/Constantan): The Type T is a very stable thermocouple and is often used in extremely low temperature applications such as cryogenics or ultra low freezers.

Temperature Range:

Thermocouple grade wire, -454 to 700F (-270 to 370C)
Extension wire, 32 to 392F (0 to 200C)

Accuracy (whichever is greater):

Standard: +/- 1.0C or +/- .75%
Special Limits of Error: +/- 0.5C or 0.4%

4 Type E Thermocouple (Nickel-Chromium/Constantan): The Type E has a stronger signal & higher accuracy than the Type K or Type J at moderate temperature ranges of 1,000F and lower. See temperature chart (linked) for details.

Temperature Range:

Thermocouple grade wire, -454 to 1600F (-270 to 870C)
Extension wire, 32 to 392F (0 to 200C)

Accuracy (whichever is greater):

Standard: +/- 1.7C or +/- 0.5%
Special Limits of Error: +/- 1.0C or 0.4%

5 Type N Thermocouple (Nicrosil / Nisil): The Type N shares the same accuracy and temperature limits as the Type K. The type N is slightly more expensive.

Temperature Range:

Thermocouple grade wire, -454 to 2300F (-270 to 392C)
Extension wire, 32 to 392F (0 to 200C)

Accuracy (whichever is greater):

Standard: +/- 2.2C or +/- .75%
Special Limits of Error: +/- 1.1C or 0.4%

NOBLE METAL THERMOCOUPLES (Type S,R, & B):
Noble Metal Thermocouples are selected for their ability to withstand extremely high temperatures while maintaining their accuracy and lifespan. They are considerably more expensive than Base Metal Thermocouples.

6 Type S Thermocouple (Platinum Rhodium – 10% / Platinum): The Type S is used in very high temperature applications. It is commonly found in the BioTech and Pharmaceutical industries. It is sometimes used in lower temperature applications because of its high accuracy and stability.

Temperature Range:

Thermocouple grade wire, -58 to 2700F (-50 to 1480C)
Extension wire, 32 to 392F (0 to 200C)

Accuracy (whichever is greater):

Standard: +/- 1.5C or +/- .25%
Special Limits of Error: +/- 0.6C or 0.1%

7 Type R Thermocouple (Platinum Rhodium -13% / Platinum): The Type R is used in very high temperature applications. It has a higher percentage of Rhodium than the Type S, which makes it more expensive. The Type R is very similar to the Type S in terms of performance. It is sometimes used in lower temperature applications because of its high accuracy and stability.

Temperature Range:

Thermocouple grade wire, -58 to 2700F (-50 to 1480C)
Extension wire, 32 to 392F (0 to 200C)

Accuracy (whichever is greater):

Standard: +/- 1.5C or +/- .25%
Special Limits of Error: +/- 0.6C or 0.1%

8 Type B Thermocouple (Platinum Rhodium – 30% / Platinum Rhodium – 6%): The Type B thermocouple is used in extremely high temperature applications. It has the highest temperature limit of all of the thermocouples listed above. It maintains a high level of accuracy and stability at very high temperatures.

Temperature Range:

Thermocouple grade wire, 32 to 3100F (0 to 1700C)
Extension wire, 32 to 212F (0 to 100C)

Accuracy (whichever is greater):

Standard: +/- 0.5%
Special Limits of Error: +/- 0.25%

Devices Used for Measuring EMF

The amount of emf developed within the thermocouple circuit is very small, usually in millivolts, therefore highly sensitive instruments should be used for measuring the emf generated in the thermocouple circuit. Two devices used commonly are the ordinary galvanometer and voltage balancing potentiometer. Of those two, a manually or automatically balancing potentiometer is used most often.

Figure 2 shows the potentiometer connected in the thermocouple circuit. The junction p is connected to the body whose temperature is to be measured. The junction q is the reference junction, whose temperature can be measured by the thermometer. In some cases the reference junctions can also be maintained at the ice temperature by connecting it to the ice bath (see figure 3). This device can be calibrated in terms of the input temperature so that its scale can give the value directly in terms of temperature.

Thermocouple vs. RTD

Temperature range:
First, consider the difference in temperature ranges. Noble Metal Thermocouples can reach 3,100 F, while standard RTDs have a limit of 600 F and extended range RTDs have a limit of 1,100 F.

Cost:
A plain stem thermocouple is 2 to 3 times less expensive than a plain stem RTD. A thermocouple head assembly is roughly 50% less expensive than an equivalent RTD head assembly.

Accuracy, Linearity, & Stability:
As a general rule, RTDs are more accurate than thermocouples. This is especially true at lower temperature ranges. RTDs are also more stable and have better linearity than thermocouples. If accuracy, linearity, and stability are your primary concerns and your application is within an RTD’s temperature limits, go with the RTD.

Durability:
In the sensors industry, RTDs are widely regarded as a less durable sensor when compared to thermocouples. However, REOTEMP has developed manufacturing techniques that have greatly improved the durability of our RTD sensors. These techniques make REOTEMP’s RTDs nearly equivalent to thermocouples in terms of durability.

Response Time:
RTDs cannot be grounded. For this reason, they have a slower response time than grounded thermocouples. Also, thermocouples can be placed inside a smaller diameter sheath than RTDs. A smaller sheath diameter will increase response time. For example, a grounded thermocouple inside a 1/16” dia. sheath will have a faster response time than a RTD inside a ¼” dia. sheath.

ArticleSource : Bright Hub Engineering ; Thermocoupleinfo

Images Courtesy

1) Book: Mechanical Measurements by Thomas G. Beckwith and N. Lewis Buck

2) https://www.tpub.com/content/doe/h1013v1/css/h1013v1_24.htm