Explain the working of a Thermocouple
What is a K Type Thermocouple?
Understanding the Accuracy of a K Type Thermocouple
Thermocouples are most widely used for monitoring temperature as they are basically electronic sensors. There are many types of thermocouples that are available in the market today, out of which K-type and J-type thermocouples are the most popular or widely used among others. Type R, T, E, B, N, and K, J, S, T, and E types of thermocouples are more widely used and can be grouped together to be known popularly as “base metal thermocouples”. While the B, R, and S types of thermocouples are made using Noble Metals. These types of thermocouples are generally found to be utilised in applications that deal with very high-temperature. The table below helps in understanding the various types of thermocouples along with their temperature ranges and their composition.
|
Type |
Positive Material | Negative Material | Sensibility at 20°C | Range of Temperature °C |
|
E |
Chromel (90% Nickel and 10% chromium) | Constantan (55 % copper and 45 % nickel) | 58.7 |
-270 to 1000 |
|
G |
Tungsten | Tungsten containing 26 % Rhenium | 19.7 (600 °C) |
0 to 2320 |
|
C |
Tungsten 5% Rhenium | Tungsten containing 26 % Rhenium | 19.7 (600 °C) |
0 to 2320 |
|
D |
Tungsten 3% Rhenium | Tungsten containing 26 % Rhenium | 19.7 (600 °C) |
0 to 2320 |
|
J |
Iron | Constantan (55 % copper and 45 % nickel) | 50.4 |
-210 to 760 |
|
K |
Chromel (90% Nickel and 10% chromium) | Alumel (Nickel 5%, along with aluminium and silicon) | 39.4 |
-270 to 1372 |
|
N (AWG 14) |
Nicrosil (84.3% Ni, 14% Cr, 1.% Si, 0.1% Mg) | Nisil (95.5% Ni, 4.4% Si, 0.1% Mg) | 39 |
-270 to 400 |
|
N (AWG 28) |
Nicrosil (84.3% Ni, 14% Cr, 1.% Si, 0.1% Mg) | Nisil (95.5% Ni, 4.4% Si, 0.1% Mg) | 26.2 |
0 to 1300 |
|
B |
Platinum containing 6% Rhodium | Platinum containing 30% Rhodium | 1.2 |
0 to 1820 |
|
R |
Platinum containing 13% Rhodium | Platinum | 5.8 |
-50 to 1768 |
|
S |
Platinum containing 10% Rhodium | Platinum | 5.9 |
-50 to 1768 |
| T | Copper | Constantan (55 % copper and 45 % nickel) | 38.7 |
-270 to 400 |
Explain the working of a Thermocouple
The main function of a thermocouple is to assist in the conversion of temperature in order to produce a small DC voltage. They are formed by the mixture of two dissimilar metal wires meeting in two or more places, as mentioned in the table above. The output voltage tends to change in a linear manner when a temperature difference arises between the two junctions. If you’re in need of high DC voltage output then it is important to remember that this is only possible when the temperature is also high. It is critical to use a suitable tube or coating if you intend to protect your thermocouple. In order to protect the thermocouple wires from getting damaged, it is recommended to use a ceramic tube or a metal sheath.
What is a K Type Thermocouple?
This type of thermocouple has made a name for itself for being the most widely used type of thermocouple. This can be credited to its good accuracy coupled with low cost that provide a wide temperature range to operate in, making it a very flexible and resourceful sensor. They also have the ability to work efficiently even when exposed continuously to temperatures between -328°F to 2,501°F (1,372°C).
There are several types of thermocouple that utilise a wide variety of metals in order to form their negative and positive wires. As K-type thermocouples have a nickel-base they tend to provide good resistance against corrosion. This makes them suitable to be used in oxidising atmospheres. K-type thermocouples are made up of a mixture of Nickel-Chromium (10% chromium) called Chromel which is used to make the Positive leg. As for the negative leg, it is made up of a mixture of Nickel-Aluminium (5% aluminium) known as Alumel. The K-type Thermocouple is known to provide a maximum accuracy of +/- 2.2°C or +/- 0.75%, depending on whichever value seems to be greater. Nevertheless, deviations in the alloys can produce dissimilarities between different thermocouples, regardless of them coming from the same production batch. Hence individual calibration is advised that the thermocouples need to undergo.
K-type thermocouples have a variety of use cases in different industries like chemical and gas, water, and food industries. They are one of the most widely used because they cost less while also providing good resistance along with impressive linearity of measurement, oxidization, and stability.
Understanding the Accuracy of a K Type Thermocouple
As K-type thermocouples are prone to drifting towards the positive direction they can offer stability only for short periods of time that too only at particular temperatures. How much they drift is usually dependent on the temperature they have been exposed to. For instance, greater drift is observed when exposed to higher temperatures. At 1,000°C the readings may differ by as little as 5°C. But exposure for a long duration of time especially to temperatures above 427°C can speed up the ageing of the thermocouple.
Heatcon Sensors is an ISO 9001:2015 certified company and has been in operation for 30 years. Heatcon has received accreditation from certifying bodies like TUV SUD, DRDO CEMILAC, NSIC, and NABL. Production of heat sensing devices like Thermocouples and RTDs has been our forte! We also produce special sensors which find application in numerous industries including the ones specializing in aeronautics, railways, turbines, nuclear plants, and research labs.
With over three decades of expertise in temperature sensor and heater manufacturing, Heatcon Sensors provides a high level of quality, accuracy, and an unbeatable commitment to customer support.
If you are interested in knowing more about thermocouples, thermocouple probes or thermocouple wires and the other services we offer, call us on +91-9844233244.
