Temperature sensors - Part 2

Temperature sensors - Thermocouples

We discussed in the previous article (Temperature Sensors definition, types, and uses) the meaning of the temperature sensor, which is a device that converts the temperature to an electrical signal. There are many temperature sensor types such as temperature switches, thermostats, RTDs, and thermocouples as well as the PT100 and PT1000 sensors. This article will discuss the thermocouples temperature sensors definitions, advantages, disadvantages, types, and applications. and the pyrometer devices used in temperature measurements will be discussed later.

What are Thermocouples?

As per the Seebeck effect which was discovered in 1821, if two metals are connected at one end (the connection is called a junction), there will be a change in the output electrical signal at the other end terminals. this electrical signal is directly proportional to the temperature difference between the two points.

In the below drawing, the two metals metal A and Metal B are connected to form a junction B. When this junction has a temperature variation, there will be a variation in the millivolts measured by a multimeter at the two terminals C and D.

What are the advantages of the thermocouples?

The advantages of the thermocouples are:

Wide range of measurements from -270 to 2329 °C.
Heavy-duty measuring device (rugged) works in a very high temperature.
It can work in different applications.
It is a low-cost and inexpensive.
Self-powered doesn't need an external power supply when working as a stand-alone device.

What are the disadvantages of thermocouples?

The disadvantages of the thermocouples are:

Low output signal and need more devices in the automatic control system such as amplifiers or transmitters.
Low accuracy compared to some temperature sensors like RTDs.
Affected by the electrical noise in some manufacturing processes.
Its materials (metals) could have a corrosion.

What Are the types of thermocouples?

There are different types of thermocouples which depend first on the working operation temperature and materials used for forming the thermocouples junction. The temperature ranges of temperature for the thermocouples start from -270 oC to 1870 oC below is a brief of the thermocouple types:

Type E.

The materials used in type E are (Chromium / Constantan). E's output is considerably high (about 68 µV/°C), which suits the low-temperature application (-270 to 1000). It is also non-magnetic.

Type K.

The materials used in type K are (Nickel-Chromium / Nickel-Aluminum). Type K is the most used type because of its temperature range. It covers the most use of temperature used in 'general purpose' thermocouples. It has a low cost and it is available in a wide variety of probes. Thermocouples are available in the -270 °C to +1372 °C range. Its Sensitivity is about 41 µV/°C.

Type J.

The materials used in type J are (Iron / Constantan) and its range is from -40 to +750 °C) type J is not widely used like type K. Type J applications are in the middle-temperature range. It cannot be used over 760 °C because of decalibration problems.

Type N.

The materials used are (Nicosia / Nisil). Its properties are that it has High stability and high-temperature oxidation resistance making it suitable for temperature applications. Its cost is lower than the types that use platinum materials with high temperatures.

Type B.

The materials used are (Platinum 30% - Rhodium / Platinum). it is used with high-temperature applications (could reach 1800). It has a low sensitivity. It cannot be used under 50 °C. and it has a very high cost because of using platinum and rhodium.

Type R.

The materials used are (Platinum 13% - Rhodium / Platinum). Used with high-temperature measurements till 1600 °C. it has Low sensitivity (10 µV/°C). Their cost is high which is why they are not used for general purposes.

Type S.

The materials used are (Platinum 10% - Rhodium / Platinum). Used for high-temperature measurements till 1600 °C. Low sensitivity (10 µV/°C). it has a high cost and cannot be used in general-purpose measurements same as type B and S. gold (1064.43°C).

When selecting thermocouple types, we have to ensure that measuring devices do not limit the range of temperatures that can be measured.

The thermoelectric series

The thermoelectric of elements can be used to build thermocouples and represent the Seebeck effect. The following list gives the materials in the thermoelectric series used in thermocouples manufacturers:

Silicon – Bismuth – Nickel – Cobalt - Palladium – Platinum - Uranium - Copper – Manganese – Titanium - Mercury – Lead – Tin – Chromium – Molybdenum – Rhodium – Iridium – Gold – Silver – Aluminum – Zinc – Tungsten - Cadmium – Iron – Arsenic – Tellurium – Germanium.

Choosing the material used in thermocouple manufacturing depends on many factors such as the operation temperature range, the measurement accuracy, and the cost (especially when there is the availability of a low-cost one. The following table shows the brief of the most widely used thermocouples, their type, and their operation temperature range.

Because of the terminal wires of the measuring device (normally copper), the connection leads and the thermocouple junction itself, there will be two more thermocouples. Those two thermocouples will cause an error in the measured temperature value by making other junctions at points C and D (it is called the reference temperature).

At these two points, the overhaul-generated mV signal is the summation of the original junction mV plus the mV results in the terminals and leads. As a result of that, there will be an error value of the temperature measured value.

To overcome that error and measure the temperature accurately, the effect of the reference temperature should be eliminated. there were two processes of correction. The first one is called the (Ice bath) where a frozen path is used at the atmospheric pressure and 0 °C to keep the reference temperature 0 at all times.

The second method is called (cold junction compensation) where the reference temperature is measured by another thermocouple and deducted from the overhaul measured value. The next drawing shows the structure of the seebeck effect configuration stating the reference junctions.

Many parameters could affect the temperature measurement such as the junction generated and the voltage drop across the cable because of the long distance or the cable condition because of the working condition.

The manufacturers developed a special cable called a compensation cable that is used for the connection with the thermocouple terminals. This cable will eliminate the error value and the output signal will be accurate. This compensation cable name takes the same name as the type of thermocouple.