Temperature measurement is the measurement of the average kinetic energy of the particles in an object. When temperature increases, the motion of these particles increases.
Temperature is one of the most widely measured properties.
Thermocouples are one of the most commonly used temperature sensors. They are found in a wide range of industries and applications.
How do thermocouples work?
A thermocouple consists of two wires of different metals. These dissimilar metals are joined together to form a measurement junction often referred to as a hot junction, this junction is placed at the location you require the temperature measuring. There is a second reference or cold junction which is located away from the temperature to be measured. A voltmeter measures the voltage generated by these dissimilar metal junctions, this is called the electromotive force. The cold junction is referenced to 0°C modern thermocouple indicators automatically by the means of an electronic circuit to compensate for this cold junction and allow for the type of thermocouple being used and display the temperature of the hot junction in the chosen unit of measurement.
Types of thermocouple
There are many types of thermocouple commercially available. They all have particular properties that make them more suitable for certain environmental conditions, temperature ranges and accuracy requirements.
The type is identified by the letter it is given e.g. Type K. The colour of the wires are specified by the International Electrotechnical Commission (IEC) and other international standards so that they can be easily identified when in the field. For example, according to the IEC requirements; Type K have a green outer sheaf, a white negative leg and a green positive leg.
We will discuss the most common types found in the industry. These are types K, J, T & N.
Type K Thermocouple (Nickel-Chromium / Nickel-Aluminum)
Type K thermocouples are the most widely used thermocouples because they have a large operating range, and are relatively low cost. Type K thermocouples are widely used in the nuclear industry because of their radiation hardness.
Type J Thermocouple (Iron / Constantan)
Type J Thermocouples are also widely used. They cost a little less than Type K but have a reduced working range and when used below ambient temperature are susceptible to corrosion of the iron leg. Type J can be used in a reducing atmosphere which most other thermocouples can’t.
Type T Thermocouple (Copper / Copper-Nickel)
Type T Thermocouples originally were called Copper-Constantan. These are ideal for low-temperature measurements and are widely used in laboratories for the temperature range -200°C to + 400°C Type T’s also have excellent repeatability of +/- 0.1°C
Type N Thermocouple ( Nickel-Chromium-Silicon / Nickel-Silicon)
Type N Thermocouples are a relative newcomer to the thermocouple family. The invention of the new thermocouple was expected to replace the Type K with its similar cost, greater oxidation resistance and increased temperature range; it can handle up to 1280°C for short periods. The repeatability at 300°C. Type N Thermocouples are widely used in Aerospace ovens and autoclaves.
Resistance Temperature Detector RTD or Resistance Thermometer
Like thermocouples these come in many different types and manufactured to different standards, we will discuss how these work and a couple of the most common types.
The basic model of operation. As the temperature increases or decreases the resistance changes of the measuring sensor.
Platinum Resistance Thermometers PT100
The most common type of RTD is the Platinum Resistance Thermometers PT100. The PT100 has a resistance of 100 ohms at 0°C hence its name. Its resistance changes by 0.392 Ohms for every degree centigrade change in temperature.
The advantages of the PT100 vs a thermocouple is for lower temperature applications it is greatly more accurate. A typical working range for a PT100 is -200 to 800°C, the actual range is dependent on the type of RTD probe and manufacturers recommendations and specifications should be followed carefully.
RTD’s do not require compensation cables and in theory, any cabling can be used, they also don’t require a cold junction measurement to calculate the actual temperature.
The disadvantages of a PT100 is that it is more expensive. Long cable runs need to be avoided and it might be necessary to use a transmitter over longer runs.
One of the other drawbacks of RTD’s is the resistance in the leads connecting it to the temperature indicator will introduce an error. To overcome this, the most basic set up of two wires can have either a 3rd or 4th wire added to the circuit to help elevate the resistance in the other leads.
3 wire RTD’s are very common in industrial applications and 4 wire is reserved for high accuracy application mainly in laboratories and calibration laboratories. The 3 or 4 wire set up is more expensive than the 2 wire for two reasons the sensor is more expensive and you will need additional cabling for the 3rd or 4th wire.
Two wire RTD connection:Three wire RTD connection:Four wire RTD connection: