RTD Working Principle

Hi friends,
In this article, I am going to discuss the RTD working principle, its construction and applications. It is also known as a resistance thermometer. A resistance temperature detector is a passive resistive transducer. It is used to measure the change in temperature.

RTD Working Principle

As we know that every metal has a definite value of resistance at a particular temperature. Also, this value of resistance changes with the change in its temperature and is very predictable. So we can calculate the temperature of metals by knowing its resistance. The resistance of an RTD at any temperature (RT) can be calculated from the following formula:
RT = Ro(1 + αT)
Where, Ro = resistance of RTD at 0oC.
α = temperature coefficient of resistance
for PT 100, Ro = 100 ohms, α = 0.00385 ohms/oC.
The resistance temperature detector uses the change in electrical resistance of the sensing element to determine its temperature. This is the basic resistance temperature detector working principle.

rtd working principle, working principle of rtd

The metals having a positive temperature coefficient is used for the construction of RTDs. Mostly, platinum is used. It is a standard material for the RTDs. It is used mostly for this purpose because:

  • It can withstand high temperatures.
  • Its stability is excellent.
  • It is a noble metal. So it is very less susceptible to contamination.
  • Its temperature resistance characteristics are most linear for the wide range of temperature. Therefore, it has a wide operating range (from – 200oC to + 850oC).

Apart from platinum, some other metals like copper and nickel are also used for the construction of RTDs. The nickel has greater sensitivity due to its higher temperature coefficient.

Requirements of Material for Sensing Element

  • Change in resistance of material per unit change in temperature should be as large as possible.
  • It should have a high value of resistivity so that a minimum volume of material is used.
  • Its resistance should have a continuous and stable relationship with temperature.
  • The effective range of an RTD mainly depends upon the material used for the sensing element.

RTD Working

It is generally used in process industries to monitor the change in temperature.  Suppose we want to monitor the temperature of steam flowing through a pipeline. To do this work, the probe of an RTD is inserted into the pipeline. Due to the temperature of the steam, the resistance of the sensing element of the RTD varies correspondingly.

rtd principle, working of rtd

The RTD (or its sensing element) is connected to a Wheatstone Bridge circuit by a suitable cable. The bridge circuit consists of three resistors and the RTD acts as the fourth resistor. This bridge circuit converts the variation in resistance into the variation in volts.
In this way, we get the variation in temperature in the form of variation in voltage. The voltmeter is calibrated in terms of temperature to show the temperature readings.  You should use only very high impedance milli-voltmeter or digital voltmeter for this purpose otherwise lead resistance of RTD will become effective and the result will be erroneous.

RTD Connection to Wheatstone Bridge

The RTD can be connected to the Wheatstone Bridge in the following three ways:

  • Two-wire connection,
  • three- wire connection,
  • four- wire connection.

Two-wire connection: This connection scheme is shown in Figure. This is a very cheap and simple method but used very rarely due to its lower accuracy. Because in this arrangement, the effect of the resistance of the connecting cable is also included in the output voltage shown by the voltmeter. 
Three wire connection: This connection scheme is shown in Figure. This is a simple modification of the two wire connection scheme. Due to this simple modification, the effect of the resistance of the connecting cable is eliminated from the output voltage shown by the voltmeter.

resistance temperature detector construction

But this method will give only accurate results if all the three connecting leads will have equal resistance. This method is mostly used in industries. Three wire RTDs are used in this method.
Four wire connection: This is a very complex and expensive scheme. It is used for high precision measurement applications only.  It is generally preferred in laboratories.

Platinum Resistance Temperature Detectors

Platinum resistance temperature detectors are mostly used in industries. These are of two types. These are:

  • PT 100
  • PT 1000

PT 100 Resistance Temperature Detector

In the case of PT 100 RTD, the term PT stands for Platinum and the term 100 stands for 100 ohms. It means the sensing element of the PT 100 RTD is made up of platinum metal and it offers 100 ohm resistance at the 0oC. Its resistance temperature coefficient is 0.00385 ohms/oC.  These are of wire wound type and their use is very common in industrial applications. They are easily interchangeable also.

PT 1000 Resistance Temperature Detector

In the case of PT 1000 RTD, the term PT stands for Platinum and the term 1000 stands for 1000 ohms. It means the sensing element of the PT 1000 RTD is made up of platinum metal and it offers 1000 ohm resistance at the 0oC.  These are thin film devices and are costlier than PT 100 RTD.
Other platinum RTDs like PT 200 and PT 500 are also available. But these are more costly and used rarely.
Thanks for reading about “rtd working principle”.

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