Electronics

Pt100 Sensor- Workings, Applications, Pros, Cons and Testing

Pt100 sensor is a type of Resistance Temperature Detector (RTD) that detects the temperature based on the change in the value of the resistance. Since it can withstand high vibration, it is used in high vibration applications such as refining, petrochemical, and power and energy industries. In health sectors, it is widely used in machines such as freezes, incubators, stability chambers, humidity chambers, water baths, ovens, etc.

Pt 100 Sensor

Here, the term Pt100 comprises two parts. The first one is the term ‘Pt’ which refers to platinum. It implies that the sensor is made up of platinum. Then the second term refers to the resistance in ohms at 0°C. In conclusion, we can say that the Pt100 sensor is the platinum sensor that measures 100 Ω at 0°C.

Pt100 Sensor Construction

Construction of Pt100 Sensor
Construction of Pt100 Sensor

Pt100 sensor consists of four main components. They are sheath, a resistance element, lead wire, and the terminator or the connector. The resistance element is the element that senses the change in the temperature and gives the result in the form of resistance. Mostly platinum, nickel, or copper is in use. The resistance element is in contact with the lead wire (head support). The lead wire extends from the sensing element to the connector. The sheath is the outer protective layer of the sensor. It protects the resistance element and the lead wire from the external environment and moisture. It also helps to stabilize and immobilize the sensor. The terminator or the connector connects the RTD to the control circuit. It can consist of two, three, or four wires.

Pt100 Sensor Working Principle

Pt100 sensor operates on the principle of temperature coefficient. For metal, there is a direct relationship between temperature and resistance. In other words, as the temperature of the metal increases, the resistance also increases.

When the probe of the sensor comes in contact with the measuring object, there will be a change in the temperature of the sensor. Due to this, there will be a change in the resistance of the sensing element. This change in the resistance gets converted into the signal with the help of the Wheatstone Bridge.

Why is Platinum Preferred

(Pt100 Sensor Temperature Range)

As stated already, the material used in making an RTD is platinum, nickel, or copper.  Even though platinum is a quite rare and expensive material, it is mostly used.

In comparison to other materials, platinum has got higher stability and linearity. Thus, it can be used for measuring a wider range of temperature. The range can approximately be around -200 to 500°C. However, it is mostly used within the range of -50 to 250°C.

Platinum has higher resistivity of 59.00 in comparison to that of nickel and copper. A material with higher resistivity implies a low volume of the material can be used as a sensing element.  Besides that, platinum is highly corrosion resistive and chemically inert. Thus, it is preferred more.

Types of Pt100 Sensors

There are a number of ways to differentiate the types of Pt100 sensors. Here we will discuss them.

1. On the Basis of Lead Configuration

(Pt100 Sensor Wiring)

Lead Configuration of the Pt100 Sensor, RTD
Lead Configuration of the Pt100 Sensor

On the basis of lead configurations, the Pt100 sensor can mainly be of two, three, or four wires.

In the given figure, Pt refers to the platinum element. B and R refer to the black and red leads. Similarly, Rb, Rr, and Rp are the resistances associated with the black lead, red lead, and platinum element. The wires can also have different color coding then this one.

Two wires sensor is the least preferred due to its least accuracy. This is because when you use a control circuit to measure the resistance of the resistive element, you will not only be measuring the resistance of the element but also will be measuring that of the lead wires. In other words, the resistance of the lead wires will give you a fault reading.

Three wires sensor is the most popular and standard form of the temperature sensor. In this case, you will have two black lead wires and a single red lead wire. You can see in the figure that the two black lead wires are in parallel to each other. Thus, when the two resistors are in parallel combination, the equivalent resistance decreases. Thus in total, the resistance delivered by the lead wires will decrease. So, it provides higher accuracy than that of the two wires lead.

Finally, there is four wires sensor. This provides better accuracy than that of the two wires or the three wires sensor.

2. On the Basis of Number of Sensing Elements

On the basis of a number of sensing elements, there are two types of Pt100 sensors available in the market. They are as follows.

  • Simplex RTD:- Simplex RTD consists of a single sensing element within. They can either be of two or three lead configurations. In conclusion, these kinds of sensors will either have two or three terminals.
  • Duplex RTD:- Duplex RTD consists of two sensing elements within. They can either be of two or three lead configurations. In conclusion, these kinds of sensors will either have four or six terminals in total.

3. On the Basis of Design of Sensing Element

There can be numerous shapes of the sensing elements. The most common design is a flat film. Other shapes can be coiled, wire-wounded, etc.

Advantages of Pt100 Sensor

Some of the advantages of the Pt100 sensor are as follows.

  • Pt100 sensor can withstand a higher range of vibration of the machines.
  • It can operate at a wider range of temperatures with higher accuracy.
  • It is easy to install and easily available.

Disadvantages of Pt100 Sensor

Some of the disadvantages of the Pt100 sensor are as follows.

  • In comparison to other types of sensors, it has got a lower sensitivity.
  • You need an extra bridge circuit to measure the temperature. You also need an extra current source.
  • It can have a higher initial cost.
  • The response time is lower than that of the thermocouple.
  • There can be a possibility of self-heating the probe when the current source is provided.

How to Calculate Temperature from Resistance?

You can find out the temperature of the measured object by knowing the resistance of the temperature sensor. For that, use the formula shown below.

A= ( Rt – R0) / α   

where, At =  Actual temperature of an object that you are trying to measure with the given RTD

Rt = Resistance of the given sensor measured with the help of the multimeter

R0 = Resistance of the given sensor at 0°C

α = Temperature coefficient of the given material

Let us take an example for a Pt100 sensor

Let Rt = 111.8 Ω

R0 = 100 Ω (at 0°C)

α= 0.385 (for platinum material )

Then, At = (111.8 – 100)/ 0.385 °C

= 30.64 °C

In this way, we will be able to calculate the temperature of the given object.

How to Test Pt100 Sensor?

There are a couple of ways to check the condition of the Pt100 sensor. Here we will discuss some of them.

  • If the Pt100 sensor is used in the machine along with the PID controller or any other display board, the temperature will be displayed on the screen. Now, hold the tip of the sensor in your hand. The sensor should display the temperature of your hand on the display board. Since it measures an external body temperature, you should get the temperature slightly below 37 °C. Also, check for any moisture on the sensor. In such a case wipe it away with the tissue paper or your hand. If the sensor is wet, it may not be able to work properly.
  • You can also test the Pt100 sensor separately. For that, take two or three separate glasses of water having different temperatures. Then, dip the tip of the sensor in one of the glasses and measure the resistance between two different colored wires. Repeat the same process with the remaining glasses as well. Now observe the value of resistance. Temperature and resistance should be directly proportional to each other. Also, check the continuity between the same colored wires (if you have).  If the sensor fails to pass both the criteria, then it can be faulty.
  • You can also take boiling water in the glass. Then dip the tip of the sensor and measure its resistance. Finally, use the formula given above to calculate the actual temperature of the water. For the boiling water, the result should be around 100 °C.

 

 

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