Instrumentation Temperature Measurement

What is a resistive transducer and what are its types?

What is a transducer?

A transducer is a device that would receive energy from one system and then this energy will be transmitted to another form. Transducer would give us a usable output as a response to the given input. The transducers can measure physical, mechanical, or optical quantity. So basically a transducer would receive a signal from one system and then this signal will be converted after that it will be transmitted. These devices can be actuated by an energizing input from one or more transmission media and in return, it would generate an equivalent signal to one or more transmission systems.

What is a resistive transducer?

A resistive transducer is a transducer whose resistance would vary because of its environmental factors. So the resistance of this transducer would change if there is any change in the physical phenomena. A resistive transducer can be used to measure many physical quantities such as temperature, pressure, force, displacement, vibrations, etc. Mostly these transducers are used as secondary transducers in which the output from the primary mechanical transducer would act as an input for the variable resistance transducer. A resistance transducer is also known as a variable resistance transducer.

How does a resistive transducer work?

The resistive transducers operating principle is that the resistance of the conductor would change if there is a mechanical change in the conductor. The physical quantity measurement is really difficult so this transducer would convert the physical quantities to variable resistance which can be easily measured by the meters. The resistive transducer can work as both the primary and the secondary transducer, the primary transducer would convert the physical quantities to a mechanical signal while the secondary transducer would convert it into an electrical signal. This type of transducer is mostly used to check the pressure, temperature, force, vibration, displacement, etc.

What are the types of resistive transducer?

Strain gauge

This type of transducer is a passive one and it can measure the physical changes with the help of the change in resistance. So basically this device would check the variation of the resistance in order to measure the strain which is produced. This type of transducer is used to measure the weight, pressure, mechanical force, and displacement. We can easily measure the strain by using a variable resistance transducer. If the conductor is stretched then it would change its resistance due to the variation in the diameter and also the length of the conductor. The resistivity of the conductor also changes if the strain is introduced to it. By using a strain gauge we can measure the mechanical surface tension, this transducer can detect and convert the force of small mechanical displacement into electrical signals. The metal would change its resistivity if it is under strain and this property of the metal is called a piezo-resistive effect.

Potentiometer

The potentiometer has a resistive element that is near to a sliding contact which can be seen in the above image. The sliding contact is called a wiper, the potentiometers are used for voltage division. The operating principle of this type of transducer is that the material which is under measurement would make a resistance change in the sensing element. The sliding contact is placed on the resistive element. This transducer can be considered as an electrical circuit and its resistance can be manually changed by the slider.

The wiper would move horizontally, the wiper movement would change the value of the resistive element and then this change will be measured. The wiper displacement is converted to an electrical signal.

Advantages

  • Low cost
  • Simple operation
  • Large amplitude displacement can be measured
  • High electrical efficiency and would also provide the required output for the control operations

Disadvantages

  • A large force is required to move the slider of a linear potentiometer
  • The sliding contact could be damaged over time, it can be misaligned, and also would produce noise.

Thermistor

A thermistor is a transducer that can be used to measure the change in temperature, so if there is a change in temperature this device would vary its resistance. This device can sense the temperature and it is composed of a semiconducting material that would quickly respond to a small change in temperature. The thermistor is of two types one type has a negative coefficient and the other type has a positive coefficient. So according to the material which is used to construct this device the resistance change would occur. Thermistors are non-metallic resistors and if the thermistor is of positive coefficient then its resistance would increase due to a rise in temperature. If the thermistor is a negative coefficient type then its resistance would decrease with a rise in temperature and this change in resistance is measured to detect the change in temperature. The selection of the thermistor would be based on the nominal resistance that we need at the operating temperature range, on the size, and also on the time constant.

Advantages

  • Quick response
  • Small in size
  • Low cost
  • High sensitivity

Disadvantages

  • It is not applicable for a wide temperature range
  • The excitation current is required to avoid self-heating
  • Resistance and temperature characteristics are non-linear

Resistance hygrometer

This transducer is capable to detect the humidity content in the atmosphere. This device is an electrical hygrometer and is an active transducer. This device working is affected by atmospheric contamination and also because of the operation in saturated conditions. This transducer uses resistive hygrometer humidity sensors and this device would change its resistance according to the humidity. So in this device, hygroscopic materials are placed between two electrodes on an insulating substrate. The hygroscopic salt is placed between the two electrodes and its resistance would change if there is any change in its environmental humidity. We must not expose this device to a hundred percent humidity because it could damage the device.

Resistance temperature detectors (RTD)

These devices are conductors which are used for sensing the temperature, these transducers are also known as resistance thermometers. The working principle of this transducer is that the metal would change its resistance according to the temperature. So the RTD operation is based upon the relation between the temperature and resistance of the metal, if the metal temperature increases then the resistivity to the current flow will increase too. Electric power will be transmitted to the metal which is an RTD element and this metal is located in the area where the temperature is to be measured. So during the change in temperature, the resistance would vary and this resistance value will be correlated to temperature according to the known resistance temperature characteristics of the RTD element. The metal element of the RTD could be platinum, nickel, or other elements whose resistance would change according to the temperature.

Thermocouples

If we place together two metals that have different work functions, then a voltage will be generated at the junction which is almost proportional to the temperature and this junction is called a thermocouple.

This type of transducer does its working according to certain effects and they are

Peltier effect

In this it would describe that the production of EMF is according to the temperature of the junction

Thomson effect

In this type it would relate the reversible thermal gradient and EMF in a homogeneous conductor.

See beck effect

This effect states that when a pair of dissimilar materials are joined at one end and there is a temperature difference between the joined ends and the open ends, thermal EMF is generated and it can be measured in the open ends.

All of these effects are used in a thermocouple if the output voltage from a thermocouple changes when its temperature changes or the temperature of the body in whose contact it is changes. Then this voltage output can be used to detect the change in the temperature of the body.

What are the advantages of a resistive transducer?

  • Quick response
  • This type of transducers are available in different sizes and they have high resistance
  • We can use alternating or direct current to calculate the variable resistance
  • Low cost
  • Very easy operation
  • Electrical efficiency is high and because of this proper output will be received for the control operations

What are the applications of a resistive transducer?

  • It can be used for temperature calculation applications
  • Displacement can be measured by using this transducer
  • The resistance property of the device would change due to the temperature so it can be used for the temperature measurement

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