Temperature Transducers

Definition of Temperature Transducer

A Temperature Transducer is a device that converts the thermal quantity into any physical quantity such as mechanical energy, pressure and electrical signals etc.
E.g. In Thermocouple the electrical potential difference is produced due to temperature difference across its terminals. So, thermocouple is an temperature transducer.

Main Features of Temperature Transducers

  • The input to them are always the thermal quantities
  • They generally converts the thermal quantity into electrical quantity
  • They are usually used for the measurement of the temperature and heat flow

Basic Scheme of Temperature Transducers

The basic scheme of temperature transducers is given below in following steps
Sensing Element
The sensing element in the temperature transducers is the element whose properties change with change in temperature. As the temperature changes the corresponding change occurs in certain property of the element.
Example- In the Resistance Thermometers the sensing element is the Platinum metal.
Desirable Conditions for Choosing the Sensing Element are as

  • Change per unit resistance of material per unit change in temperature should be large
  • The material should have a high resistivity so that minimum volume of material is used for its construction
  • The material should have continuous and stable relationship with temperature
  • Transduction Element
    It is the element that transforms the output of the sensing element into electrical quantity. The change in the property of the of sensing element acts as the output for it. It measures the change in the property of sensing element. The output is of transduction element is then calibrated to give output which represents the change in the thermal quantity.

Example- In the thermocouple the potential difference produced across the two terminal is being measured by voltmeter and magnitude of voltage produced after calibration gives idea of change in temperature.

Types of Temperature Transducers

Contact Temperature Sensor Types

In these the sensing element is in direct contact with the thermal source. They use the conduction for transfer of thermal energy.

Non-contact Temperature Sensor Types

In these the sensing element is not in direct contact with the thermal source. They use principle of convection for heat flow. Various temperature transducers that are generally Used,

Thermistor

The word thermistor can be termed as Thermal Resistor. So as the name indicates it is a device whose resistance changes with the change of the temperature. Due to there high sensitivity they are widely used for the measurements of the temperature.They are usually called the ideal temperature transducer. Thermistors are generally composed of mixture of metallic oxides.
thermistor

Properties of Thermistors

  • They have Negative Thermal Coefficient i.e. resistance of the thermistor decreases with increase in temperature
  • They are made up of the semiconductor materials
  • They are made sensitive than RTD (Resistance Thermometres) and Thermocouples
  • There resistance lies between 0.5Ω to 0.75 MΩ
  • They are generally used in applications where measurement range of temperature -60oC to 15oC.

Resistance Thermometers

Another type of temperature transducer is the Resistance Temperature Detector or RTD. RTD’s are precision temperature sensors made from high-purity conducting metals such as platinum, copper or nickel wound into a coil and whose electrical resistance changes as change of temperature, similar to that of the thermistor.
rtd
There resistance changes with following relation as,

R = Resistance of element at given temperature
α = Thermal coefficient of element
Ro = Resistance of element at 0oC

Main Features of RTD’s

  • They are highly sensitive and very cheap as compared to thermistors and thermocouples
  • They can measure the temperature from -182.96oC to 630.74oC

Thermocouples

Thermocouples are temperature transducers that are basically consists of two junctions of dissimilar metals, such as copper and constantan that are welded. One junction is kept at a constant temperature called the reference (Cold) junction, while the other the measuring (Hot) junction. When the two junctions are at different temperatures, a voltage is developed across the junction which is used to measure the temperature.

Principle of Thermocouple

thermocouple
When the junctions of two metals such as copper and constantan are connected together the potential difference is produced is produced between them. The phenomenon is called the Seebeck effect as a temperature gradient is generated along the conducting wires producing an emf. Then the output voltage from a thermocouple is a function of the temperature changes.

Main Features of Thermocouples

  • Extreme temperatures of range between -200oC to over +2000oC can be measured with thermocouples which is an advantage over both RTD and Thermistor
  • They are the Active Transducers so they don’t require any external source for measuring of temperature as like RTD’s and Thermistors.
  • They are the cheaper than both RTD’s and Thermistors.
  • These have small accuracy as compared to RTD’s and Thermistors so generally they are not used for high precision work.

Integrated Circuit Temperature Transducers

These are the temperature transducers which use the temperature sensing element with monolithic electronic circuits as a combination for the measurement of temperature.
They have following type

  • LM 335 – it provides an output of 10 mV/oK
  • LM 34 – it provides an output of 10 mV/oF
  • AD 592 – it provides a current output of 1µA/oK

Description of LM 335 Series

It is an temperature sensitive zener diode, which reverse biased into its breakdown region when sensor senses any temperature changes and gives output as,

θ = Temperature in oC

Main Features of Integrated Temperature Transducers

  • They are linear temperature transducers
  • They are very cheap
  • They have operating range is small between 0 – 200oC which is the main disadvantage of them.

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