Contact Sensors vs Non-Contact Sensors

Sometimes a sensor is directly attached to another component while some sensors are placed away from the component and use light or radiation for sensing. Sensors are classified into contact and non-contact sensors based on the mounting of the sensor. This classification of sensors is further discussed in the below article.


  1. Overview of Contact Sensors and Non-contact Sensors
  2. Comparison of Contact and Non-contact Sensors
  3. Liquid in Glass Thermometers
  4. Bimetallic Thermometers
  5. Float Level Sensors
  6. Optical Pyrometers
  7. Fiber Optic Temperature Sensors
  8. Radar Level Sensors

Overview of Contact Sensors and Non-Contact Sensors

Contact sensors are devices that are directly attached to the object itself. For example, if the temperature of a liquid is to be measured, there are sensors which can be placed inside the container itself. Such sensors are known as contact sensors.

Non-Contact sensors are sensors that are not attached to the object, but they are placed at a distance and emit radiation or light for measurement. The same temperature measurement can also be done by non-contact sensors which are placed far away from the container. Such sensors are non-contact sensors.

Comparison of Contact and Non-Contact Sensors

The below table compares the structure, advantages and disadvantages of contact and non-contact sensors.

Parameter Contact Sensors Non-Contact Sensor
Construction Simple A little complicated
Working principles Directly attached to the body Placed far away from the body
Average Cost Relatively lesser as it doesn’t involve complex construction Higher as it uses a radiation or light for measurement
Efficiency Easy to operate but not accurate Accurate and precise
Major Applications Small scale applications Industrial applications
Examples Liquid Thermometers, Thermistors, etc. Pyrometers, RADAR Level Sensors, etc.

Liquid in Glass Thermometers

Liquid glass thermometers are the most commonly used thermometers. This type of thermometer is placed inside the container or on the human body for temperature measurement making it a contact type sensor. The construction and working of this sensor are explained below.

  • This type of thermometer consists of a reservoir that contains a liquid that expands according to the temperature of the surrounding area, typically mercury.
  • When the temperature around the reservoir is increased, the temperature of the liquid increases and it starts to expand.
  • There is a scale that is mounted near the tube in which the liquid is expanded.
  • The temperature of the surrounding can be found using the amount of liquid expanded and is seen in the scale.
  • These types of thermometers are mostly used to find human body temperature.

Bimetallic Thermometers

A bimetallic thermometer is a contact type temperature measurement device that is used in air conditioners, heaters etc. for measuring the temperature. The working construction and the working of the bimetallic thermometer is as follows:

  • Most of the metals expand or shrink when the temperature of the metal changes. This fact is the working of the bimetallic thermometers.
  • But not every metal change at the same rate. The amount of expansion or shrinkage varies from metal to metal.
  • A bimetallic thermometer uses two different metals with variable bending coefficients glued together into a single strip.
  • When the temperature increases, the metal with lower temperature coefficient shrinks and metal with a higher temperature coefficient expands.
  • As a result, the whole metallic strip will bend to turn in the direction of the metal that shrinks.
  • The amount of bending produced in the strip is directly proportional to the temperature and the temperature can be calculated by calculating the bending effect.

Float Level Sensors

Float level sensors are also contact type sensors that are used in level measurement. These sensors are most commonly used when the viscosity of the liquid is high. The working of this type of sensor is explained below.

  • A small material or a float with very little density is allowed to float inside on the surface of the liquid inside the container.
  • There are two types of float sensors – the manual type and the electronic sensor.
  • The manual type electronic sensor consists of a tape that is connected to the float and by the reading on the tape, the level can be measured.
  • However, this float sensor is not widely used because sometimes it can be practically impossible for someone to climb to the top of the tank or a container.
  • The electronic float sensor consists of a cable that is connected to an external circuit.
  • From the length of the cable, the level of liquid in the container can be measured.

Optical Pyrometers

An optical pyrometer is a non-contact type sensor that is used to measure the temperature. When the temperature of the material is so high, it is tough to measure its temperature by a contact-type thermometer as it may damage the instrument. The optical pyrometers are used in such cases. The construction and working of such sensors are as follows:

  • The main component of the optical pyrometer is a filament that is placed inside a lamp.
  • The source whose temperature is to be measured is focused by an objective lens.
  • There is a fixed temperature for the filament in which the filament will not be visible at all.
  • If the temperature of the source is more than the threshold temperature, the filament will be brighter and thin.
  • If the temperature of the source is lesser than the threshold temperature, the filament will be darker and thick.
  • By identifying the thickness of the filament, the temperature of the source can be measured.

Fiber Optic Temperature Sensors

The fiber optic temperature sensors are devices that measure the wavelength of the light emitted from the source in order to calculate the temperature of the source. It can be used in almost every application where temperature measurement is needed. The construction and working of the fiber optic temperature sensor is as follows.

  • This sensor also uses the thermal expansion principle just like the bimetallic temperature sensor.
  • It uses a certain type of gratings which are also known as fiber Bragg gratings for measurement.
  • The wavelength of the light through these gratings is known as the Bragg’s wavelength.
  • Bragg’s wavelength is proportional to the temperature of the source object.
  • By analyzing the Braggs wavelength, the temperature of the source can be measured.
  • The main advantage of this type of sensor is that the measurement range of temperature is so wide that it can be used in many applications.

RADAR Level Sensor

A RADAR level sensor is a non-contact level measurement device. This is a simple yet efficient device. The overview of a RADAR level sensor is explained below.

  • This sensor consists of a rod antenna that emits microwaves into the container whose level is to be measured.
  • The transmitter microwave touches the surface of the liquid inside the container and reflects back.
  • The reflected wave is then received by the sensor and the time taken for the wave to reach back to the sensor is measured.
  • The time taken is inversely proportional to the level of the liquid inside the container.
  • The main advantage of RADAR level sensors is that the efficiency of the sensor is not easily affected by the temperature of the liquid.
  • Thus, it is widely used in many industrial applications.

Key Points to Remember

Here are the key points we need to remember about “Contact Sensors Vs Non-Contact Sensors”

  • A sensor is a contact sensor if it is directly placed on or inside the body whose parameters are to be measured.
  • If it is placed far away from the sensed material, then the sensor would be a non-contact sensor.
  • The liquid glass thermometers use a thermally expandable liquid for measurement.
  • The bimetallic thermometer consists of two metal strips glued together for measuring the temperature.
  • The float level sensor allows a small low-density material to float on the liquid surface to measure the level.
  • The optical pyrometers are non-contact type temperature sensors that uses a filament and a lens.
  • The fibre optic temperature sensor consists of a Bragg’s grating whose wavelength is used to measure the temperature.
  • The RADAR level sensor uses microwaves for level measurement.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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