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How Does a Temperature Sensor Work?

Temperature sensors are one of the most widely used sensors across various applications such as consumer electronics, automobiles, industrial equipment, IoT devices, and more. They allow monitoring and controlling temperature which is a key parameter affecting performance, safety and reliability.

This article provides a comprehensive overview of the inner workings of various types of temperature sensors. We examine the underlying sensing principles, signal conditioning circuits, packaging considerations and output interfacing for common temperature sensor technologies.

Temperature Sensor Types

Different physical effects utilized by sensors to determine temperature:

Thermocouples – Measure voltage generated due to junction of two dissimilar metals

RTDs – Measure change in electrical resistance of metal element

Thermistors – Utilize semiconductor material’s resistance variation with temperature

IC Sensors – On-chip amplification and conditioning converts voltage/current into digital output

Pyrometers – Measure infrared energy emitted by an object

Bimetallic – Physical deformation of two bonded metals with different expansion coefficients

Thermocouple Temperature Sensors

Operating Principle

Thermocouples rely on the Seebeck effect – generation of voltage due to a temperature gradient along the junctions of two dissimilar metal wires. Common types:

  • K type (Nickel-Chromium and Nickel-Aluminum)
  • J type (Iron and Constantan)
  • T type (Copper and Constantan)
  • E type (Nickel-Chromium and Constantan)

The voltage generated is proportional to the temperature difference. Reference junctions provide cold junction compensation.

Structure

Thermocouples have a simple structure with two wires made of specific alloys joined at one end to form the hot junction. This junction is brought in contact with the target being measured while the open wire ends connect to the measurement system.

Working

When the hot junction experiences a temperature change, a voltage difference is created which causes current to flow through the loop. This voltage is linearly proportional to the temperature gradient. The reference junction provides a baseline cold temperature reading. The system measures and amplifies the small voltage signals in the microvolt range.

Characteristics

ParameterCharacteristic
Operating range-200°C to 2320°C
Accuracy0.5°C to 1°C
Response timeMedium (~200 ms)
CostLow
ProsHigh temperature range, low cost
ConsLow voltage output, noise pickup

Applications

  • Industrial processes running at high temperatures
  • HVAC and refrigeration systems
  • Automotive under-the-hood monitoring

RTD Temperature Sensors

Operating Principle

RTDs or Resistance Temperature Detectors operate on the principle that metals change electrical resistance linearly in proportion to temperature. RTD elements are made of metals like platinum, copper or nickel.

Structure

RTDs contain a fine coiled metal wire encapsulated within a ceramic or glass tube. The entire assembly is enclosed in a protective sheath for mechanical stability. Leads connect the coil to the measurement system.

Working

As the sensing element is subjected to a temperature change, its electrical resistance varies predictably. This change in resistance is measured using a Wheatstone bridge circuit. The variation follows a positive temperature coefficient (PTC) curve. Platinum shows near ideal linearity across a wide range.

Characteristics

ParameterCharacteristic
Operating range-200°C to 850°C
Accuracy±0.1°C to ±0.3°C
Response timeSlow (~10 sec)
CostMedium
ProsExcellent linearity and stability
ConsFragile, slower response

Applications

Thermistors Temperature Sensors

Operating Principle

Thermistors are thermally sensitive resistors made from semiconductor materials like oxides of manganese, nickel or cobalt. Their resistance changes exponentially with temperature as per the material’s unique curve.

Structure

They contain a sintered semiconductor material pellet or chip encapsulated in epoxy, glass or metal housing with lead wires. Different housing styles are available like beads, probes, discs etc.

Working

Thermistors have a large negative temperature coefficient (NTC) implying their resistance decreases rapidly with increase in temperature. The nonlinear change is measured by passing a current and determining the voltage drop. No amplifier is required to condition the output.

Characteristics

ParameterCharacteristic
Operating range-50°C to 300°C
Accuracy0.1°C to 1°C
Response timeFast (<5 sec)
CostVery low
ProsInexpensive, fast response
ConsLimited range, nonlinear

Applications

  • Consumer electronics like mobile phones
  • Battery temperature monitoring

-Automotive sensor circuits

IC Temperature Sensors

Operating Principle

Integrated circuit sensors incorporate amplification, analog-to-digital conversion, compensation and calibration on-chip to provide fully conditioned digital temperature readings.

Different sensing mechanisms are utilized:

  • Voltage difference of diode junctions (silicon bandgap)
  • Change in base-emitter voltage (VBE) of BJT transistors
  • Variation in mobility of charge carriers within transistors

Structure

IC sensors come in tiny surface mount packages like SOT23 containing the silicon microchip. Leads provide power and digital interfaces like I2C/SPI.

Working

The raw on-chip sensor converts temperature into corresponding voltage or current. Support circuits amplify, linearize, digitize and calibrate the signal. The microcontroller interface allows easy integration.

Characteristics

ParameterCharacteristic
Operating range-55°C to 150°C
Accuracy±0.25°C to ±2°C
Response time<500 ms
CostLow
ProsDigital output, fast, integrated
ConsLimited range, power consumption

Applications

  • Smartphones, tablets, laptops
  • IoT and wearable electronics
  • Drones and robotics
  • Smart home automation
  • HVAC and weather stations

Infrared Pyrometer Temperature Sensors

Operating Principle

Pyrometers calculate temperature by detecting the infrared radiation emitted by an object based on its emissivity. They work on the principle that hotter surfaces emit higher infrared energy.

Structure

Pyrometers contain a lens to capture infrared, spectral filter, infrared detector and signal processing circuits. The housing includes a sighting scope for aiming at the measurement target.

Working

The infrared radiation emitted passes through an optical window and is focused onto the detector. The photonic energy generates current that gets converted into voltage and amplified. Calibration curves compensate for emissivity variance.

Characteristics

ParameterCharacteristic
Operating range0°C to 3000°C
Accuracy±1°C
Response timeVery fast (10 ms)
CostMedium
ProsNon-contact, very fast response
ConsEmissivity dependence, distance

Applications

  • Monitoring high temperature surfaces like molten metals
  • Glass and plastic manufacturing
  • Furnaces and kilns
  • Welding monitoring

Bimetallic Temperature Sensors

Operating Principle

Bimetallic strips convert temperature into mechanical deflection by exploiting the different thermal expansion coefficients of bonded metals. Common material pairs are brass-steel and invar-steel.

Structure

They contain two thin strips of dissimilar metals joined together. The bonded assembly is shaped into a coil spiral for enhanced movement. The free end has an indicator, switch or potentiometer.

Working

When heated, one metal expands more than the other causing the bi-metal to bend. The mechanical displacement is proportional to the temperature change. This motion can toggle switches or move a wiper over a resistive element to produce an electrical signal.

Characteristics

ParameterCharacteristic
Operating range-20°C to 150°C
Accuracy±3°C
Response timeSlow (~60 sec)
CostVery low
ProsInexpensive, simple
ConsLow sensitivity, mechanical

Applications

  • Water heaters and cooking appliances
  • HVAC thermostats
  • Automotive engine monitoring
  • Irons, kettles and coffee machines

Temperature Sensor Signal Conditioning

Sensor output signals require conditioning before feeding to instrumentation. Common conditioning circuits:

Linearization

Linearizes nonlinear sensor outputs like in thermistors for consistent readings over full scale. Done using op-amps, A/D converters and linearization equations.

Amplification

Amplifies small sensor voltages up to usable levels. Important for thermocouples. Uses instrumentation amplifiers to minimize noise.

Filtering

Removes noise through low pass filters. Prevents aliasing errors in subsequent digitization.

Digitization

Analog to Digital Converters (ADCs) convert conditioned sensor voltage into digital values proportional to measured temperature. Provides interface to processors.

Calibration

Applies correction factors to eliminate sensor inaccuracies over temperature range. Done by characterizing sensor behavior and supplying coefficients.

Output Interfacing

Standard serial interfaces like SPI, I2C provided for connecting to microcontrollers or networks.

Temperature Sensor Packaging

Packaging plays an important role in:

  • Protecting the sensing element from environmental damage
  • Preventing contact with measured medium
  • Allowing rapid response to temperature changes
  • Isolating sensor electrically and thermally
  • Enabling straightforward integration

Common packaging types:

Plastic housing – Low cost, lightweight, resistance to chemicals

Ceramic casing – Withstands high temperatures, inert to chemicals

Metal sheaths – Robust protection in fluid applications

IC packagesSMD styles like SOIC, SOT-23 easy to integrate

Specialized probes – Shapes optimized for specific uses like air, surface and penetration

Temperature Sensor Selection Criteria

Factors to consider when selecting a temperature sensor:

  • Measurement range required
  • Desired accuracy and repeatability
  • Speed of response
  • Size constraints
  • Measurement environment – heat, humidity, pressure etc.
  • Electrical characteristics – analog vs digital output
  • Integration requirements – conditioning circuits, interface etc.
  • Application operating conditions – vibration, shock, EMI etc.
  • Compatibility with processing system
  • Calibration needs
  • Cost considerations

By carefully weighing these aspects, the optimal temperature sensing solution can be identified for any application need.

Common Temperature Sensor Applications

Temperature sensors find ubiquitous use across industrial, commercial and consumer applications:

Process monitoring – Chemical plants, oil refineries, pharmaceutical equipment etc.

HVAC/R – Air conditioners, heaters, thermostats, refrigerators etc.

Automotive – Engine control units, cabin climate control etc.

Consumer electronics – Mobile phones, computers, home appliances etc.

Medical – Diagnostics equipment, sterilization systems etc.

Food/chemical – Food processing, chemical synthesis, cold storage etc.

IoT – Smart devices, wireless sensor networks etc.

Frequently Asked Questions

What is the main difference in working of RTD and thermocouple?

RTDs measure temperature by change in electrical resistance of the sensor element while thermocouples generate voltage based on junction of two dissimilar metals.

How does emissivity affect pyrometer sensors?

Emissivity is a material property defining how efficiently it emits infrared energy. Pyrometers need to be calibrated for target emissivity for accurate non-contact temperature measurement.

Why are linearization circuits required for some temperature sensors?

Sensors like thermistors have an inherent nonlinear relationship between temperature and electrical parameters. Linearization converts this to a linear scale for consistent measurements.

What are the main considerations in temperature sensor packaging?

Important packaging considerations are protection from environment, fast response time, isolation from measured medium, robustness for the application, integration with electronics and cables/connectors.

What are some key selection criteria for choosing a temperature sensor?

Important parameters are measurement range, accuracy, speed of response, size, measurement environment, electrical interface type, application operating conditions, calibration needs and costs.

 

 

 

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