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
| Parameter | Characteristic |
|---|---|
| Operating range | -200°C to 2320°C |
| Accuracy | 0.5°C to 1°C |
| Response time | Medium (~200 ms) |
| Cost | Low |
| Pros | High temperature range, low cost |
| Cons | Low 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
| Parameter | Characteristic |
|---|---|
| Operating range | -200°C to 850°C |
| Accuracy | ±0.1°C to ±0.3°C |
| Response time | Slow (~10 sec) |
| Cost | Medium |
| Pros | Excellent linearity and stability |
| Cons | Fragile, slower response |
Applications
- Medical instruments
- Food processing systems
- Chemical reactors
- Aerospace electronics
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
| Parameter | Characteristic |
|---|---|
| Operating range | -50°C to 300°C |
| Accuracy | 0.1°C to 1°C |
| Response time | Fast (<5 sec) |
| Cost | Very low |
| Pros | Inexpensive, fast response |
| Cons | Limited range, nonlinear |
Applications
- Consumer electronics like mobile phones
- Battery temperature monitoring
-Automotive sensor circuits
- IoT sensor nodes and devices
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
| Parameter | Characteristic |
|---|---|
| Operating range | -55°C to 150°C |
| Accuracy | ±0.25°C to ±2°C |
| Response time | <500 ms |
| Cost | Low |
| Pros | Digital output, fast, integrated |
| Cons | Limited 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
| Parameter | Characteristic |
|---|---|
| Operating range | 0°C to 3000°C |
| Accuracy | ±1°C |
| Response time | Very fast (10 ms) |
| Cost | Medium |
| Pros | Non-contact, very fast response |
| Cons | Emissivity 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
| Parameter | Characteristic |
|---|---|
| Operating range | -20°C to 150°C |
| Accuracy | ±3°C |
| Response time | Slow (~60 sec) |
| Cost | Very low |
| Pros | Inexpensive, simple |
| Cons | Low 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 packages – SMD 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.