Temperature Sensors - A221

Temperature Sensors
Document Type	Article
Document Identifier	221
Themes	Thermal management
Relevant Class	Equipment
Tags	Conductive heating Convective heating Equipment Thermal transformation
Factory Cells	Thermal transformation
Prerequisites	Equipment

Introduction

Temperature sensors are devices that are used to measure the temperature of a part. Most temperature sensors work on the principle of detecting changes in temperature by measuring changes in other physical parameters that are effected by temperature changes, such as voltage or resistance.

Scope

This page covers the fundamentals of temperature sensors commonly used in composites processing/manufacturing. It discusses different types of sensors, examples of use cases for each, and the advantages and disadvantages associated with them.

Significance

The mechanical and chemical properties of a material depend on the thermal history during processing. Hence, to get a better understanding of the thermal history of a material during processing, temperature sensors are frequently used for data collection. In composites manufacturing, temperature sensors are essential for monitoring the temperatures of the parts during curing processes. An accurate measurement of temperature during these processes is essential for certification in industries such as aerospace.

Prerequisites

Recommended documents to review before, or in parallel with this document:

• Equipment
• Sensors

Overview

There are two basic categories of temperature sensors:

• Contact Sensors: These sensors must be in physical contact with the object being measured and can be used to measure the thermal history of solids, liquids, and gases over a wide range of temperatures.
  
• Non-contact Sensors: These sensors interpret the radiant energy emitted by an object as infrared energy within the electromagnetic spectrum. Widely used in hazardous environments, these devices are effective for monitoring non-reflective solids and liquids but tend to be less accurate with gases, primarily due to their low density.
  

There are four types of sensors that are widely used in measuring temperatures, each with their own use cases:

1. Thermocouple
   
2. Thermistor
   
3. Thermal camera (infrared, IR)
   
4. Resistance Temperature Detector (RTD)
   

Types of temperature sensors

Thermocouples

Thermocouples are a type of temperature sensor that uses principles of the Seebeck (thermoelectric) effect to measure temperature. It is simply a junction of two wires composed of different metals with different thermoelectric properties. This junction creates a voltage that is a function of the temperature at the junction. The voltage can be measured and correlated to the temperature of the junction using the Seebeck effect.

The Seebeck effect describes the resultant thermoelectric voltage \(E\) developed between two points in a conductor or semi-conductor due to a temperature difference \(ΔT\) between those two points . The Seebeck coefficient \(\alpha\), defined as the thermoelectric voltage developed per unit temperature difference, can be expressed as:

\(E = \alpha \Delta T \)

where:
\(\alpha \) = Seebeck coefficient, [V/K],
\(E\) = Voltage [V], and
\(T\) = Temperature [K],

See: Thermocouple for more information.

Thermistor

A thermistor is a temperature sensor which operates on the principle that the resistance of a material changes with temperature. There are two types of thermistors: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient) . In NTC thermistors, resistance decreases as the temperature increases, whereas in PTC thermistors, resistance increases with rising temperature.

Resistance vs temperature for a typical NTC thermistor

Thermal Camera

Thermal cameras are a type of non-contact sensors which operate based on the principle that all objects emit infrared radiation if their temperature is above absolute zero temperature . The most commonly used type of thermal camera is equipped with an infrared focal plane array (IRFPA), which captures this radiation and produces a two-dimensional image.

There are generally two categories of detectors in thermal cameras:

• Photon Detectors: These detectors convert absorbed electromagnetic radiation directly into changes in electronic energy distribution within a semiconductor. This occurs through alterations in the concentration of free charge carriers.
  

• Thermal Detectors: These devices capture electromagnetic radiation and convert it into thermal energy, which then causes a rise in the detector’s temperature. The thermal energy is subsequently converted into an electrical output by exploiting changes in a physical property of the material, such as the temperature-dependent electrical resistance found in a bolometer.
  

Example of a thermal image of the outside of an autoclave during an active run

Resistance Temperature Detector (RTD)

A Resistance Temperature Detector (RTD) measures the changes in temperature by correlating changes in electrical resistance. Unlike thermistors, which use semiconductor materials, RTDs use pure metal wires, typically platinum or copper. These metals consistently show an increase in resistance with temperature. RTDs are known for their linear response to temperature changes but are less sensitive compared to the highly sensitive and non-linear thermistors.

Comparison of different sensors

The advantages and disadvantages for the different sensors can be compared as:

Sensor Type	Advantages	Disadvantages	Typical cost
Thermocouple	Inexpensive Wide temperature range	Relatively low accuracy (1°C) Non-linear relationship Susceptible to noise	~10$
Thermistor	Higher sensitivity Inexpensive Fast response time	Non-linear relationship Fragile	~30$
Thermal camera	Large assessment area Produces real time feedback	Inaccurate at very close range Expensive Only surface temperature readings given	~15,000$
Resistance Temperature Detector (RTD)	Linear relationship High accuracy	Expensive Relatively high response time Self-heating properties	~300$

In composites manufacturing processes, temperature sensors are used both in the design of process cycles as well as in-process control. Depending on the particular application, different sensors can be used. For example, to measure the part temperatures accurately, thermocouples are often embedded in sacrificial zones of laminates to accurately measure the temperature of the material during curing. On the other hand, when full field temperature measurements are required, thermal cameras are typically used.

This page links to:

• Thermocouple

Explore this area further

• Temperature Sensors - A221
  • Thermocouple - A227

References

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