Thermocouple - A227

Thermocouple
Document Type	Article
Document Identifier	227
Themes	Thermal management
Relevant Class	Equipment
Tags	Conductive heating Convective heating Equipment Thermal transformation
Factory Cells	Thermal transformation
Prerequisites	Equipment

Introduction

Thermocouples (TCs) are devices used to measure temperature based on the Seebeck (thermoelectric) effect. The fundamental mechanism is the conversion of a thermal signal (temperature change) to an electric signal (voltage change). They are one of the commonly used sensors used for monitoring the thermal history of composite parts during manufacturing.

Scope

This page provides an overview of the fundamental principles of thermocouples, explaining how they operate and the different types available. It also discusses the specific applications of thermocouples in the field of composites and offers a preliminary discussion on how they should be installed.

Significance

The thermal history of a composite part is an essential measure for quality control and certification processes. Hence, temperature sensors like thermocouples are used to measure and monitor the thermal history if a part during production.

Prerequisites

• Sensors
• Temperature Sensors

Level I

Level II

Object Description

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],

A thermocouple consists of two wires of made of two dissimilar metals or alloys with different thermoelectric properties joined at one end. When a temperature difference is applied between the two ends, the Seebeck effect results in different voltages developed across the two wires. The difference in the developed voltage is calibrated and used to measure the temperature drop across the two ends of a wire.

Types of thermocouples

Thermocouples can be manufactured with different combinations of metals or alloys which make suitable for different applications based on their temperature sensitivities.

Comparison of different types of commonly used thermocouples* :

TC Type (ANSI Color Code)	Material		Color Code (ANSI/ASTM E230)		Typical Maximum Useful Thermocouple Grade Temperature Range# + (°C)	Standard Limits of Error (above 0°C)	Special Limits of Error (above 0°C)
	+ Lead	- Lead	+ Lead	- Lead
J (Black)	Iron	Constantan (Copper-Nickel)	White	Red	0 to 750°C	greater of 2.2°C or 0.75%	greater of 1.1°C or 0.4%
K (Yellow)	Nickel-Chromium	Nickel-Aluminum	Yellow	Red	-200 to 1250°C	greater of 2.2°C or 0.75%	greater of 1.1°C or 0.4%
T (Blue)	Copper	Constantan (Copper-Nickel)	Blue	Red	-250 to 350°C	greater of 1.0°C or 0.75%	greater of 0.5°C or 0.4%
N (Purple)	Nicrosil (Nickel-Chromium-Silicon)	Nisil (Nickel-Silicon-Magnesium)	Purple	Red	-200 to 900°C	greater of 2.2°C or 0.75%	greater of 1.1°C or 0.4%
E (Orange)	Nickel-Chromium	Constantan (Copper-Nickel)	Orange	Red	-270 to 1300°C	greater of 1.7°C or 0.5%	greater of 1.0°C or 0.4%

Notes:

• ^* This list is not exhaustive of all types of thermocouples.
• ^# The sensing point (or probe part) of the thermocouple is made out of thermocouple grade wire. Extension grade wire is used to extend a thermocouple signal from the probe to the instrument reading the signal. Hence, the extension grade temperature range is typically lower than the thermocouple grade temperature range and range might vary between different thermocouple manufacturers and suppliers. Please refer to the specifications supplied by the manufacturer for the actual temperature ranges.
• ⁺ The thermocouple grade temperature range denotes the temperature range for the thermocouples wire only. The actual temperature range will depend on the temperature ratings for the sheath and connectors as well.

Advantages

The major advantages in using thermocouples are:

• Economical - For example, typically J type thermocouples wires cost about $2 per foot and each connector costs about $5-7. This enables using multiple thermocouples at low costs, enabling more accurate temperature measurements for large parts as well.
• Fast response times - The response time of thermocouples are typically in the order of seconds. The response time of a thermocouple type depends on the diameter of the probe or sheath as well as the actual configuration. For example, the response time of butt-welded thermocouple would be lower than a beaded-type thermocouple, for the same wire diameter.
• Wide temperature range - The different temperature range for different types of thermocouples make them versatile to be used for a variety of applications.

Limitations

• Non-linear response - The Seebeck coefficient is a temperature dependent property which leads to non-linear response for thermocouples. Hence, it becomes essential to calibrate thermocouples before use.
• Susceptible to electrical noise - The measurements obtained from thermocouples can often be affected by electrical interference. This necessitates proper shielding and grounding of the thermocouple wires.
• Local measurement - thermocouples are unable to provide full field temperature measurements. However, this can be compensated by installing a large number of thermocouples to replicate a full field measurement.

Applications

Thermocouples are used in a variety of applications where it is necessary to measure and monitor temperature. The temperature data is often used for control setups, quality management, as well as safety sensors (with appropriate design). In the context of composites manufacturing, TCs are widely used in:

• Autoclave processes - To measure the tool and part temperatures
• Hot press - To measure the temperature of the moulds as well as parts

Example of a tool-part configuration instrumented with thermocouples for measuring temperature during an autoclave cycle

Safety

The temperature rating of the thermocouple wires, wire sheath as well as connectors need to be noted before selecting any thermocouple type for a specific application. In case the likely operating temperatures are higher than the temperature rating for any of the components of a thermocouple, there might be thermal degradation or risk of a fire hazard while using the thermocouple.

Key considerations during usage

The following are some of the key aspects to focus on to ensure proper usage of thermocouples for measuring temperatures:

Procurement

Thermocouples are available in a wide range of types and can be procured in different configurations of wire gauges, insulation material, and cable lengths. Each choice has an effect on the sensitivity of the thermocouple during use and needs to be considered before procurement. Some of the key aspects to be considered are:

• Type: The type of thermocouple to be procured depends on the application, particularly the chemical environment and the range of process temperatures possible. Since different types of thermocouples consist of wires made of different materials, it needs to be verified if any of the wires might react with the environment during the process. The thermocouple type used during the process should also be able to measure the range of possible temperatures.
• Wire gauge: The thickness of the thermocouple wire can affect the accuracy of measurements. Thicker wires can act as heat sinks leading to an error in measuring the actual temperature.
• Conductor insulating material: The choice of conductor insulating material has an effect on the useful temperature range for the thermocouple.
• Standard or Special Limit of Error (SLE): Thermocouples with Special Limit of Error (SLE) are made of higher grade of thermocouple wire which leads to increased accuracy in measurements. Consequently, they are more expensive as well.

For example, the thermocouple grade temperature range of K type thermocouples is typically -200 to 1250°C. For a thermocouple with polyimide insulation, the useful temperature range would be 0 to 260°C, while for PVC insulation, it would be 0 to 105°C. If it is a standard thermocouple, the error would be the greater of ±2.2°C or 0.75%, and if it is an SLE thermocouple, the error would be the greater of ±1.1°C or 0.4%

Storage & Handling

The storage of thermocouples is an important consideration, especially if multiple types are being used at the same facility. The representative color codes of thermocouple types can be extended to storage bins for the thermocouples as well.

• 

  Example of appropriate storage of J type thermocouple wires

• 

  Example of appropriate storage of K type thermocouples

Selection of appropriate TC type

The type of thermocouple that can be used for a particular application depends on a number of factors. The primary criteria would be to verify if the operating temperatures are within the bounds of the useful range of the thermocouple type. It is important to note that while the thermocouple grade temperature range is typically higher than the extension grade temperature range, it is the extension grade temperature range that will be the deciding factor. Also, the temperature rating of connectors or DAQs might limit the useful range of a thermocouple type.

Calibration

The calibration of thermocouples is important to ensure accurate measurements during use. In applications where thermocouple measurements are used in certifying quality requirements, regular and proper calibration of thermocouples is a critical activity. Calibrating the thermocouple by itself as well as with any connectors/data acquisition systems used during the actual application should be done according to need. There are different methods of calibrating thermocouples which are based on comparing the reading from the thermocouple to actual known temperature values.

Installation

The installation of thermocouples depends on the physical object which is to be measured as well as its surroundings. The key considerations while installing and using thermocouples include:

• Ensuring proper contact of the thermocouple with the object to be measured: This is applicable especially while measuring temperatures of solid objects, it is essential to ensure the thermocouple remains in contact with the object throughout the time of interest.
• Ensuring insulation from surroundings: In cases where the object itself is at a very different temperature from its surroundings, it is essential to ensure that the thermocouple is sufficiently insulated from the surroundings to ensure it is able to measure the true object temperature.

Error in measured data

The data obtained from thermocouples can often be erroneous or noisy. The different sources of error include, but are not limited to:

• Intermittent or improper contact: If the thermocouple is not installed correctly, it may lose contact entirely or intermittently during an application leading to erroneous data measurement.
• Contamination: When used in corrosive or reactive environments, the thermocouple might get contaminated leading to a change in the thermoelectric properties which will lead to erroneous measurements.
• Improper grounding: Thermocouples are designed to be grounded at only one location. In case there are multiple ground locations, the measurement accuracy will be impacted.

A quick test to check the accuracy and calibration of thermocouples is to measure the temperature of ice-water(0°C) and boiling water(100°C) individually.

Suppliers

Common providers of thermocouples include:

• Omega [1]
• TE Wire [2]

References

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