Thermocouple elements are passive temperature sensing elements that are useful over a wide range of temperatures. The sensing element contains a junction of two dissimilar metals. The bimetallic junction is based off of specific alloys that have a predictable and repeatable relationship between temperature and voltage. They are highly sensitive to small temperature fluctuations and respond quickly to temperature changes. Since thermocouples sense temperature differences and not absolute temperatures they are known to suffer from temperature drift and require recalibration to remain accurate temperature readings.

Working Principle

A thermoelectric circuit with a continuous current is established by joining the ends of two dissimilar wires and heating one of the junctions. An open circuit voltage, referred to as a Seebeck voltage, is observed if the circuit is broken at the center. The Seebeck voltage is the thermocouples sensor signal which correlates to the temperature differential.

Seebeck effect

The thermoelectric effect used to produce a thermocouple is known as the Seebeck effect. Discovered by Thomas Johann Seebeck in 1821, the Seebeck effect is the production of an electromotive force (emf) or Seebeck voltage when a conductor is subjected to a temperature gradient. Thermocouples operate by adjoining two materials that generate different voltages under the same temperature gradient. Measurement of the open circuit voltage across the bimetal junction yields a voltage that corresponds to the observed temperature gradient. The relative temperature measured by the thermocouple element is the difference between the absolute temperature at the bimetal junction and the other end of the thermocouple wire. A reference source is used in conjunction with thermocouple elements to produce a temperature reading.

Seebeck coefficient

The Seebeck coefficient, also known as thermoelectric sensitivity, of a material is a measurement of the magnitude of the induced electrical potential in response to the thermal gradient. The Seebeck coefficient is represented by the SI units’ volts per Kelvin, (V/K), which is more commonly represented as microvolts per Kelvin, (µV/K). To create and open circuit voltage conductors with contrasting Seebeck coefficients are paired, one with a positive and the other a negative Seebeck coefficient operating as a cathode and anode.

Metal type

Metal type is an important consideration when selecting thermocouple elements. Each thermocouple type denotes a pair of conductors that operate linearly within a known temperature range. Selection of thermocouple types is dependent on their temperature range, accuracy, and environmental compatibility.

Thermocouple types include base metal thermocouples, noble metal thermocouples, and refractory metal thermocouples.

• Base metal thermocouples are those composed of common metals and metal alloys and may also be described as general purpose thermocouples. They are the most abundant thermocouples types and include types E, J, K, N, and T.
• Noble metal thermocouples are manufactured using alloys of platinum and rhodium. They are designed for use in higher temperature applications and include types B, R, and S.
• Refractory metal thermocouples, also known as type W thermocouples, are manufactured from the exotic metals tungsten and rhenium. They are expensive, brittle, and have little to no oxidation resistance. They have the highest operating temperature and are commonly used in vacuum furnaces. They include types G, D, and C. 

The following chart show the conductors used, the Seebeck coefficient, and the operating temperature range for the most common thermocouple types:

Wire Gauge

Wire gauge is another important consideration when selecting thermocouple elements. The wire gauge represents the nominal diameter of the thermocouple wire. Thermocouples may be available in multiple or optional gauges. Gauge choices include the following:

• 8 (nominal diameter 0.16 in)
• 14 (nominal diameter 0.08 in)
• 16 (nominal diameter 0.064 in)
• 18 (nominal diameter 0.048 in)
• 20 (nominal diameter 0.036 in)
• 24 (nominal diameter 0.022 in) 

Wire gauge is known to limit the maximum temperature range. Selection of thermocouple elements with larger nominal diameter wires increases its response across the full temperature range.

Insulation / Sheath Material

Insulation or sheath material often dictates environmental compatibility, response time, and the maximum operating temperature. The sheath material should be selected based on these criteria. Available sheath materials include:

• Bare / No Insulation
• Alumina
• Ceramic
• Fiberglass
• Mullite
• PVC
• Teflon 

Thermocouple Junction

Sheathed thermocouple elements are available with one of three junction types, each with its own benefits and limitations.

Grounded elements are welded or joined to the sheathing. A grounded junction decreases the response time and is recommended for high-pressure applications, oxidizing environments, and most general applications.

Ungrounded elements have a welded junction that is electrically isolated from the sheath material. Ungrounded elements have a longer response time, although they allow for increased accuracy when stray EMF jeopardizes the temperature reading.

Exposed elements have the fastest response time and remain isolated from the sheath material as the exposed junction extends beyond the sheath. Exposed junctions are sensitive to environmental conditions and may fail due to contact with liquids, contaminants, or other oxidizing agents.

Styles

Thermocouples are available in several different styles including bare wire, full-length 2-hole, full-length 1-hole, and flexible.

• Bare wire devices consist of a bare wire element with no protective insulation.
• Full-length 2-hole insulators position both wires in a single insulator.
• Full-length 1-hole insulators provide each wire with its own insulator.
• Flexible thermocouples incorporate a flexible sheathing or contain flexible sections. 

Performance Specification

Performance criteria used to select a thermocouple element includes the following:

Operating temperature is the full ambient temperature range over which there is a linear output without jeopardizing the materials of construction.

Response time is the required time for a sensor to reach 63.2% of a step change in temperature under normal operating conditions. Media flow rates, installation, and contact area can all affect the response time of the thermocouple element.

Accuracy is the difference between the sensors measurement and the absolute value of the parameter being measured, usually expressed as ±°C.

Standards

IEC 61515 - This International Standard establishes the requirements for mineral insulated thermocouple cables and for mineral insulated thermocouples but does not specify cold end seals, terminations, connections and other accessories. This standard deals only with cables and thermocouples having one pair of base-metal conductors and is intended for use in general industrial applications.

ASTM E2846 - This guide describes tests that may be applied to new or previously used thermocouples for the purpose of verification. Some of the tests perform a suitable verification by themselves, but many tests merely alert the user to serious problems if the thermocouple fails the test. Some of the tests examine inhomogeneity and others detect wire or measuring-junction breakage. For Style U mineral-insulated metal-sheathed (MIMS) thermocouples with ungrounded measuring junctions, this guide includes tests that examine the electrical isolation of the sheath as well as sheath deterioration.

ESDU 06018 - This is one of a series of data items concerned with temperature measurement providing background information and practical guidance on the design of temperature measuring systems using thermocouples.

Resources

How to Choose a Thermocouple

Thermocouples

Thermocouple Theory (pdf)

Image Credits:

Thermometrics Corporation | National Instruments | Acrolab Ltd.