Introduction to thermocouples

Despite being around for over 80 years, the selection and application of thermocouples continues to confound many users. Here's a bit about the basics of thermocouples. A thermocouple is two dissimilar electrical conductors joined together to produce a thermal electromagnetic force (EMF) when the measuring and reference junctions are at different temperatures.

11/01/2003


Despite being around for over 80 years, the selection and application of thermocouples continues to confound many users. Here's a bit about the basics of thermocouples.

A thermocouple is two dissimilar electrical conductors joined together to produce a thermal electromagnetic force (EMF) when the measuring and reference junctions are at different temperatures.

Junction purpose

The measuring junction is the part of a thermocouple junction subjected to the temperature to be measured.

The reference junction is the part of a thermocouple junction maintained at a known temperature, or for which temperature variations are automatically compensated.

NOTE : In normal industry practice, the thermocouple element is terminated at the connection head. However, the reference junction is seldom located in the connection head but is transferred to a controlled environment by the use of appropriate thermocouple type extension wire.

Junction types

Sheathed thermocouples are available in three junction types.


Sheathed thermcouples are available with three junciton types.

The tip of a grounded junction probe is physically attached (welded) to the inside of the probe. This results in good heat transfer from the outside, through the probe wall to the thermocouple junction. The grounded junction is recommended for measuring temperatures of static or flowing corrosive gas and liquid temperatures and for high-pressure applications.

In an ungrounded probe, the thermocouple junction is detached from the probe wall and surrounded by a soft powder. Ungrounded junction types respond slower than the grounded style but do provide electrical isolation. Thermocouples with ungrounded junctions are recommended for measurements in corrosive environments where it is desirable to have the thermocouple electronically isolated from and shielded by the sheath.

The exposed junction type thermocouple permits the tip to extend into the surrounding environment. This type offers the best response time, but is limited in use to non-corrosive, non-hazardous, and non-pressurized applications.

Response time

Response time is measured in terms of time constants. One time constant is defined as the time required by a sensor to reach 63.2% of a step change under controlled conditions. Exposed junction thermocouples are the fastest responding. Also, the smaller the probe's sheath diameter, the faster the response, however the maximum allowable temperature may be lower.

Extension wires

Thermocouple extension wire is a pair of wires that exhibit the same temperature electromagtic frequency characteristics as the thermocouple to which they are connected. When properly connected, the extension wires transfer the reference junction point from the thermocouple to the other end of the wires, which is usually a controlled environment.

Choosing a thermocouple

Consider the following when selecting a thermocouple:

  • Temperature range;

  • Required response time;

  • Junction type;

  • Chemical resistance of the thermocouple or sheath material;

  • Abrasion and vibration resistance; and

  • Installation requirements and restrictions.

To learn more about thermocouples, visit www.controleng.com, obtain a copy of ISA-MC96.1-1982 standard "Temperature measurement thermocouples," and/or visit www.omega.com/techref/themointro.html

Comments? Email dharrold@reedbusiness.com





No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Sensor-to-cloud interoperability; PID and digital control efficiency; Alarm management system design; Automotive industry advances
Make Big Data and Industrial Internet of Things work for you, 2017 Engineers' Choice Finalists, Avoid control design pitfalls, Managing IIoT processes
Engineering Leaders Under 40; System integration improving packaging operation; Process sensing; PID velocity; Cybersecurity and functional safety
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Flexible offshore fire protection; Big Data's impact on operations; Bridging the skills gap; Identifying security risks
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
click me