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.

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.

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 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.

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.

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

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