Thermocouple temperature probes are bimetallic probes that are used in various temperature-sensing applications. They consist of two wires, each of which is made of a different metallic element or alloy. The wires are joined at one end to form a measuring junction that is exposed to the fluid or medium under measurement. The other end of the wire is terminated at a reference junction.

Current flows through a circuit when the two junctions of thermocouple temperature probes are at different temperatures. The voltage resulting from this current flow is then measured to determine the temperature.

Types of Thermocouple Temperature Probes

Grounded thermocouples have a tip with wires that are physically attached to the inside of the probe wall. This results in good heat transfer from the outside, through the probe wall to the thermocouple junction.

Ungrounded thermocouples have a junction which is detached from the probe wall. Consequently, the response time is slower than with grounded thermocouples. An advantage of ungrounded thermocouples, however, is that they offer electrical isolation of 1.5 M1/2 at 500 VDC in all diameters

Exposed thermocouples have an exposed junction that protrudes from the tip of the sheath and extends into the surrounding environment. These thermocouple temperature probes provide the best response time, but are limited to only non-corrosive and non-pressurized applications.

Specifications

• Base, noble, and refractory metal type is an important specification to consider when selecting thermocouple temperature probes.
• Lettered choices for thermocouple type include E, J, K, N, T, B, S, R, W, and C.
• The element configuration for thermocouple temperature probes may be single, double, or more than two.

There are many choices for probe configuration. Examples include:

• Straight
• Penetration
• Flexible
• Drop
• Curved
• Roller or moving
• Air
• Angled
• Surface
• Clamp
• Strap sensor
• Flat or ribbon

Additional specifications for thermocouple temperature probes include diameter or width, sensitive length, and temperature range.

Parameters

Parameters for thermocouple temperature probes include process connection and sensor termination type.

Process Connection 

• None or hand held
• Bayonet
• Threaded fitting
• Compression
• Brazed
• Welded
• Soldered

Sensor Termination Type 

• Bare or insulated lead wires
• Metal braided, armor, and flexible conduit leads
• Terminal blocks
• Conduit connectors, and screw tabs or lugs
• Integral connection heads or thermoheads
• Plugs or quick connects.

Thermocouple temperature probes for specific applications are also available.

Standards

ASTM MNL12 - Manual on the use of thermocouples in temperature measurement

ASTM E230/E230M - Standard specification and temperature-electromotive force (EMF) tables for standardized thermocouples

ASTM E2846 - Standard guide for thermocouple verification

Image credits:

CCI Thermal Technologies, Inc. | Automationdirect.com | Glas-Col, LLC