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How to Choose a Thermocouple

When selecting a thermocouple, the following items will need to be considered: temperature range, reaction time, abrasion / vibration resistance, chemical resistance, calibrations, installation and compatibility.

There are many types of thermocouples available in several different calibrations. The most commonly used thermocouples today are K, T, J, E, but are also available in other calibrations. Each calibration offers a specific temperature range and is selected carefully for the environment in which it is to be used.

Thermocouple Material Selection

{Compacted MgO Thermocouples}

All Temprel MgO insulated thermocouples are made using the highest purity MgO for temperatures up to 2300°F (1260°C). The thermo-elements are all ANSI special limits of error to give your measurements the best possible results. The various sheath materials are dependent on the application and the following will help you make the best selection.

304 SS Maximum temperature of 1650°F (900°C) and is the most widely used low temperature sheath material. It offers good corrosion resistance but is subject to carbide precipitation in the 900°F to 1600°F (480 to 870°C) range.

310 SS Maximum temperature of 2100°F (1150°C) and offers good mechanical and corrosion resistance similar to 304 SS. Very good heat resistance. Not as ductile as 304 SS.

316 SS Maximum temperature of 1650°F (900°C) and has the best corrosion resistance of the austenitic stainless steels. Subject to carbide precipitation in the 900°F to 1600°F (480 to 870°C)

Inconel® Maximum temperature 2150°F (1175°C) and is the most widely used thermocouple sheath material. Good high temperature strength, corrosion resistance and is resistant to chloride-ion stress corrosion, cracking and oxidation. Do not use in sulfur bearing environments.

Hastelloy X Maximum temperature 2200°F (1205°C) widely used in aerospace applications. Resistant to oxidizing, reducing and neutral atmospheric conditions. Excellent high temperature strength.

Thermocouple Diameter Selection

Standard Sheath Diameters Suggested Upper Temperature Limits
.020” +.001 -.0005” 1290°F (700°C)
.032” +.001 -.0005” 1290°F (700°C)
.040” +.001 -.0005” 1290°F (700°C)
.063” ±.001” 1690°F (920°C)
.090” ±.001” 1830°F (1000°C)
.125” +.002 -.001” 1960°F (1070°C)
.188” +.002 -.001” 2100°F (1150°C)
.250” +.003 -.001” 2100°F (1150°C)

{All MgO compacted thermocouples can be bent on a radius of twice the sheath diameter.}

Selection of the Measuring Junction

Sheathed thermocouples are available in 3 junction types: grounded, ungrounded, and exposed. Each design offers specific advantages as well as disadvantages and must considered carefully when placing an order.

The thermo-elements are welded into the end cap using the same weld rod as the sheath material. Fast time response. Recommended for high-pressure applications, liquids, moisture, contaminating atmospheres, and most general uses. -Least expensive.

The thermo-elements are welded together and are electrically isolated from the sheath. Recommended for applications where stray EMF’s from electrical apparatus would affect the reading. -More expensive.

The thermo-elements are welded together outside of the sheath. This provides the fastest time response, but exposes the elements to contamination. Electrically isolated from the sheath.

Inconel® is the registered trademark of INCO Alloys International.

Hastelloy X is the registered trademark of Haynes International.

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Temprel, Inc., Controls, Control Sys & Regulators, Boyne City, MI