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Selecting MIL-SPEC Thermocouple Contacts: Calibration, Temperature Range, Resistance

When selecting a thermocouple contact, the following data need to be considered:

  • Thermocouple temperature range
  • Reaction time
  • Abrasion/vibration resistance
  • Material chemical resistance
  • Calibrations
  • Installation specifics
  • Compatibility

Thermocouple Calibration

 Different thermocouple types are best suited for different applications and are found in various calibrations. They are usually selected on the basis of the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions.  The most common thermocouples today are K, T, J and E. Each calibration offers a specific temperature range and must be selected carefully to match the environment in which it will operate.

Temperature range selection – Thermocouple Calibration

    Material

    ASTM E230

    Description

    Color Code

    Connector Operating Temperature

    Alumel

    K

    Type Kn Thermocouple

    200 Deg C

    Chromel

    K

    Type Kp Thermocouple

    200 Deg C

    Constantan

    J

    TypeJn Thermocouple

    200 Deg C

    Iron

    J

    Type Jp Thermocouple

    200 Deg C

    Copper TC

    T

    ASTM E 230 Type T

    200 Deg C

    Be Cu

    ASTM B

    Copper Alloy

    200 Deg C

    leaded Nickel Copper

    ASTM B

    Copper Alloy

    200 Deg C

Thermocouple Material Selection

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

 

  • 304 SS
    Maximum temperature of 1650F (900C) and is the most widely used low temperature sheath material. It offers good corrosion resistance but is subject to carbide precipitation in the 900F to 1600F (480 to 870C) range.
  • 310 SS
    Maximum temperature of 2100F (1150C) and offers good mechanical and corrosion resistance similar to 304 SS. Very good heat resistance. Not as ductile as 304 SS.
  • 316 S
    Maximum temperature of 1650F (900C) and has the best corrosion resistance of the austenitic stainless steels. Subject to carbide precipitation in the 900F to 1600F (480 to 870C)
  • Inconel®
    Maximum temperature 2150F (1175C) 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 2200F (1205C) 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” 1290F (700C)

    1290F (700C)

    .032” +.001 -.0005” 1290F (700C)

    1290F (700C)

    .040” +.001 -.0005” 1290F (700C)

    1290F (700C)

    .063” .001” 1690F (920C)

    1690F (920C)

    .090” .001” 1830F (1000C)

    1830F (1000C)

    .125” +.002 -.001” 1960F (1070C)

    1960F (1070C)

    .188” +.002 -.001” 2100F (1150C)

    2100F (1150C)

    .250” +.003 -.001” 2100F (1150C)

    2100F (1150C)

(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.

  • Grounded
    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.
  • Ungrounded
    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.
  • Exposed
    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®
    The registered trademark of INCO Alloys International.
  • Hastelloy X
    The registered trademark of Haynes International.

Other Topics to Consider:  thermocouple accuracy, thermocouple temperature range, type of thermocouple and range, thermocouple connectors, thermocouple connector types, thermocouple cable, thermocouple cable manufacturers, thermocouple materials, thermocouple junction, calibration of thermocouples.

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