Thermocouple Options & Descriptions

  • Thermocouple Overview

     

    Thermocouples Overview

     

     

     

     

     

    Beaded Wire Thermocouple

     

    A beaded wire thermocouple is the simplest form of thermocouple. It consists of two pieces of thermocouple wire joined together with a welded bead.  Because the bead of the thermocouple is exposed, there are several application limitations.  The beaded wire thermocouple should not be used with liquids that could corrode or oxidize the thermocouple alloy.  Metal surfaces can also be problematic.  Often metal surfaces, especially pipes are used to ground electrical systems.  The indirect connection to an electrical system could impact the thermocouple measurement.  In general, beaded wire thermocouples are a good choice for the measurement of gas temperatures.  Since they can be made very small, they also provide very fast response time.

     

    Thermocouple Probe

     

    A thermocouple probe consists of thermocouple wire housed inside a metallic tube.  The wall of the tube is referred to as the sheath of the probe.  Common sheath materials include stainless steel and Inconel.  Inconel supports higher temperature ranges than stainless steel, however, stainless steel is often preferred because of its broad chemical compatibility.  For very high temperatures, other exotic sheath materials are also available.

     

     

    A Thermocouple is a sensor used for measuring temperature. They consist of two dissimilar metals, joined together at one end.  When the junction of the two metals is heated or cooled a voltage is produced that relates to the temperature.

  • Understanding & Choosing Thermocouples

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

    Understanding & Choosing Thermocouples

     

    Choosing a thermocouple type should be determined based on:

    • Required Temperature Range (Calibration Types)

    • Mechanical or Chemical Application Process  (Junction Types)

    • Accuracy

    • Additional Info

     

     

    Thermocouples Calibration Types

    Thermocouples are available in different combinations of metals commonly referred to as calibrations.  The three calibrations that our clients most commonly utilize are J, K, N.  Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple.  The thinner the wire lower the max not temperature range.

     

     

    Type J

    Type J is the second most common calibration type and is a good choice for general purpose applications where moisture is not present.  The Type J thermocouple may be used, exposed or unexposed, where there is a deficiency of free oxygen.  For cleanliness and longer life, a protection tube is recommended. Since iron (JP) wire will oxidize rapidly at temperatures over 1000°F (540°C), it is recommended that larger gage wires be used to compensate.  Maximum recommended operating temperature is 1400°F (760°C).

     

     

    Type K

    Type K thermocouples usually work in most applications as they are nickel based and exhibit good corrosion resistance.  It is the most common sensor calibration type providing the widest operating temperature range.  Due to its reliability and accuracy the Type K thermocouple is used extensively at temperatures up to 2300°F (1260°C).  This type of thermocouple should be protected with a suitable metal or ceramic protection tube, especially in reducing atmospheres.  In oxidizing atmospheres, such as electric furnaces, tube protection is not always necessary when other conditions are suitable; however, it is recommended for cleanliness and general mechanical protection.  Type K will generally outlast Type J because the J wire rapidly oxidizes, especially at higher temperatures.

     

     

    Type N

    This nickel-based thermocouple alloy is used primarily at high temperatures up to 2300°F (1260°C).  While not a direct replacement for Type K, Type N provides better resistance to oxidation at high temperatures and longer life in applications where sulfur is present.  It also outperforms Type K in K’s aging range.

     

     

    Thermocouples Junction Types

     

    The tip of the thermocouple probe is available in three different styles. Grounded, Ungrounded and Exposed.

     

    While the terms Grounded and Ungrounded do have some electrical implications, this is not what these terms are determining.  Forget the common electrical-circuit notion of a “grounded-circuit.”  A thermocouple couple has two wires regardless of whether it is “grounded” or “ungrounded.”  You can’t just look at a thermocouple and tell if it is “grounded” or “ungrounded.”

     

    You can, however, use a meter to do a continuity test between the lead ends and the sheathing of the probe.  If the thermocouple is “grounded” you will have a closed circuit; where as “ungrounded” will result in an open circuit.  Most Thermocouples are “Grounded.”  Usually, if a thermocouple is ungrounded it is stated in the description or there is a “U” in the part number, or you are aware of it due to some reason existing for that feature to be present.

     

    Exposed:  An exposed junction is recommended for the measurement of static or flowing non-corrosive gas temperatures where fast response time is required.  The tip of the thermocouple protrudes outside the sheath wall with an exposed junction.  Exposed junction thermocouples offer the best response time but not intended for pressurized or corrosive environments and are best suited for air measurement.

     

    Grounded:  The grounded junction is recommended for the measurement of static or flowing corrosive gas and liquid temperatures and for high pressure applications.  With a grounded tip the thermocouple is in contact with the sheath wall.  A grounded junction provides a fast response time but it is most susceptible to electrical ground loops.  Measuring the temperature of a solid surface is difficult for most types of temperature sensors.  In order to assure an accurate measurement, the entire measurement area of the sensor must be in contact with the surface.  This is difficult when working with a rigid sensor and a rigid surface.  Since thermocouples are made of pliable metals, the junction can be formed flat and thin to provide maximum contact with a rigid solid surface.  These thermocouples are an excellent choice for surface measurement.  The thermocouple can even be built in a mechanism which rotates, making it suitable for measuring the temperature of a moving surface.

     

    Ungrounded:  An ungrounded junction is recommended for measurements in corrosive environments where it is desirable to have the thermocouple electronically isolated from and shielded by the sheath.  In ungrounded junctions, the thermocouple is electrically isolated from the sheath wall by a layer of insulation.  This results in a slower response time.  This electrical isolation of ungrounded thermocouples gives them some important advantages over grounded versions.  They interface more easily with instrumentation and offer higher reliability with microprocessor based controls.

     

     

     

    Thermocouple Accuracy

     

    Response time refers to a time constant defined as the time required by a sensor to reach 63.2% of a step change in temperature under a specified set of conditions.  Five time constants are required for the sensor to approach 100% of the step change value.  An exposed junction thermocouple is the fastest responding.  Also, the smaller the probe sheath diameter, the faster the response, but the maximum temperature may be lower.  Be aware, however, that sometimes the probe sheath cannot withstand the full temperature range of the thermocouple type.

     

     

     

    Thermocouple Additional Info:

     

    Standard Diameters: 0.010", 0.020", 0.032", 0.040", 1/16", 1/8", 3/16", and 1/4" with two wires.

     

    Length: Standard thermocouples have 12 inch immersion lengths.  Other lengths are available.

     

    Sheaths: 304 stainless steel and Inconel are standard. Other sheath materials are available.

     

    Insulation: High purity Magnesium Oxide (MgO) is standard. Minimum insulation resistance wire to wire or wire to sheath is 1 Gohm at 500 volts DC in all diameters above 1.5mm (1/16").

     

    Extension Wire: Thermocouple alloy wire must always be used to connect a thermocouple sensor to the instrumentation to assure accurate measurements.

  • Thermocouple Options

     

     

     

    Armored Thermocouple

    Armored thermocouples feature a heavy duty stainless steel armor over the thermocouple wire. The armor protects the wires from damage.  Armored thermocouples are well suited for industrial environments where an unprotected thermocouple could end up damaged.

    Spring Adjustable Thermocouple

    Spring Adjustable Thermocouples are designed to measure the surface temperatures of moving or stationary walls by direct contact. The spring-loaded sensing tip maintains contact even if the wall is moving.  Some of the primary applications include plates, bearings, rotating shafts, and molding machines.  Spring Adjustable Thermocouples are available as small as 3/4" overall length and J, K, E and T type.

    Thermocouples Options

     

    Thermocouples are sensors used for measuring temperature.  They are made in a vast array of configurations for an equally diverse set of processes and applications.  Below are three of the most versatile and commonly used thermocouples by our clients.  CONTACT US today so we can help you to determine the ideal design and fit for your individual needs.

    Metal Transition Thermocouple

    Heavy duty transition joint thermocouple probes are available with either a spiral armor cable or stainless steel over-braided PFA insulated lead wire, fitted with an SMPW miniature plug. With Inconel 600 or 304 stainless steel sheath.  150, 300 mm and 1 meter Probe Lengths Standard.

     

    CONTACT US directly for additional lengths available.

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