ThermocouplesHow to measure temperature with a thermocouple?To measure athermocoupleSeebeck voltage, you cannot simply connect the thermocouple to a voltmeter or other measurement system because connecting the thermocouple wires to the measurement system creates additional thermoelectric circuits.

Consider the circuit illustrated in the figure, in which a J-type thermocouple is in a candle flame that has a temperature you want to measure. The two thermocouple wires are connected to the copper leads of a voltmeter. Notice that the circuit contains three dissimilar metal junctionsJ1, J2 and J3. J1, the thermocouple junction, generates a Seebeck voltage proportional to the temperature of the candle flame.J2 and J3 each have their own Seebeck coefficient and generate their own thermoelectric voltage proportional to the temperature at the data acquisition terminals. To determine the voltage contribution from J1,you need to know the temperatures of junctions J2 and J3 as well as the voltage-to-temperature relationships for these junctions. You can then subtract the contributions of the parasitic junctions at J2 and J3 from the measured voltage at junction J1.Thermocouples require some form of temperature reference to compensate for these unwanted parasitic "cold" junctions. The most common method is to measure the temperature at the reference junction with a direct-reading temperature sensor and subtract the parasitic junction voltage contributions. This process is calledcold-junction compensation.You can simplify computing cold-junction compensation by taking advantage of some thermocouple characteristics.By using the Thermocouple Law of Intermediate Metals and making some simple assumptions, you can see that the voltage a data acquisition system measures depends only on the thermocouple type, the thermocouple voltage, and the cold-junction temperature.The measured voltage is in fact independent of the composition of the measurement leads and the cold junctions, J2 and J3.What is the law of intermediate metals?According to the Thermocouple Law of Intermediate Metals, illustrated in the figure, inserting any type of wire into a thermocouple circuit has no effect on the output as long as both ends of that wire are the same temperature, or isothermal

Consider the circuit in the next figure.

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Both circuits are quite similarbut a short length of constantan wire has been inserted just before junction J3 and the junctions are assumed to be held atidentical temperatures.Assuming that junctions J3 and J4 are the same temperature, the Thermocouple Law of Intermediate Metals indicates that the circuit inthe figureon leftis electrically equivalent to the circuitof thefigure on right. Consequently, any result taken from the circuitin thefigure on leftalso applies to the circuit illustrated in thefigure on right.What is the typical internal construction of a thermocouple?The leads of the thermocouple are encased in a rigid metal sheath. The measuring junction is normally formed at the bottom of the thermocouple housing. Magnesium oxide surrounds the thermocouple wires to prevent vibration that could damage the fine wires and to enhance heat transfer between the measuring junction and the medium surrounding the thermocouple.

Beaded wire thermocouplesA 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 temperature. Since they can be made very small, they also provide very fast response time.Thermocouple probesA 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. View our line of high temperature exotic thermocouple probes.The tip of the thermocouple probe is available in three different styles. Grounded, ungrounded and exposed. 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. In ungrounded junctions, the thermocouple is separated from the sheath wall by a layer of insulation. The tip of the thermocouple protrudes outside the sheath wall with an exposed junction. Exposed junction thermocouples are best suited for air measurement.

Surface probesMeasuring 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.What are the different types of thermocouples?Athermocoupleis available in different combinations of metals or calibrations. The four most common calibrations are J, K, T and E. There are high temperature calibrations R, S, C and GB.Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple.Although the thermocouple calibration dictates the temperature range, the maximum range is also limited by the diameter of the thermocouple wire. That is, a very thin thermocouple may not reach the full temperature range.

Whatare the thermocouples wiring codes?Thermocouple wiringiscolor codedby thermocouple types. Different countries utilize different color coding. Jacket coloring is sometimes a colored stripe instead of a solid color as shown. For thecomplete color table(including American and Canada),click here.

How do I choose a thermocouple?Because athermocouplemeasures in wide temperature ranges and can be relatively rugged, thermocouples are very often used in industry. The following criteria are used in selecting a thermocouple:Temperature rangeChemical resistance of the thermocouple or sheath materialAbrasion and vibration resistanceInstallation requirements (may need to be compatible with existing equipment; existing holes may determine probe diameter)We are making thermocouples from 30 years :contact usfor further information.What is the response time?