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Thermocouples, Thermocouple Probes and Sensors

What is a thermocouple sensor?


A thermocouple is a sensor for measuring temperature. It consists 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 can be referrenced back to the temperature. The thermocouple alloys are commonly available as wire.


What are the different thermocouple types?


Thermocouples are available in different combinations of metals or calibrations to suit different applications. The three most common calibrations are K, T and J; of which Type K is the most popular due to it's wide temperature range and low cost. Type K has a Nickel-Chromium positive conductor and a Nickel-Aluminium negative conductor. There are high temperature calibrations R, S, B, G, C, and D which offer performance up to 2320°C. These are made from precious metals (Platinum/Rhodium and Tungsten/Rhenium) and are therefore relatively expensive.

Each calibration has a different temperature range and intended environment. 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. Our Thermocouple Maximum Temperature Guide shows maximum temperatures for each thermocouple type and wire diameter. This guide also provides upper temperature limits for mineral insulated thermocouple probes at common sheath diameters.



How do I choose a thermocouple type?


Because a thermocouple measures 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 range
    Chemical resistance of the thermocouple or sheath material
    Abrasion and vibration resistance
    Installation requirements (compatibility with existing equipment)

How do I know which junction type to choose?


Sheathed thermocouple probes are available with one of three junction types: grounded, insulated or exposed (see graphic below: "Thermocouple Tip Styles"). At the tip of a grounded junction probe, the thermocouple wires 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. In an insulated probe, the thermocouple junction is detached and insulated from the probe wall. Response time is slower than the grounded style, but the insulation offers electrical isolation (see table below).



Room-Temperature Insulation Resistance
Insulated Junction Thermocouple
Nominal Sheath Diameter Applied dc Voltage min. Insulation Resistance min.
Less than 0.90mm (0.03in.) 50V 100M Ohms
0.80-1.5mm (0.030 to 0.059in.) 50V 500M Ohms
Greater than 1.5mm (0.059in.) 500V 1000M Ohms

The thermocouple in the exposed junction style protrudes out of the tip of the sheath and is exposed to the surrounding environment. This type offers the best response time, but is limited in use to dry, non corrosive and non pressurised applications.

What is response time?


A time constant has been 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.



Material Maximum Temperature Application Atmosphere
Oxidising Hydrogen Vacuum Inert
304 SS 898°C
(1650°F)
Very Good Good Very Good Very Good
Inconel 600 1148°C
(2100°F)
Very Good Good Very Good Very Good

Thermocouple Ranges and Tolerances


The IEC and ANSI accuracy specifications and temperature ranges are published in our Thermocouple Tolerance Guide for the most common types.

Wire colour codes and limits of error (Thermocouple Colour Codes)



Thermocouple Reference Tables


Thermocouples produce a voltage output that can be correlated to the temperature that the thermocouple is measuring. The documents in the table below provide the thermoelectric voltage and corresponding temperature for a given thermocouple type. Most of the documents also provide the thermocouple temperature range, limits of error and environmental considerations.



Click on a link below to see the related document
° Thermocouple Type K ° Thermocouple Type R
° Thermocouple Type T ° Thermocouple Type S
° Thermocouple Type J ° Thermocouple Type B
° Thermocouple Type N ° Thermocouple Type C
° Thermocouple Type E ° Tungsten and Tungsten/
Rhenium
° Nickel-Chromium vs. Gold-0.07
Atomic Percent Iron
 

 

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