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 (may need to be compatible with existing equipment; existing holes may determine probe diameter)

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).

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.

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.

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.

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 is exposed, there are several application limitations. The beaded wire thermocouple should not be used with liquids that could corrode or oxidise the thermocouple alloy. Metal surfaces can also be problematic. Often metal surfaces, especially pipes are used to earth 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 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.

Click here for more information on high temperature exotic thermocouple probes.

The tip of the thermocouple probe is available in three different styles. Grounded, insulated 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 earth loops. In insulated 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 Probe

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 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.