What is a thermocouple?

A thermocouple consists of two conductor wires, each made of a different metal. At one end, the two wires are joined (the “couple” of “thermocouple”) at the measuring point. When this point is heated, a voltage is produced due to the different electron densities of the two dissimilar metals. A voltmeter at the other end – the cold junction – measures this thermoelectric voltage. Because the relationship between temperature and the two wires’ voltage is known, the measured voltage is used to determine the temperature reading. In short, a thermocouple measures the differential temperature between the measuring point and the cold junction, not the medium’s absolute temperature.

When should I use a thermowell with a temperature sensor?

Whether the temperature sensor is a thermocouple or resistance thermometer (RTD), operators should place the probe inside a protective thermowell when measuring the temperature of aggressive media and other harsh process conditions. Also use a thermowell in sanitary applications, as this protective device allows for easy sensor removal – for periodic calibrations, repairs, or replacements – without having to open the process, thus eliminating a source of potential contamination.

Use WIKA’s wake frequency calculator to design thermowells that avoid the formation of wake-induced vortices that create vibrations and cause mechanical fatigue.

Can thermocouples be installed directly into the process media?

Of course! In fact, most thermocouples are mounted this way. Insert temperature probes directly into the process if the media is not abrasive or chemically aggressive.

Can thermocouples detect surface temperatures as well?

Absolutely. WIKA offers almost 40 different models of high-quality thermocouples, including several with surface probes:

• TC50 surface thermocouple for flat areas or the outside of pipes

• TC47-RL ring lug thermocouple and TC47-AB adjustable bayonet thermocouple for plastics machinery

• Furnace tube temperature monitoring with tubeskin thermocouples

Thermocouples for the plastics industry

WIKA manufactures hot runner thermocouples specifically for plastic injection molding machines. These specialty sensors are press-fitted into a grooved channel along the machined parts, or the metal sensor tip is fitted directly into a drilled hole.

Producers of low-density polyethylene (LDPE) plastics benefit from thermocouples that can withstand high-pressure polymerization. With conductors sheathed within a mineral-insulated (MI) cable made of Inconel 600, model TC90 thermocouple (types K, J, and E) is designed to operate reliably under:

• Extremely high pressures – up to around 87,025 psi (6,000 bar) in test conditions

• High process temperatures – up to around 660°F (350°C)

• Heavy vibration

In addition, this specialized thermocouple has a very fast response time, thanks to the MI cable’s small diameter and the ability to insert the sensor directly into the process.

Long-lasting thermocouples for extreme applications

Even though typical thermocouples withstand temperatures as high as 2,192°F (1,200°C), they are prone to drifting and aging with repeated exposure to high temperatures, causing them to become less accurate over time. A hydrogen-rich environment, such as what is found in partial oxidation units, leads to embrittlement in measuring instruments and a shortened service life.

For these reasons, WIKA has come up with model TC84, featuring a patented sapphire design. This innovative thermocouple has a gas-tight monocrystalline protection tube that shields the thermocouple’s sensitive wires from hydrogen permeation and temperatures as high as 3,092°F (1,700°C).

Thermocouples with output signals

As processes become more interconnected and digital, remote monitoring is a must for today’s smart factories. Many of our thermocouples offer 4 … 20 mA output signals and come with bus communication protocols.

Quality thermocouples for any application

For whatever the process, temperature range, pressure range, mounting configuration, or output signal, WIKA has the just-right thermocouple for your applications. We understand that with all the available options in temperature sensing and the variables in your refinery or manufacturing facility, it can be difficult to select the right instrument. To ensure that you choose the appropriate temperature measuring solutions, contact the specialists at WIKA USA for personalized advice. And if an off-the-shelf product doesn’t quite fit your needs, we can engineer a custom temperature solution for your specific application.