Thermocouples are an easy and reliable way to measure temperature; they can perform well in most environments, across extreme temperatures, and even when ionizing radiation is present. However, they can’t measure temperatures well in regions where electromagnetic fields fluctuate.
This White Paper from Omega Engineering discusses the problems with using thermocouples in electromagnetic environments and recommends alternative measurement techniques.
Thermocouple Theory and Application
Thermocouples utilize the Seebeck Effect, first observed in 1821. Electrical current will in a circuit made from different metals when the junction between those metals is held at different temperatures. However, the metals need to be good thermoelectric metals – the electrons need to diffuse through the material; then, at high temperatures when the electrons gain kinetic energy, this influences the electrical potential. Nickel-based alloys are a typical substance used for this and are in most thermocouple wires. For example, the Type K thermocouple uses junctions of Chromel and Alumel, both of which include significant proportions of nickel. Platinum-rhodium and tungsten-rhenium also possess thermoelectric properties and can be used.
Thermocouples Probes with Connectors.
The current and voltage produced are proportional to the temperature difference although the effect isn’t always linear, and the induced voltages are very small - 41 mV per degree Celsius in the widely-used type K thermocouple, and this is typical for thermocouples. Consequently, to measure small changes, the signal has to be amplified – but this means any noise present in the voltage is amplified as well.
Electromagnetic fields are ubiquitous and difficult to avoid in totality. Power lines carry high voltages; induction heating is used throughout industry and temperature; transformers see high loads and can overheat in a way that needs to be measured. Spark plugs used in engines generate EM transients that can be measured; these fields can induce voltage in the thermocouple wires or cause inductive heating of the thermocouple. Additionally, common-mode voltage relative to earth ground will add voltage to the thermocouple signal. These problems are worse in AC cases compared to DC.
Motion of electrical conductors in magnetic fields induces a voltage; this is Faraday’s law, and it’s the basis for almost all modern power generation. In thermocouple wires, Faraday’s law means that changing electromagnetic fields can induce voltages in the thermocouple wires, and even small fields can shift the small voltages in the Seebeck effect noticeably to throw off temperature measurements.
Alternating electromagnetic fields induce voltages, but this also induces heating that has the secondary effect of changing the temperature; the Nickel in the thermocouple itself will heat up in an alternating EM field and change the temperature you’re trying to measure. This can be a problem in large magnetic fields that alternate – such as those produced by generators – and you’re no longer measuring the ambient temperature in this case.
Alternative Temperature Measurement Devices
Given these problems, alternative devices such as the Pt100-type resistance temperature devices (RTDs) and the detection of infrared (IR) emission.
Infrared Temperature Sensors/Transmitter.
Infrared Temperature Sensors/Transmitter.
RTDs use the change in resistance of a length of platinum wire; they’re highly accurate and immune to EM fields, but the fragility of the wire means they can’t always be used in industrial environments.
Planck’s Law tells us how bodies emit IR radiation depending on their temperature; we can therefore indirectly measure the temperature by looking at the radiation; this can be performed at distances of several feet away, depending on the size and power of the emitter. Different surfaces at the same temperature radiate at different rates, so this property – the “emissivity” of the substance – should also be taken into account by the device that measures temperature.
Omega Engineering offers several IR temperature sensors/ transmitters suitable for use in a wide range of industrial situations. The OS137 comes in a NEMA 4 rated 1" diameter stainless-steel housing and can be used at distances up to 48" (although, for this to work, the full field of view of the detector should be filled by the temperature you want to measure.)
There are three temperature ranges available for the OS137 which can cover up to 538 °C (1000 °F). Laser sighting accessories can be mounted to the front during set-up to ensure accurate alignment with the target. Output type should be specified; voltage, current or Type K thermocouple outputs. You can set alarms for the sensor and adjust the emissivity profile.
The OS136 is a more compact infrared sensor/ transmitter; it works just like the OS137 except the viewing angle is wider. Unlike the OS137 you cannot change the emissivity profile – it is set to 0.95 as standard – so you must correct for different substances.
Thermocouples measure temperature in microvolts per degrees Celsius. Since the signals are small and must be amplified to be easily read, measurement errors due to drifts in voltage or temperature when associated electromagnetic fields are present can be significant. Voltages can be induced in the thermocouple wires, induction heating can raise the temperature of the thermocouple, and grounding issues can increase the voltage measured.
You can try various filters and shielding methods, but these may not always be successful; depending on the application, it could be worth switching to RTDs or IR emission methods. Both of these methods have their own advantages and disadvantages, but are generally less sensitive to ambient EM fields than thermocouples. RTDs are often too fragile for industrial environments, but IR measurement devices don’t have to be in contact with the output options and they can be housed in protective casing.
This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.
For more information on this source, please visit OMEGA Engineering Ltd.