Temperature can be easily measured and recorded using a thermocouple. Thermocouples work consistently in most environments, tolerating vibration, temperature extremes and even ionizing radiation. However, they are prone to the effects of electromagnetic fields, so should be used cautiously, or not at all, in such places.
This article from Omega Engineering discusses the problems with employing thermocouples in electromagnetic environments and makes recommendations for alternative types of temperature instrumentation.
Thermocouple Theory and Application
In 1821, Thomas Johann Seebeck discovered the Seebeck Effect, which is currently being used by these thermocouples. This is the phenomenon by which electrical current passes in a circuit made from dissimilar metals, when their two junctions are at different temperatures.
Thermocouples Probes with Connectors
The metals employed in a thermocouple should have thermoelectric properties. This is when the electrons are capable of diffusing through the material. The electrons gain kinetic energy at higher temperatures, becoming more movable and increasing the extent to which they move, so creating changes in electrical potential. A number of nickel-based alloys have such characteristics and are employed in most frequent thermocouple wires.
For instance, the Type K thermocouple employs junctions of Alumel and Chromel, both of which include considerable proportions of nickel. Other material combinations employed in thermocouples are based on tungsten-rhenium and platinum-rhodium, which also have thermoelectric properties.
Although the relationship is not exactly linear, the generated voltage and current are proportional to the difference in temperature between the two junctions. The actual voltages are very minute. The change is 41 mV/°C in a Type K thermocouple (widely used owing to its broad temperature range and low cost). Other thermocouple types generate changes of a similar magnitude. As a result, thermocouple signals should be amplified for use in measurement systems. Unavoidably, any extra voltage in the signals on account of external causes gets amplified simultaneously.
High voltages are common in a number of situations where temperature measurements are required, and electromagnetic fields are inevitable. Induction heating is employed throughout industry and temperature must be measured to ensure reliable processes. High voltages are carried by electrical power lines. Transformers see high loads and can become extremely hot. Transient electromagnetic signals are produced even by spark plugs employed in internal combustion engines (not only automobile engines but large generator sets).
The thermocouple readings are affected by the electromagnetic fields in two ways, they may:
- Cause inductive heating of the thermocouple
- Induce voltage in the thermocouple wires
In addition, common-mode voltage comparative to earth ground will add voltage to the thermocouple signal. These problems can occur in dc environments but are more rigorous in the presence of ac.
Faraday’s Law describes the phenomenon by which an electrical potential is produced when an electrical conductor is moved via a magnetic field. The same effect can produce voltage in thermocouple wires, particularly if the wires are aligned perpendicular to a changing field. Since the Seebeck effect generates extremely small voltages, even a small field can change the temperature reading.
Exposing a conductor to an alternating electromagnetic field produces eddies giving rise to heating. Thus with nickel being electrically conductive, an alternating magnetic field which might be found around a large generator or motor, will heat the temperature measurement device itself. This will result in a signal that does not accurately show the temperature being measured.
Common-mode Voltage Issues
When a thermocouple is employed along with or as part of electrical equipment it is usually connected to that supply. Once electrically energized it is possible for a difference between equipment ground and earth ground to affect the thermocouple signal voltage. The solution in such cases is to offer galvanic separation of the temperature measurement system, or alternatively, to look at other temperature measurement techniques.
Alternative Temperature Measurement Devices
Infrared Temperature Sensor/Transmitter
Infrared Temperature Sensor/Transmitter
Pt100-type RTDs (resistance temperature devices) and the detection of infrared (IR) emission are the two technologies to be explored.
RTDs (where the measuring principle is the change in resistance of a length of platinum wire) have good immunity to electromagnetic fields and are famous for high accuracy. Still, they tend to be fragile and are not always ideal for industrial environments.
IR emission has the advantage of being a non-contact measurement and can be carried out at distances of several feet or more, based on the emitter size. It exploits Planck’s Law that describes how a body radiates energy in proportion to its temperature. One issue that has to be overcome is that different surfaces at the same temperature will radiate at varying rates. Described as a difference in emissivity, this should be considered when measuring temperature with any kind of IR detector.
Omega Engineering provides multiple IR temperature sensors/ transmitters ideal for use in an extensive range of industrial situations. The OS137 is available as a NEMA 4 rated 1" diameter stainless-steel housing and can be employed at distances up to 48" (Note: the measurement target should fill the field of view of the sensor. If not, the measured temperature will not be accurate).
Three temperature ranges of OS137 can cover temperatures up to 538 °C (1000 °F). A laser sighting accessory can be fitted to the front during set-up for ensuring accurate alignment with the target. Output type must be specified when ordering: choose from voltage, current or Type K thermocouple outputs. Emissivity is adjustable and facility exists for an alarm set point.
The OS136, at 3/4" diameter is a more compact infrared sensor/ transmitter. Although the viewing angle is wider (which may require closer placement), performance is similar to the OS137. Unlike the OS137, emissivity is fixed at 0.95, so corrections should be made for targets that differ.
Temperature is measured in microvolts per degrees Celsius by thermocouples. These signals require amplification to be productive, which makes them prone to measurement errors when employed in electromagnetic environments. Voltages can be triggered in the thermocouple wires, grounding issues can increase the voltage measured and induction heating can raise the temperature of the thermocouple.
While various shielding methods and various filters can be employed another approach is to switch measurement technology. Both RTDs and IR emission detection have good tolerance to electromagnetic fields even though RTDs are usually though extremely fragile for industrial environments. IR sensor/transmitters provide non-contact measurement with a range of output options and are available in robust protective housings.
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.