Thermocouples are very simple temperature measurement sensors. They consist of two dissimilar metals joined together typically by welding at the tip. When different metals are joined together and exposed to a temperature gradient, a voltage is produced according to the Seeback Effect. The voltage can be correlated to the measured temperature. Since the wires used to construct a thermocouple are typically specialized alloys, the question often arises if it is possible to extend a thermocouple? Thermocouples can be extended but certain practices should be followed to maintain the integrity of the measurement.
When Extending a Thermocouple It is Best Practice to Use Thermocouple Wire.
A thermocouple should be extended using the same wire type as the thermocouple. A thermocouple does not technically measure the temperature of a given point; it measures the temperature difference between two points. The temperature of the measurement point can be calculated as long as the temperature at the other end of the thermocouple is known.
In figure 1 we show a simple temperature measurement setup consisting of a K thermocouple and measurement instrument. The voltage produced by the thermocouple will be a function of the temperature difference between T1 to T2. If either T1 or T2 changes the voltage produced by the thermocouple will change. The instrument is able to determine T1 because all thermocouple instruments have a built-in temperature sensor that measures the temperature at the point the thermocouple is connected to the instrument(T2) and uses that temperature in a process called cold junction compensation to determine T1. If T2 changes, the instrument will compensate for the change and still calculate T1 accurately.
Figure 1 – A Basic K Thermocouple Measurement Setup
Tip for testing a thermocouple instrument.
A quick test of thermocouple instrument is to put a short across the input terminals of the instrument. Any wire can be used for this test. The short produces 0 Vdc which the instrument will interpret as a room temperature reading because of the instruments cold junction compensation circuitry.
In Figure 2 we have modified the setup in Figure 1 by extending the thermocouple with copper wire. This introduces two new thermocouples into the system. A thermocouple formed by Copper/ Chromel and one of Alumel/Copper. The measured thermocouple voltage now is a function of temperature T1, T2 and T3 where T3 is the temperature at the point where the copper wire is joined to the K thermocouple. According to the Law of Intermediate Metals if T2 is equal to T3 the introduction of the copper wire will not impact the temperature measurement but in most practical applications it is unlikely that T2 will be equal to T3 thus introducing an unknown error into the application. Since the measurement is only as good as the known accuracy it is considered best practice to always extend a thermocouple with wire made of the same thermocouple alloys.
Figure 2 Extending a Thermocouple with Wire Made from Different Alloys than the Thermocouple- Errors will be introduced if T3 ≠ T2
How Far Can a Thermocouple be Extended?
The factors influencing the distance a thermocouple may be extended depend on the resistance of the wire, the electrical noise in environment (RF noise) and whether there is there is a potential for a ground loop. Each one of these factors is addressed below. As a general rule of thumb, it is normally safe to extend a thermocouple 100 ft but all of the factors discussed below should be considered.
The Impact of Wire Resistance on Thermocouple Temperature Measurement.
A thermocouple is a voltage source and as a voltage source the resistance of the wire leads will produce a voltage drop along the thermocouple length. Depending on the magnitude of the voltage drop, it could impact the temperature measurement. The amount of voltage drop is a is function of the resistance of the wire as well as the input impedance of the measurement instrument. As an example, let’s consider a K thermocouple measuring a 500°C temperature connected to an instrument with 1MΩ input impedance with 24AWG wire. Assuming the instrument is at 22°C ambient temperature the thermocouple will produce 19.806 mv but because of the voltage drop across the wire, the instrument will 19.762 mV. The equivalent circuit diagram is shown in Figure 3 below.
Figure 3 – Equivalent Circuit of a K thermocouple with 100 ft of 24 AWG Wire
|See the IOthrifty blog Thermocouple Wire Resistance for other thermocouple wire resistances.
In this example the impact of the wire resistance is a 0.07 °C deviation from the actual measurement which is probably insignificant as compared to the system accuracy. Of course, instruments with lower input impedances and higher lead wire resistance due to thinner wire and/or longer lead lengths will have a larger impact on the measurement, however, RF noise and ground loop are normally a much larger concern rather than the impact of wire resistance.
The Impact of RF(Radio Frequency) Noise on Thermocouples
Electrical noise in the vicinity of the thermocouple can impact the temperature measurement. RF noise is produced by any electrical device that contains an oscillating circuit. This includes wireless transmitters, motors, ovens, and even fluorescent lights as well as many other devices. The magnitude of the noise may be small, but a thermocouple produces such a low-level voltage, it does not take much noise to impact the sensor reading. The voltage a thermocouple may be as low as 0.04mv per degree. A thermocouple running through a “noisy” environment may pick up noise and the longer the wire length the greater the possibility the thermocouple will pick up the noise, just as a long antenna will pick up a signal better than a short antenna. The error typically shows up as erratic readings. It is difficult to determine the length of wire which will be susceptible to noise. Even a short thermocouple can be affected by noise if the noise is severe enough. Best practice is to always locate a thermocouple away from equipment that generates electrical noise. If that is not possible using wire with an electrical shield or running the wire through a metal conduit which acts as a shield is a good solution.
Thermocouple Ground Loops
A ground loop occurs when a voltage potential exists between the sensor measurement point and the ground of the measurement instrument. These voltage potentials can be caused by electrical wiring practices, faulty equipment, and even natural current flows within the Earth. Although ground loops can occur with short thermocouple lengths, the longer the length of wire the more likely they will exist which is why they need to be considered when extending thermocouples.
If both the sensor and instrument are grounded and there is a ground voltage potential difference, an unwanted current flowing through the thermocouple circuit may result causing erroneous readings. The errors can be a small deviation from the expected values but more often it is drastically inaccurate readings. Figure 4 shows a thermocouple measurement setup with a ground loop.
Figure 4 - A Thermocouple Measurement Application with a Ground Loop
Eliminating a ground loop can be accomplished by removing the connection to ground at the sensor, the instrument or both. Although this is theoretically a simple solution, practically it can be challenging since it often involves changing the sensor, the instrument or adding equipment like an isolator. Here are a few approaches to resolving a ground loop.
- Physically Remove the Thermocouple from the Grounded Point - A ground loop will only occur if the thermocouple is attached to a conductive surface or immersed in a conductive liquid. Removing the thermocouple from the surface or liquid will break the ground loop. This is a simple solution but since it is the temperature of surface or liquid that needs to be measured it is not a practical solution.
- Mechanically Isolate the Thermocouple from the Ground Point - Putting a non-conductive material between the thermocouple and the ground point will break the ground loop. Examples of a non-conductive material could be a non-conducting tape or epoxy. The problem with this solution is that it may also impact the temperature reading isolating material does not conduct the heat adequately.
- Use an Un-grounded Thermocouple Probe – Un-grounded thermocouple probes have a thin layer of insulating material between the thermocouple and probe wall. This is normally one of the best solutions for eliminating a ground loop.
- Use an Electrical Isolator – Electrical isolators are devices that break the physical connection between two circuits while still allowing the signal to flow. They do this by converting and transmitting the signal in an optical or inductive form. Isolators are a fault proof way to protect ground loops and are often used in industrial installations. The downside ofusing isolators is the expense they add to a system.
Thermocouples may be extended but it is best practice to use wire made of the same alloy to extend the thermocouple. The distance a thermocouple may be extended can be more than 100 ft, but long lengths of thermocouples may introduce errors due to noise and ground loops but with the proper precautions the impact of these factors can be minimized.