Solving the Industrial Temperature Measurement Challenge: How to Keep Resistance Temperature Detectors Away from Strong Electromagnetic Field Interference

In the field of industrial automation control, temperature measurement is crucial, and resistance temperature detectors (RTDs), as the most commonly used temperature detectors in the medium and low temperature range, directly impact production safety and product quality. However, interference from strong magnetic and electric fields has long been a technical challenge for many engineers. Today, we will delve into how to effectively avoid installing RTDs in areas with strong magnetic or electric fields to ensure accurate and reliable temperature measurements.

Why should RTDs be kept away from strong electromagnetic fields?

RTDs measure temperature based on the characteristic that the resistance of a metal conductor changes with temperature. This precise measurement of resistance changes is highly susceptible to interference from the external electromagnetic environment.

In industrial environments such as hydropower plants and power plants, generator port voltages typically reach kilovolts. The resulting strong electric fields and the strong magnetic fields generated by rotor leakage flux severely interfere with RTDs and temperature measurement systems. Actual measurement data shows that the leakage flux voltage signal induced by some thrust bearing RTDs can even reach 110V, rendering the temperature measurement meaningless and potentially damaging other components in the circuit.

Electromagnetic interference (EMI) can cause unstable or fluctuating temperature readings, and even false alarms. In severe cases, it can lead to unnecessary unit shutdowns and significant economic losses.

Practical Methods to Effectively Avoid Electromagnetic Interference

**Optimal Installation Location**

When installing a resistance temperature detector (RTD), the measurement point location should be carefully selected, avoiding areas with strong magnetic and electric fields. The RTD should be installed as far away as possible from heat sources, electric fields, and strong magnetic fields to prevent external factors from interfering with the measurement results.

For existing strong electromagnetic environments, if complete avoidance is not possible, consider changing the RTD's installation method. For example, use suspended installation to prevent the RTD from contacting the refractory bricks of the furnace wall, and use insulation between the RTD and the support.

**Comprehensive Shielding Measures**

Shielding is an effective means of resisting EMI. Specifically, the following methods can be used:

The compensating wires of the RTD can be shielded by running them through an iron pipe or other metal shielding material to prevent interference from electromagnetic interference and high-voltage electric fields. When using this method, ensure that the iron pipe and shielding material are properly grounded, and that the compensating wires are twisted together.

For strong magnetic field interference, integrated mesh shielding is the most effective solution. The cables for RTDs (Resistor Temperature Detectors) have many long leads and numerous intermediate links, requiring meticulous wiring. At each link, the shielded wire of the lead must be reliably connected to the common ground. The lead wires should be three-core (or four-core) cables with a copper wire mesh shield and silver-plated cores, specifically designed for measurement.

Changing the Signal Transmission Method

If shielding measures are insufficient to effectively resolve the interference problem, changing the signal transmission method can be considered.

In the field or at the field station's PC room, a Pt100 RTD temperature converter can be installed to convert the Pt100 resistance signal to a standard DC 4-20mA signal, correspondingly changing the computer's input signal channel. Because the DC 4-20mA signal has very strong anti-interference capabilities, this method can effectively eliminate interference generated during signal transmission.

Proper Grounding

Grounding is an important method for introducing earth to eliminate interference. There are two specific grounding methods: grounding the RTD reference terminal and grounding the RTD measurement terminal.

The reference-end grounding method involves grounding one end of the RTD (or compensating wire) output terminal through a sufficiently large capacitor (the larger the capacitor, the better, if conditions permit). The measuring-end grounding method involves leading a metal wire from the measuring end of the RTD to ground, which effectively prevents high-temperature leakage interference. When selecting the metal wire, choose one that is high-temperature resistant and harmless to the RTD electrodes.

Optimizing Wiring Methods

The wiring method of the RTD also affects its anti-interference capability. In industrial process control, a three-wire wiring method is commonly used, which can effectively eliminate the influence of lead resistance. A four-wire system can completely eliminate the influence of lead resistance and is mainly used for high-precision temperature detection.

Avoid using a two-wire wiring method, because a two-wire system will add the lead resistance to the RTD's resistance value, leading to increased measurement error. For example, with a 20-meter cable, the wire resistance is approximately 3 ohms, which translates to a temperature error of up to 6°C, a very large error.

Additional Protective Measures in Special Environments

In certain special environments, additional protective measures are required:

High-Temperature and High-Pressure Environments: For outdoor RTDs installed at high altitudes, ensure proper waterproofing of the conduit and leads to prevent water ingress and short circuits. RTDs installed at high temperatures and pressures require adequate insulation for the leads.

Oil-Immersed Environments: For RTDs that are immersed in oil for extended periods (such as thrust bearings in hydroelectric power plants), oil-resistant and temperature-resistant polytetrafluoroethylene (FEP) insulated cables should be selected. This material can be used continuously in temperatures ranging from -250°C to 250°C and has excellent oil resistance, corrosion resistance, and heat resistance.

Vibration Environments: In environments with strong vibrations, consider adding protective devices at the junction of the cable leads and the RTD, such as tapered spring protective tubes, corrugated pipes, and armored wire extension protection, to prevent root breakage.