RTD Wiring Guide: Material, Length, and Resistance Value Must All Be Correct

In industrial temperature measurement, resistance temperature detectors (RTDs) are the most commonly used temperature detectors in the medium- and low-temperature range. Their measurement accuracy depends not only on the quality of the sensor itself but also closely on the choice of connecting wires. Many errors in field temperature measurements often originate from those seemingly ordinary connecting wires. This article will analyze in detail the key requirements for wire material, outer diameter, length, and resistance value in RTD wiring.

Wire Material: Not Just Conductivity, But Stability

The connecting wires of RTDs must first meet specific material requirements. Copper, due to its high conductivity and good solderability, is the most commonly used wire material. For industrial platinum RTDs, the leads are typically made of silver wire with a diameter of about 1mm to minimize lead resistance and increase mechanical strength and connection reliability. Standard platinum RTDs can use 0.3mm platinum wire as the lead.

When selecting the wire material, its high-temperature resistance and corrosion resistance characteristics must be considered. Especially in harsh industrial environments, the outer sheath material of the wire needs to have good insulation properties and protective capabilities to protect the wire from mechanical damage and chemical corrosion.

It is worth noting that thermocouple compensating leads must not be used for connecting RTDs; this is a common mistake in practice.

Wire Outer Diameter: Strict Requirements for Cross-sectional Area

The cross-sectional area of ​​the connecting wires for RTDs has clear specifications. Generally, the wire cross-sectional area should not be less than 1.0 square millimeters, which is a basic requirement for most industrial instruments. Some applications are more stringent, requiring the use of 1.5 square millimeter wires to ensure sufficient mechanical strength and low line resistance.

Wires with too small a cross-sectional area will increase line resistance, introducing measurement errors; while those with too large a cross-sectional area may lead to inconvenient wiring and increased costs. Therefore, when selecting wires, it is necessary to follow the technical requirements of the instrument and find a balance.

Wire Length: The Principle of Short and Precise

Wire length has a direct impact on the accuracy of RTD measurements. In principle, the wiring length of the RTD should be as short as possible to reduce the influence of factors such as electromagnetic interference and temperature drift on the measurement results.

For three-wire connections, a crucial requirement is that the material, diameter, and length of the three wires must be identical, and ideally, they should operate at the same temperature to effectively compensate for lead resistance. Case studies have shown that when one wire is 2 meters longer than the other two, it leads to a 3°C measurement error.

For long-distance transmission, it's necessary to appropriately increase the wire diameter (e.g., using 1.5 square millimeter wire) and use shielded cables to reduce interference.

Resistance Value: The Key to Accuracy Control

The resistance value of the wires is a core parameter of the entire system. Display instruments typically have specific specifications for wire resistance values, generally requiring the resistance of a single wire to be controlled within 2-5Ω.

Why is wire resistance so critical? Taking a Pt100 RTD as an example, its standard resistance at 0°C is 100Ω, with a temperature coefficient of approximately 0.385Ω/°C. If each connecting wire has a resistance of 2Ω, for a two-wire connection, it will result in a measurement error of approximately 5.3°C.

Three-wire connection, through the bridge balancing principle, can offset the influence of lead resistance, reducing the error to ±0.5℃~±1℃. To completely eliminate the influence of lead resistance, a four-wire connection is required, primarily used in high-precision temperature measurement applications.

Wire Connection Selection: Two-wire, Three-wire, or Four-wire?

There are three main lead connection methods for RTDs, each with its applicable scenarios:

Two-wire is the simplest, but unsuitable for applications requiring high measurement accuracy because lead resistance is completely superimposed on the measurement result.

Three-wire is the most commonly used method in industrial process control. It effectively eliminates the influence of lead resistance, representing an ideal balance between accuracy and cost.

Four-wire completely eliminates the influence of lead resistance, offering the highest measurement accuracy, and is mainly used in laboratories and high-precision measurement applications.

Wiring Practice Recommendations

In actual wiring, the following key recommendations are provided:

Always use shielded cables, such as KVVRP or RVVP type cables, to effectively prevent signal interference. Before wiring, check the insulation performance of the wires to ensure that the insulation resistance between wires and between wires and the protective sleeve is greater than 100MΩ.

For long-distance transmission, consider using a three-core shielded cable with a wire diameter of 1.5 square millimeters, and ensure minimal external interference. Regularly check the wire connections to prevent increased contact resistance due to oxidation or loosening.