No more worries about long-wire temperature measurement: Three-wire and four-wire technologies accurately eliminate errors in resistance temperature detector (RTD) measurements.

In industrial automation, temperature measurement is a crucial aspect of process control. Resistance temperature detectors (RTDs) are the most commonly used temperature sensors in the medium- and low-temperature range due to their high accuracy and stable performance. However, when using long connecting wires to RTDs, the wire resistance can lead to significant measurement errors. So, how can this error be avoided?

Measurement Error Due to Wire Resistance

In the actual connection between RTDs and display instruments, because the connecting wires are relatively long, if only two wires are used to connect the two ends of the RTD, the resistance of the wires themselves will be in series with the RTD, causing measurement errors. If the resistance of each wire is *r*, the absolute error added to the RTD is 2*r*, and this error is not constant; it varies with the ambient temperature of the wires. For a Pt100 RTD, every 0.38Ω error is equivalent to a 1℃ temperature deviation, demonstrating that the influence of wire resistance cannot be ignored.

Three-Wire Connection: A Common Choice for Industrial Applications

The three-wire connection method connects one wire to one end of the RTD and two wires to the other end. When used in conjunction with a bridge circuit, the three wires connected to the RTD have the same thickness, length, and resistance value *r*.

When the bridge circuit is balanced, if the condition R1=R2 is met, the influence of the wire resistance r can be completely eliminated. In this case, changes in wire resistance have no effect on the RTD measurement. It is important to note that the influence of wire resistance can only be completely eliminated when the bridge is in a balanced state with all four arms having equal resistance. Otherwise, although it cannot be completely eliminated, the three-wire connection can significantly reduce its influence.

The three-wire connection is currently the most common connection method in industrial applications, achieving a good balance between measurement accuracy and cost.

Four-wire connection: The ideal choice for high-precision measurement

The four-wire connection uses two wires at each end of the RTD to connect to the instrument, typically used for high-precision instruments requiring voltage or potential input.

The working principle of the four-wire connection is as follows: a known current I is supplied by a constant current source flowing through the RTD, generating a voltage drop U. Rt can be obtained by measuring U (Rt=U/I). In this connection method, although the current-carrying wires have resistance *r*, their voltage drop is outside the measurement range. While the wires connecting to the measuring instrument have resistance, no current flows through them (current is not measured in high-impedance measurements). Therefore, the resistance of all four wires has no effect on the measurement.

As long as the current from the constant current source remains stable, the four-wire system is a relatively perfect method, unrestricted by any conditions, and can completely eliminate the influence of the connecting wire resistance on the measurement. This method is particularly suitable for laboratory and high-precision measurement applications.

Two-wire connection method: Limited to applications with low precision requirements

The two-wire connection method is the simplest connection method, connecting a wire to each end of the RTD to extract the resistance signal. This connection method is only suitable for applications with low measurement precision requirements, and the wire length should not be too long. The error in the two-wire system comes directly from the wire resistance; the longer and thinner the wire, the greater the error.

Key Points for Correct Installation

Regardless of whether a three-wire or four-wire connection is used, for accurate measurements, the lead wire must originate from the base of the resistor element, i.e., from the internal lead, and not from the terminals of the RTD's junction box. This is because the internal lead is located in a region of drastic temperature changes. Although the internal lead within the protective conduit is short, its resistance effect cannot be ignored during accurate measurements.

Furthermore, the following should be noted when installing an RTD:

The temperature measurement point should be selected at a location representative of the medium temperature, avoiding installation near valves, bends, or dead zones in equipment.

The RTD should have sufficient insertion depth, generally inserted into the center of the pipe.

In high-temperature, high-pressure, and high-speed fluid conditions, shallow-insertion or heat-shrinkable RTDs can be used.

Hequan Instrument Technology Co., Ltd.'s Temperature Measurement Solutions

In the field of industrial temperature measurement, accuracy and reliability are key indicators for evaluating the quality of technical solutions. Hequan Instrument Technology Co., Ltd. has been deeply involved in the temperature measurement field for many years and deeply understands the impact of resistance errors in long lead wire connections on measurement results. Our resistance temperature detectors (RTDs) employ standard three-wire and four-wire connection methods, with the wires extending directly from the base of the resistor element, effectively eliminating measurement errors caused by the resistance of the connecting wires.

To meet the specific needs of different industrial scenarios, we offer various models of platinum and copper RTDs, along with appropriate wiring instructions, ensuring our customers obtain the most accurate temperature measurement data. Whether for routine industrial applications or high-precision laboratory measurements, Hequan Instrument Technology Co., Ltd. provides professional and reliable solutions.