Converting 2-Wire and 3-Wire RTDs into 4-Wire RTDs

Process plants require Resistance Temperature Detectors (RTDs) in order to accurately monitor temperature. This comprehensive guide explores the step-by-step procedures for modifying 2-wire and 3-wire RTDs to 4-wire RTDs to improve temperature measurement accuracy. The discussion is going to center around an understanding of the various types of RTDs, the difficulties associated with spare RTDs, and the importance of these conversions.

Before getting into the conversion processes, let’s create a fundamental understanding of the three main types of RTDs based on their wire configuration:

Different Types of RTDs

The simplest arrangement has two lead wires.

Highly susceptible to lead wire resistance, which might affect accuracy.

Contains three lead wires in its structure.

Designed to reduce lead wire resistance and increase overall accuracy.

four lead wires are used, providing improved accuracy.

reduces resistance caused by lead wire, providing accurate temperature readings.

When a spare RTD is not compatible with the type placed in the field, instrumentation engineers frequently run into problems. An example of this would be having a spare 2-wire RTD in the event that a 3-wire RTD fails, or vice versa. In order to ensure ongoing plant operation, a temporary solution to these problems is changing the available spare RTD to match the current configuration.

Steps for Converting 2-Wire RTD into 3-Wire RTD:
  • For the purpose of connecting an additional cable, select one of the lead wires from the two-wire RTD.
  • One pair of lead wires in a three-wire RTD shows resistance that is close to one ohm.
  • Connect the additional cable to the transmitter terminal and the selected lead wire.
  • Note: This eliminates lead wire compensation, resulting in reduced accuracy compared to the original 3-wire configuration.
  • Set up the temperature transmitter to function as a three-wire RTD sensor.
  • It should be acknowledged that this method is a short-term solution that has a lower degree of accuracy but still preserves the functionality of the plant

While the procedure of converting a 2-wire RTD into a 4-wire RTD is more complicated, it is essential in situations where a spare 4-wire RTD is not readily available. For a successful conversion, make sure to follow these specific steps:

Steps for Converting 2-Wire RTD into 4-Wire RTD:
  • Be aware that a 4-wire RTD is made up of four lead wires that are arranged in two pairs and have a resistance that is generally close to 1 ohm.
  • Connect a cable parallel to both existing cables of the 2-wire RTD.
  • Ensure proper connections at all terminals, following the recommended configurations.
  • Even though you are utilizing a 2-wire RTD, configure the temperature transmitter with a 4-wire RTD sensor configuration.
  • Please be aware that although this technique lowers measurement error, it is not advised to use it in place of a specific 4-wire RTD sensor.
  • The accuracy of the temperature measurement will be lower than it was with the original 4-wire arrangement, although the fact that these conversions assist reduce the probability of temperature measurement errors.
Steps for Converting 3-Wire RTD into 4-Wire RTD:
  • In a 3-wire RTD, one lead wire serves as a common connection for two resistive elements. Identify this common lead and the other two.
  • Introduce a fourth wire, connecting it in parallel with one of the two existing lead wires. 
  • This creates two parallel paths, similar to the configuration in a 4-wire RTD.
  • Connect the additional wire to the transmitter terminal, ensuring proper identification and insulation to prevent interference.
  • Configure the temperature transmitter to operate with a 4-wire RTD sensor setting. 
  • This adjustment is crucial for compatibility and accurate temperature readings.
  • Double-check all connections to ensure that there are no loose or incorrect terminations. 
  • Proper insulation should be maintained to avoid any signal interference.
  • While the conversion minimizes the impact of lead wire resistance, it is important to acknowledge that long lead lengths can still introduce some error. Keeping lead lengths short is advisable for optimal accuracy.
  • Proper calibration of the temperature transmitter is essential to account for any changes in the configuration. Regular calibration checks should be included into the maintenance schedule.
  • Understand that, while the conversion improves accuracy compared to a 3-wire configuration, it may not achieve the same accuracy as an original 4-wire RTD. In applications where great accuracy is essential, take this balance into consideration.

Converting 2-wire and 3-wire RTDs to 4-wire RTDs improves temperature measurement accuracy in process plants. However, it’s crucial to acknowledge that these conversions may result in lower accuracy than original configurations.

  • This ASTM standard provides specifications for industrial platinum resistance thermometers, which include RTDs. It covers various aspects, including construction, tolerances, calibration, and testing procedures.
  • This International Electrotechnical Commission (IEC) standard specifies the characteristics and testing of industrial platinum resistance thermometers, which are commonly used as RTDs. It covers different classes of RTDs, including Class A and Class B, based on their tolerance and temperature coefficient.
  1. Click here for RTD Temperature Coefficient Calculation to learn about RTD temperature coefficients and their impact on resistance variations.
  2. Click here for RTD Calculator: Converting Measured Resistance to Process Temperature to utilize this calculator to convert RTD measured resistance to accurate process temperature readings.
  3. Click here for RTD Calculator: Converting Process Temperature to measured Output Resistance to access this calculator to convert process temperature values into corresponding RTD measured output resistance for precise temperature measurements.

Frequently asked questions

Yes, a 4-wire RTD can function as a 2-wire RTD. Connect two wires (e.g., 2x red) and leave the unused wires aside. Accuracy may vary.

2-wire, 3-wire, and 4-wire RTDs differ in wiring configuration. 2-wire is simpler but prone to lead wire resistance. 3-wire minimizes this impact. 4-wire further enhances accuracy by isolating lead resistance, vital for precise temperature measurements.

RTDs are named based on their wiring configuration. 2-wire RTDs have two lead wires, 3-wire RTDs use three, and 4-wire RTDs employ four wires, each configuration impacting accuracy in temperature measurements.

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