Temperature Measurement

Why 4 wire RTD’s measurement accuracy is better than 2 and 3 wire RTD?

RTD Measurement: An Overview 

Why 4 wire RTD’s measurement accuracy is better than 2 and 3 wire RTD? 1
  • RTD (Resistance Temperature Detector) measurement is based on the idea that the electrical resistance of a metal changes predictably with temperature. 
  • The resistance of RTDs, which are frequently made of platinum, increases linearly with temperature. A known current runs through the RTD, causing a voltage drop across it. 
  • The resistance of the RTD and subsequently the temperature are inversely correlated with this voltage. 
  • Several wiring layouts, including 2, 3, and 4-wire arrangements, are used to increase accuracy. The most precise arrangement has four wires, which completely eliminates the effects of lead wire resistance. 
  • For accurate temperature sensing in applications including industrial processes, HVAC systems, and medical devices, RTD measurements are essential in order to account for non-linearities and produce accurate temperature measurements, calibration and compensation are conducted.

The Requirement for Compensation

  • In temperature sensing systems, compensation is essential because of the many variables that might change measurement accuracy. 
  • RTDs and other thermal sensors have inherent problems with lead wire resistance and less-than-ideal properties. 
  • These problems are reduced through compensation strategies. Readings can be considerably distorted by lead wire resistance, especially in low-resistance sensors. 
  • Particularly in applications with long wire lengths or low resistance values, this variation has an impact on the level of accuracy of the temperature measurement and can cause large error.
  • Accuracy is improved via compensation, which usually involves using 3- or 4-wire designs to eliminate the effect of lead wires. For accurate measurements, nonlinearities in sensor output must also be taken into consideration.

Benefits of 4-Wire RTD Measurement

In comparison to the 2-wire and 3-wire configurations, the 4-wire RTD measurement configuration is clearly superior, principally because lead wire resistance is completely eliminated. 

With a 4 wire RTD, resistance is not used to determine the temperature. The two lead wires of a four wire RTD are connected in series with a constant current source. We take a voltage reading across the additional two lead wires of the four wire RTD.

Ohm’s law states that V = IR.

The current value is constant (extremely low). The change in resistance, which is dependent on the temperature being measured, will be used to generate the voltage.

As a result, 4 wire RTDs are more accurate than 2 or 3 wire RTDs since they monitor temperature using a voltage signal rather than a resistance.

The four wire RTD circuit schematic is displayed below.

Why 4 wire RTD’s measurement accuracy is better than 2 and 3 wire RTD? 2

Here are some benefits of the 4-wire configuration:

Lead Wire Resistance Elimination

  •  Lead wire resistance, particularly with low-resistance RTDs, can have a considerable impact on measurement accuracy in a 2-wire configuration.
  •  Temperature measurements are extremely accurate because to the 4-wire arrangement, which fully eliminates this influence.
  • Two additional wires are utilized only for measuring the RTD element’s resistance in a 4-wire RTD measurement system. 
  • These wires have very little current flowing through them, and their sole function is to detect the RTD’s true resistance without being influenced by lead resistance. 
  • Lead resistance mistakes are efficiently minimized by separating the RTD element from the lead wires.

Higher Accuracy

  • Out of the three configurations, the 4-wire layout provides the most accurate readings. 
  • It eliminates mistakes brought on by voltage drops across lead wires by using separate wires for current excitation and voltage measurement, resulting in accurate temperature data.

Performance of Low-Resistance RTDs

  • The 4-wire arrangement is essential for low-resistance RTD applications, such as platinum RTDs. 
  • Accurate readings are practically difficult with 2 or 3-wire setups because the resistance of the lead wires can be equivalent to or even greater than the resistance of the RTD.

Consistency

  • The 4-wire arrangement keeps precision even when wire resistances alter as a result of temperature changes or variations in production. 
  • This consistency is especially crucial in critical processes or applications that require accurate temperature monitoring.

Wide Operating Range

  • Because the 4-wire arrangement works well in a wide range of temperatures, it may be used in a variety of environments and applications where temperature accuracy is crucial.

Process Control and Calibration

  • Industries needing strict process control and precise calibration, such pharmaceuticals and aerospace, greatly benefit from the accuracy offered by 4-wire RTD measurements.

4 wire RTD vs 2 and 3 wire RTD

Aspect4-Wire RTD2-Wire RTD3-Wire RTD
Lead Wire ResistanceEffectively EliminatedSignificant ImpactPartially Compensated
AccuracyHigh AccuracyModerate AccuracyImproved Accuracy
Low-Resistance RTDsExcellent PerformancePoor PerformanceImproved Performance
Wire Resistance VariationMinimal ImpactSignificant ImpactModerate Impact
ConsistencyHigh ConsistencyModerate ConsistencyImproved Consistency
CalibrationPrecise CalibrationLess PreciseImproved Calibration
Temperature RangeWide RangeModerate RangeModerate Range
Suitable for Critical ApplicationsYesLimited SuitabilityImproved Suitability
ComplexityMore Complex WiringSimple WiringModerate Complexity

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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