The three wiring methods for Pt100 thermal resistors differ in their working principles and impact on measurement accuracy. Understanding these differences is crucial for ensuring reliable temperature readings in industrial and scientific applications.
The two-wire system and the three-wire system both use a bridge method to measure resistance, which is then converted into a temperature value. However, this method is susceptible to errors caused by the resistance of the connecting wires and contact resistance. In contrast, the four-wire system uses a constant current source to send current through the resistor and measures the voltage directly, eliminating the influence of lead resistance and providing higher accuracy.
The three-wire system improves upon the two-wire setup by using an additional wire to separate the current loop from the voltage measurement loop. This configuration helps reduce errors caused by lead resistance, making it more accurate than the two-wire method but less precise than the four-wire system.
The two-wire system, while simple and cost-effective, is the least accurate because it combines the current and voltage measurement loops into one. This means that any resistance in the connecting wires directly affects the measured resistance, leading to potential inaccuracies in temperature readings.
For high-precision applications, the four-wire system is often preferred due to its ability to isolate the current and voltage measurement paths. Most modern test equipment, such as temperature controllers and PLC inputs, are designed with four terminals: I+ (current input), I- (current output), V+ (voltage input), and V- (voltage output). These terminals allow for accurate measurement by supplying a constant current and measuring the voltage across the resistor independently.
In practice, choosing the right wiring method depends on the required level of accuracy, the length of the wiring, and the type of equipment being used. If you're working with long-distance measurements or need high precision, the four-wire system is the best choice. For most general applications, the three-wire system offers a good balance between simplicity and accuracy.
To better visualize the differences between these wiring methods, refer to the image below:
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