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WHY 2 OR 3 WIRES FOR AN RTD INPUT The RTD (Resistance Temperature Detector) is commonly used to achieve an accurate repeatable temperature measurement. The most common RTD is made of platinum wire and has a resistance of 100 ohms at 0 degrees Celsius, 32 degrees Fahrenheit. For each change of 1 degree Celsius the resistance changes 0.00385 ohm. Today most signal conditioners pass a fixed relegated current through the RTD and measure the voltage generated by the rules of Ohms Law E=IR Voltage (E) equals Current (I) times Resistance (R). If we know the current we pass through the resistor and we can measure the voltage then we can calculate the resistance. As the resistance changes with temperature, the voltage changes proportionately and we have our temperature measurement. This type of measurement can be useful where changes in lead length and ambient temperature can be ignored. If the lead length were fixed and compensated for and the ambient temperature did not vary then the lead resistance values would remain constant. But in the real world this is not so and changes in temperature can affect your measurement. Therefore the most common measurement is made using 3 wires instead of 2. The third wire is used to compensate for lead resistance and temperature changes in lead resistance (see fig. 1). It’s in a high impedance circuit so that no significant current can pass through it and therefore its' resistance is not a factor. It can measure the voltage at the connection of L(2) and L(3) at the RTD. ER(2) is now the voltage of one leg, L(2) in the circuit. If L(1) is the same size wire and same length as L(2) then we consider the voltage to be equal. Therefore the correct temperature voltage E(T) is equal to the measured voltage E(M) minus the correction voltage ER(2), E(T)=E(M)-ER(2). In this manner, the signal conditioner can measure the correct temperature at the RTD by using 3 leads, hence a 3 wire RTD. This change in RTD resistance can now be quantified and indicated digitally as a temperature or converted to a DC output through a transmitter proportional to the RTD input. This stable signal can now be utilized in many types of process control instruments, such as chart recorders, as a compensation for flow signals or input into systems for process control. fig. 1
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