Question

Draw a lead resistance loop connection for connecting a sensor to a Wheatstone bridge. Where should the sensor be put to properly test for heat?

Answer 1

A wheatstone bridge circuit is given by,

The Wheatstone Bridge circuit consists of two basic series-parallel configurations of the resistances linked between the voltage supply terminal and the ground generating zero voltage differential between the two parallel branches when balanced. The Wheatstone bridge circuit consists of two input terminals and two output terminals consisting of four resistors. When balanced, the Wheatstone bridge can be analyzed simply as two strings in parallel.  

So in order to measue temperature a Resistance Temperature Detector(RTD) can be used.

Resistance Temperature Detector(RTD)

The RTD (Resistance Temperature Detector) is a sensor whose resistance varies as the temperature changes. Resistance increases as the temperature of the sensor rises. Resistance vs. temperature ratio is well established and can be replicated over time. The RTD is a passive instrument. It does not generate any production of its own. External electronic instruments are used to test the sensor's resistance by sending a tiny electrical current through the sensor to produce a voltage. Typically 1 mA or less measuring current, maximum 5 mA without risk of self-heating.

The most popular approach for calculating RTD resistance is to use the Wheatstone Bridge Circuit.

The electrical excitation current is passed via the bridge, and the RTD and bridge output voltage are an indication of RTD resistance. The circuit uses a very robust power supply, three high-precision resistors with a very low temperature coefficient, and High-input impedance amplifier to calculate RTD resistance adjustment with temperature changes.

The RTD is usually situated on the plant equipment or tubing, and the measuring system can be situated hundreds of feet from the control center. As both the slope and the absolute resistance value of the standard RTD are high, the length of the wire from the RTD to the Wheatstone bridge circuit may be important. This is particularly true when we take into account that the measuring wires that lead to the sensor may be quite a few ohms or even tens of ohms. A small resistance to lead can result in a significant error in the measurement of the output temperature.

The RTD and its two lead wires make up a bridge leg. The R1 and R2 resistors are of equal resistance, whereas the R3 resistor is adjustable and is used as a reference. The DC voltage source provides the RTD with an excitation current. The differential amplifier generates a voltage proportional to the bridge output voltage. T he resistance R3 is initially balanced to achieve a zero voltage (0 V) at the output of the differential amplifier. In this condition, the bridge is said to be zero balanced. When the bridge is zero balanced, the output voltage of the amplifier should differ in direct relation to the temperature.