An RTD forms one arm of a Wheatstone bridge, as shown in Figure 8.38. The RTD is used to measure a constant temperature, with the bridge operated in a balanced mode. The RTD has a resistance of 25 V at a temperature of 0°C, and a thermal coefficient of resistance, a = 0.003925°C^-1. The value of the variable resistance R1 must be set to 37 V to balance the bridge circuit.

a. Determine the temperature of the RTD.

b. Compare this circuit to the equal-arm bridge in Example 8.2. Which circuit provides the greater static sensitivity?

Example 8.2

Consider the bridge circuit and RTD of Example 8.1. To select or design a bridge circuit for measuring the resistance of the RTD in this example, the required uncertainty in temperature would be specified. If the required uncertainty in the measured temperature is 0:5°C, would a 1% total uncertainty in each of the resistors that make up the bridge be acceptable? Neglect the effects of lead wire resistances for this example.

KNOWN A required uncertainty in temperature of 0.5°C, measured with the RTD and bridge circuit of Example 8.1.

FIND The uncertainty in the measured temperature for a 1% total uncertainty in each of the resistors that make up the bridge circuit.

ASSUMPTION All uncertainties are provided and evaluated at the 95% confidence level.

Example 8.1

An RTD forms one arm of an equal-arm Wheatstone bridge, as shown in Figure 8.8. The fixed resistances, R2 and R3 are equal to 25 V. The RTD has a resistance of 25 V at a temperature of 0°C and is used to measure a temperature that is steady in time. The resistance of the RTD over a small temperature range may be expressed, as in Equation 8.5:

Figure 8.8

Figure 8.38