A platium RTD (alpha = 0.004^degree - 1, for ease of calculation) is used to mea

Question

A platium RTD (alpha = 0.004^degree - 1, for ease of calculation) is used to measure the temperature via a four-wire bridge scheme in balanced mode, as shown in Fig. 2. It is known that the RTD has a resistance of 25 ohm at temperature 0 degree and is used to measure temperature at steady-state. The bridge was balanced in the two measurements at R_3 = 200 ohm and R'_3 = 300 ohm. Determine the measured temperature. Assuming that R_3 has a 1% (95%) uncertainty, and r_1 and r_3 has an uncertainty of (u_d)_r1 = 1 ohm (95%) and (u_d)_r3 = 2 ohm (95%). Find the uncertainty in the measured RTD resistance.

Explanation / Answer

Soln.

1. Temperature can be obtained using bridge balance condition R2*(r3+Rrtd)=R2*(R3+r1)

r3+Rrtdo(1+T)=R3+r1

T=[((R3+r1-r3)/Rrtd0)-1]/

So for R3=200, T=1750

R3'=300, T=2750

2. % Uncertainity = ((%uR3)^2+(%ur1)^2+(%ur3)^2)^0.5/(Rrtd0)

=(0.01^2+2*0.95*2)^0.5/(0.004*25)

=13.435

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