28 Feb 2000ISAT 3001 Resistance Temperature Detectors (RTDs)

28 Feb 2000ISAT 3002 Bridge Circuits ISAT 300 Spring 1999

28 Feb 2000ISAT 3003 Wheatstone Bridge n A circuit designed to measure changes in resistance n In Instrumentation it is used as signal conditioning for strain gages

28 Feb 2000ISAT 3004 VsVs R1R1 R2R2 R3R3 R4R VoVo A B C D

28 Feb 2000ISAT 3005 Build a Wheatstone Bridge VsVs + - R2R2 R3R3 V2V2 V3V3

28 Feb 2000ISAT 3006 Build A Wheatstone Bridge VsVs + - R2R2 R3R3 V2V2 V3V3 R1R1 R4R4 V1V1 V4V4 VoVo Or Apply Kirchoff’s Voltage Law:

28 Feb 2000ISAT 3007 Balancing the Bridge Governing Equation Multiply by a common denominator Simplify

28 Feb 2000ISAT 3008 Balance The Bridge The bridge is balanced if the output is zero

28 Feb 2000ISAT 3009 If R3 is the RTD, then With some algebra, For RTD circuits we can get R RTD as a function of V o Start with Design with R1=R4, then

28 Feb 2000ISAT RTDs: Characteristics and Applications n Characteristics: u Resistive device, active, linear u Large range: -200 to +850 o C for Platinum u High accuracy:  o C u Low sensitivity: 0.39 % per o C u Don’t need reference temperature n Applications: u Industries and laboratories where high accuracy of temperature measurements are required.

28 Feb 2000ISAT Thin-Film RTDs Thin-film RTD design is a newer technology and is gaining favor due to lower cost. It is designed to minimize strain on the platinum due to thermal expansion since strain also cause changes in resistance, R =  (L/A).

28 Feb 2000ISAT Calendar-Van Dusen Equation For platinum, the resistance temperature relationship is given by the Calendar-Van Dusen equation: For the U. S. calibration curve,  = /°C (U.S. calibration curve, text p 248)

28 Feb 2000ISAT Platinum RTD: R versus T (U.S. Calibration)

28 Feb 2000ISAT RTD’s small resistance change requires n Bridge circuit: u Can detect small resistance changes u If R 1 =R 4, R RTD = R 2 (V s -2V o )/(V s +2V o ) (eq. 9.11) R1R1 R2R2 R RTD R4R4 VoVo VsVs “Supply” Voltage

28 Feb 2000ISAT Circuits Used to Determine the Resistance of an RTD n Two-wire: Non-linear relationship between the measured voltage and the RTD resistance. n Three-wire: Better results. n Four-wire: Resistance is a linear function of the measured voltage.  Four Wire Design

28 Feb 2000ISAT Example: An RTD probe has a resistance of 100  at 0 o C. The Calendar-Van Dusen constants are  = ,  = 1.49, and  = 0 for T > 0 o C. What will be the resistance at 350 o C. Alternatively, we could use table 9.3 (p248) and obtain R T = . (R T =R RTD )

28 Feb 2000ISAT Summary n Thermocouples u Passive, non-linear, increase temperature increase voltages, big temperature range. u Types K and T are common devices. u Need reference temperature n Thermistors u Active, highly non-linear, increase temperature decrease resistance. u Medical use, not available above 300 o C. n RTD’s u Requires a Bridge, Linear by nature. u High accuracy, use in industry & laboratory. n ALL: time constant of a first order system