1 / 17

Resistance Temperature Detectors (RTDs)

Resistance Temperature Detectors (RTDs). Bridge Circuits. ISAT 300 Spring 1999. Wheatstone Bridge. A circuit designed to measure changes in resistance In Instrumentation it is used as signal conditioning for strain gages. R 1. R 2. +. -. +. V s. V o. -. R 3. R 4. A. B. D. C.

corbin
Download Presentation

Resistance Temperature Detectors (RTDs)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Resistance Temperature Detectors(RTDs) ISAT 300

  2. Bridge Circuits ISAT 300 Spring 1999 ISAT 300

  3. Wheatstone Bridge • A circuit designed to measure changes in resistance • In Instrumentation it is used as signal conditioning for strain gages ISAT 300

  4. R1 R2 + - + Vs Vo - R3 R4 A B D C ISAT 300

  5. V2 R2 + Vs - R3 V3 Build a Wheatstone Bridge ISAT 300

  6. Build A Wheatstone Bridge Apply Kirchoff’s Voltage Law: V2 V1 R1 R2 + Vo Vs - Or R4 V4 R3 V3 ISAT 300

  7. Balancing the Bridge Governing Equation Multiply by a common denominator Simplify ISAT 300

  8. Balance The Bridge The bridge is balanced if the output is zero ISAT 300

  9. For RTD circuits we can get RRTD as a function of Vo Start with If R3 is the RTD, then Design with R1=R4, then With some algebra, ISAT 300

  10. RTDs: Characteristics and Applications • Characteristics: • Resistive device, active, linear • Large range: -200 to +850oC for Platinum • High accuracy: 0.001oC • Low sensitivity: 0.39 % per oC • Don’t need reference temperature • Applications: • Industries and laboratories where high accuracy of temperature measurements are required. ISAT 300

  11. 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). ISAT 300

  12. Calendar-Van Dusen Equation For platinum, the resistance temperature relationship is given by the Calendar-Van Dusen equation: (U.S. calibration curve, text p 248) For the U. S. calibration curve, a = 0.003851/°C ISAT 300

  13. Platinum RTD: R versus T(U.S. Calibration) ISAT 300

  14. R1 R2 Vo Vs R4 RRTD RTD’s small resistance change requires • Bridge circuit: • Can detect small resistance changes • If R1=R4, RRTD= R2(Vs-2Vo)/(Vs+2Vo)(eq. 9.11) “Supply” Voltage ISAT 300

  15. Circuits Used to Determine the Resistance of an RTD • Two-wire: Non-linear relationship between the measured voltage and the RTD resistance. • Three-wire: Better results. • Four-wire: Resistance is a linear function of the measured voltage.  Four Wire Design ISAT 300

  16. Example: An RTD probe has a resistance of 100  at 0oC. The Calendar-Van Dusen constants are  = 0.00392,  = 1.49, and  = 0 for T > 0oC. What will be the resistance at 350oC. (RT=RRTD) Alternatively, we could use table 9.3 (p248) and obtain RT = 231.89 . ISAT 300

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

More Related