1. 1 RTD (Resistance temperature detector) n Varying resistance with temperature n Good for measurement of small temperature differences n General equation for RTD material Rt = Ro(1+ At + Bt 2 + Ct 3 - - - - - Wt 4 ) B,C - - -W are negligibly small. Rt = Ro(1+ At)

2. 2 RTDs: Characteristics and Applications n Characteristics: u Resistive device, linear u Large range: -200 to +850 o C for Platinum u High accuracy:  0.001 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.

3. 3 Requirements for RTD material n High temperature coefficient n High resistivity to ensure small wire length n Linearity of relation between resistance and temperature n Sufficient mechanical strength n Stability of electrical characteristics

4. 4 Construction n A thin Platinum wire wound in form of a free spiral by an insulated carrier such as mica or ceramic. n Diameter of wire should be 0.02mm to 0.2mm. n Wire should be smooth, free from defects, ensurity of constancy of resistance to avoid resistance changes due to dimension changes. n Wire generally enclosed in a protective tube made of glass, quartz etc.

5. 5 Thin-Film RTDs

6. 6 Calendar-Van Dusen Equation For platinum, the resistance temperature relationship is given by the following equation: For the U. S. calibration curve,  = 0.003851/°C (U.S. calibration curve

7. 7 Platinum RTD: R versus T (U.S. Calibration)

8. 8 Graph for different materials

9. 9 RTD’s small resistance change requires n Bridge circuit: u Can detect small resistance changes R1R1 R2R2 R RTD R4R4 VoVo VsVs “Supply” Voltage

10. 10 RTD Materials Gold & Silver: Rarely used because of low resistivity Tungsten: Very high resistivity Gold Silver alloy: Give same charac s As Platinum below 125 0 C. Phosphor Bronze alloy: Better for low temperature measurement Nickel: Most used, cheaper than platinum Platinum: More suitable but expensive, high range, high precision

11. 11 n Very accurate n Very stable n Standardized among vendors n Large variety of packaging options RTD Advantages

12. 12 RTD Disadvantages n Costly n Require current source or voltage source n Low resistance/small change n 4-wire measurement n Slow

13. 13 Latest development :Miniature RTDs