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Resistance Temperature Detectors (RTDs). Characteristics and Applications. Characteristics: Resistive device, active, linear Large range: -200 to +850 o C for Platinum High accuracy: 0.001 o C Low sensitivity: 0.39 % per o C. Applications:
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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 • Applications: • Industries and laboratories where high accuracy of temperature measurements are required. ISAT 300
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
Calendar-Van Dusen Equation For platinum, the resistance temperature relationship is given by the Calendar-Van Dusen equation: For the U. S. calibration curve, a = 0.003921/°C ISAT 300
R1 R2 Vout Vin R4 RRTD RTD’s Small Resistance Change • Bridge circuit: • Can detect small resistance changes • Adjust R3 so that R1/R4 = R2/RRTD More on bridge circuits next lecture! ISAT 300
Circuits Used to Determine the Resistance of an RTD • Two-wire: Non-linear relationship between the measured votage and the RTD resistance. • Three-wire: Better results. • Four-wire: Resistance is a linear function of the measured volatge. ISAT 300
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. Alternatively, we could use table 9.3 and obtain RT = 231.89 . ISAT 300
Summary • Thermocouples • Passive, non-linear, increase temperature increase voltages, big temperature range. • Types K and T are common devices. • Time Constant of a first order system • 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. ISAT 300