Instrument-10 - Cryogenic Instrumentation I Thermometry...

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1 OUTLINE 1. Thermometry 2. Temperature Ranges of Application 3. Constant Volume Thermometer 4. Thermocouples 5. Thermometer Time Response Data 6. 4 Terminal Resistance Measurement 8. Pt (pure metal) Thermometer 9. Typical Resistive Thermal Sensor 10. Typical Resistance and Sensitivity Curves 11. Thermal Regulation 12. Data Collection/Wheatstone Bridge Cryogenic Instrumentation I Thermometry Temperature Ranges of Application Constant Volume Thermometer •Named after Sir Fancis Simon •Helium is an ideal gas down to 5 K •Modern versions use in situ pressure gauges at low T with electrical read-out (no gas line from RT to Low T) Low T RT Helium gas Gas line Thermocouples Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 530 Wires of 2 different metals (pure or alloy) when joined and connected to a volt meter produce a voltage related to temperature. At right is the sensitivity of various common thermocouples, perhaps the simplest, least expensive, and most common thermometer in use. Usually a reference junction in an ice bath is used to make the measurement absolute. Calibrations are tabulated (e.g. type K) Thermometer Time Response Data Differences between wet and dry can be exploited for level detection Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 495
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2 4 Terminal Resistance Measurement Thermometer at low T Leads from RT to Low T must have low heat leak (alloy) Meter at RT must have high input impedance Current source must be stable and reversible. Average of readings with current flowing in opposite directions gives correct voltage drop, canceling thermal emfs. Often, low frequency ac source is used, with lock-in detector as volt meter, to improve sensitivity. Pt (pure metal) thermometer •Resistance thermometer—use 4 terminal set-up •R is almost proportional to T: Callender-VanDusen Equation: R(T)=R 0 [1+AT+BT 2 +C(T-10)T 3 ] (0 K<T<300K) Other calibrations available down to 20 K •Purity determines calibration (A, B, C)-no individual calibration required •Transfer standard used by NIST Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 512 Typical Resistive Thermal Sensor Cross-section:Details of construction showing strain-free mounting of chip (#4) inside hermetically sealed container Photo of sensor with leads 0.01
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Instrument-10 - Cryogenic Instrumentation I Thermometry...

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