Cryogenic Instrumentation I Thermometry OUTLINE Thermometry Pt (pure metal) Temperature Ranges of Thermometer Application Typical Resistive Thermal

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Geraldine Brittney Hoover
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Cryogenic Instrumentation I 1. Thermometry 2.

Typical esistance and Sensitivity Curves 11.

Data Collection/Wheatstone Bridge Thermometry anges

Fancis Simon Helium is an ideal gas down to 5 K

T with electrical read-out (no gas line from T to

different metals (pure or alloy) when joined and

thermocouples, perhaps the simplest, least

Usually a reference junction in an ice bath is used

Thermocouples Calibrations are tabulated (e.g.

1 Cryogenic Instrumentation I 1. Thermometry 2. anges of Application 3. Constant Volume 4. Thermocouples 5. Time esponse Data 6. 4 Terminal esistance Measurement OUTLINE 8. Pt (pure metal) 9. Typical esistive Thermal Sensor 10. Typical esistance and Sensitivity Curves 11. Thermal egulation 12. Data Collection/Wheatstone Bridge Thermometry anges of Application Constant Volume 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 T to Low T) T Gas line Low T Helium gas 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. Thermocouples Calibrations are tabulated (e.g. type K) Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 530 Time esponse 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

4 Terminal esistance Measurement Meter at T must have high input impedance at low T Leads from T to Low T must have low heat leak (alloy) Pt (pure metal) thermometer esistance thermometer use 4

C)-no individual calibration required Transfer standard used by NIST Current source must be stable and reversible.

Often, low frequency ac source is used, with lock-in detector as volt meter, to improve sensitivity. Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p.

2 4 Terminal esistance Measurement Meter at T must have high input impedance at low T Leads from T to Low T must have low heat leak (alloy) Pt (pure metal) thermometer esistance thermometer use 4 terminal set-up is almost proportional to T: Callender-VanDusen Equation: (T)= 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 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. Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 512 Typical esistive Thermal Sensor Cross-section:Details of construction showing strain-free mounting of chip (#4) inside hermetically sealed container esistance ( ) Typical esistance and Sensitivity Curves TT-D TT-G Ge on GaAs substrates Produced in development of Maximum sensitivity at 20 K (liquid hydrogen) Photo of sensor with leads (K) Dimensionless Sensitivity (S) TT-G 3 TT-D (K) PID Heater esponse Temp. Monitor Input-set point Thermal egulation Dewar Heater Computer data acquisition board may be used for all electronics Computer with data acquisition board Data Collection/Wheatstone Bridge Input- Current level measurement and control is easily automated Computer board Break-out box source detector sensor Decade 2

Cryogenic Instrumentation II 1. Pressure Measurements 2. Pressure Transducers Characteristics 3.

Level Detection-Continuous 7. Time esponse Data-again 8.

conversion of a force measurement to some measurable parameter Converters, or transducers may be: Electrical:

These devices may often be connected to low temperature volumes using small tubes, avoiding cooling the

changed Low temperature devices often work better than at room temperature because the environment is more

Piezoelectric Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p.

3 Cryogenic Instrumentation II 1. Pressure Measurements 2. Pressure Transducers Characteristics 3. Thermal Conductivity Measurements 4. Example: Ortho-Para atiometer OUTLINE 5. Level Detection-Point 6. Level Detection-Continuous 7. Time esponse Data-again 8. Flow Metering Pressure Measurement at Low Pressure is a force applied to an area, so its measure involves the conversion of a force measurement to some measurable parameter Converters, or transducers may be: Electrical: Capacitive-diaphragm Inductive-reluctive esistive-strain gauge Mechanical-spring Piezoelectric-quartz crystal These devices may often be connected to low temperature volumes using small tubes, avoiding cooling the transducer Most transducers operate at low temperature, although sensitivity and calibration are usually changed Low temperature devices often work better than at room temperature because the environment is more controlled Typical Pressure Transducers Mechanical Pressure Gauge Bourdon Tube Capacitive Inductive Piezoelectric Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 457 Characteristics of Transducers Dependence of Transducer Output Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 458 Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p

C Ultra-sensitive/Low Pressure Transducer Developed strain-type (similar to UF Straty-Adams gauge) in situ capacitive probe with resolution

3 liters Screen Push od 10 cm Membrane which Flexes Under Pressure Capacitive Manometer Connections to Capacitance Bridge C Capacitor Plates

Technology for Sensor Construction blockkkkkk Perfect size range Design Of Piezo-resistive Pressure Sensors Typical design: 4 piezo-resistors

surface Wheatstone bridge Piezo-resistive Pressure Sensor SM5108 Piezo-resistive Pressure Sensor SM5108 Semiconductor resistors joined by

4 C Ultra-sensitive/Low Pressure Transducer Developed strain-type (similar to UF Straty-Adams gauge) in situ capacitive probe with resolution of 1 Pa equires at least 15 J for 1 measurement (cap(capacitor plate moves 0.01 Å)) Supports Maximum Volume displaced ~1.3 liters Screen Push od 10 cm Membrane which Flexes Under Pressure Capacitive Manometer Connections to Capacitance Bridge C Capacitor Plates Pressure Transducer Test Apparatus Tested in liquid nitrogen and liquid helium Differential Pressure Gauge 25 cm Test Port eference Port MEMS Technology for Sensor Construction blockkkkkk Perfect size range Design Of Piezo-resistive Pressure Sensors Typical design: 4 piezo-resistors in Wheatstone bridge on a diaphragm diaphragm deflects from applied pressure causing the deformation of the piezo-resistors mounted on the surface Wheatstone bridge Piezo-resistive Pressure Sensor SM5108 Piezo-resistive Pressure Sensor SM5108 Semiconductor resistors joined by aluminum conductors in bridge configuration s placed on diaphragm Two strained parallel to I Two strained perpendicular to I Manufactured by Silicon Microstructures, Inc. 4

Drawbacks of Piezo-resistive Pressure Sensors-esults elatively low sensitivity Large temperature dependence temperature compensation necessary Voltage (mv) Voltage vs Pressure for Piezoresistive

5 Drawbacks of Piezo-resistive Pressure Sensors-esults elatively low sensitivity Large temperature dependence temperature compensation necessary Voltage (mv) Voltage vs Pressure for Piezoresistive Transducer at varying temperatures K K K 64.4 K K K K K K 29.9 K K K K K Pressure (Bar) Principle of Gas Thermal Conductivity Measurement egulate the temperature of a pure metal film separated by the H 2 gas from a constant T heat-sink Metal film is both heater and thermometer Qtotal=QH 2 conduction +Qconvection +Qradiation +Qstructural supports H 2 gas: Q ortho-para mixture T=constant Design makes the hydrogen conduction dominate Planar design linear heat conduction T Q T Conductivity Cell Cross Section Ortho -Para Hydrogen atio- Meter Expanded View of Actual Cell Inlet Needle Valve Inner Wire connec. l m n Wires & Connector k Outlet Needle Valve f j S.S. Cap Second Thermal Insulation Layer Nylon Screw g c b a Cold Cu Plate L N 2 d e I In Seal S.S. Can Mylor Spring Pure Ni heater & Quartz Spacer (Low Ther. Ex. & low Thermoconductivity Thermal Insulation A Pure Metal Makes a Good and Heater Electrical esistance of Nickel micro-ohm-cm T(C) Use Feed-back Loop to hold esistance Constant Heater -set Principle of atio Measurement egulate the temperature of a pure metal film separated by the H 2 gas from a constant T heat-sink Metal film is both heater and thermometer Qtotal=QH 2 conduction H +Qconvection 2 gas: Q ortho-para +Qradiation mixture +Qstructural supports T=constant Our design makes the hydrogen conduction dominate Planar design linear heat conduction T Q T 5

6 Level Detection-Point Point (discrete) level detectors Any device which changes property when submerged from vapor to liquid (wet or dry) can be a point level detector. Ex. 1 Device is heated, and the property change (e.g. resistance) is a result of temperature change caused by the large difference in thermal contact and hence heat conduction between the device and its surroundings. Ex. 2 The Q of a piezo-electric oscillator depends on loading Ex. 3 Capacitance depends on dielectric between plates Ex. 4 Motion of paddle or wire depends on viscous drag Ex. 5 Float and switch Drawbacks Indication does not change when level is between discrete sensors. Fluid can not be multiply connected, such as when there is a lack of gravity Level Detection-Continuous Continuous level detectors Any device which changes property when submerged from vapor to liquid (wet or dry) and can be made long (1- dimensional) can be a continuous level detector. Ex. 1 esistive strip or Superconducting wire carrying enough current to self heat Ex. 2 Optical or Acoustic beam which suffers different attenuation in liquid and gas or reflects from interface Ex. 3 Long tubular Capacitor Drawbacks May produce unnecessary heat Advantanges Higher resolution of level determination Fluid can be multiply connected in Ex. 2, such as when there is a lack of gravity Time esponse Data-again Differences between wet and dry can be exploited for level detection Flow Metering All types of flow meters may be adapted to cryogenic liquids: Positive displacement-measures volume displacement Pressure drop-may cause cavitation Turbine-measures volume displacement Momentum-measures mass directly Vortex shedding-velocity and no moving parts! Doppler shift-difficult Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p

Strain, Force, and Pressure

10-1 10-1 Strain, Force, and Pressure Force is that which results in acceleration (when forces don t cancel). Strain is the change in shape of an object...... usually due to some force. (Force is usually