Cryogenic Instrumentation I
Thermometry
OUTLINE
1.
2.
3.
4.
5.
6.
Thermometry
Temperature Ranges of
Application
Constant Volume
Thermometer
Thermocouples
Thermometer Time
Response Data
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
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)
RT
Gas line
Low T
Helium
gas
Thermocouples
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.
Thermometer Time Response Data
Differences between wet and dry can be exploited for level detection
Calibrations are tabulated (e.g. type K)
Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 530
Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 495
1
Pt (pure metal) thermometer
4 Terminal Resistance Measurement
•Resistance thermometer—use 4 terminal set-up
•R is almost proportional to T:
Callender-VanDusen Equation: R(T)=R0[1+AT+BT2+C(T-10)T3]
(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
Meter at RT must have high input impedance
Leads from RT to
Low T must have
low heat leak (alloy)
Thermometer
at low T
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 Resistive Thermal Sensor
Typical Resistance and Sensitivity Curves
106
Cross-section:Details of
construction showing strain-free
mounting of chip (#4) inside
hermetically sealed container
Ge on GaAs substrates
Produced in development of
Maximum sensitivity at 20 K
(liquid hydrogen)
Resistance ()
105
104
TTR-G
TTR-D
103
101
0.01
0.1
1
10
100
Temperature (K)
Photo of sensor with leads
Temp.
Monitor
Temperature
Input-set point
TTR-G
3
TTR-D
2
1
0
0.01
0.1
1
10
100
Temperature (K)
Data Collection/Wheatstone Bridge
Thermal Regulation
PID Heater
Response
Dimensionless Sensitivity (S)
102
Dewar
Computer
with data
acquisition
board
Computer board
Break-out box
source
detector
Resistor
Heater
Thermometer
Computer data acquisition
board may be used for all
electronics
InputCurrent
level
Temperature
sensor
Decade
Resistor
Resistor
R
Resistor
R
Temperature
Temperature measurement
and control is easily automated
2
Cryogenic Instrumentation II
OUTLINE
1.
2.
3.
4.
Pressure Measurements
Pressure Transducers
Characteristics
Thermal Conductivity
Measurements
Example: Ortho-Para
Ratiometer
5.
6.
7.
8.
Level Detection-Point
Level Detection-Continuous
Thermometer Time
Response Data-again
Flow Metering
Typical Pressure Transducers
Pressure Measurement at Low
Temperature
•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
 Resistive-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
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
Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 458
Temperature Dependence of Transducer Output
Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 459
3
Ultra-sensitive/Low Temperature
Pressure Transducer
Capacitive Manometer
Push
Rod
Connections to Capacitance Bridge
Supports
Developed strain-type (similar to UF
“Straty-Adams” gauge) in situ capacitive
probe with resolution of 1 Pa
Requires at least 15 J for 1 measurement
(cap(capacitor plate moves 0.01 Å))
Maximum
Volume
displaced
~1.3 liters
10 cm
Membrane which Flexes
Under Pressure
Capacitor Plates
Screen
C
Differential
Pressure
Gauge
25 cm
C
Pressure Transducer Test Apparatus
Tested in liquid nitrogen
and liquid helium
Test Port
Reference Port
MEMS Technology for Sensor Construction
Perfect size range
blockkkkkk
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
Resistors 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-Results
 Relatively low sensitivity
 Large temperature dependence temperature compensation necessary
Voltage vs Pressure for Piezoresistive Transducer
at varying temperatures
55
300 K
91.2 K - 94.6 K
64.4 K - 66.0 K
49.8 K - 51.0 K
40.4 K - 43.5 K
29.9 K - 30.4 K
22.9 K - 26.0 K
26.2 K
50
45
Voltage (mV)
40
35
30
Principle of Gas Thermal Conductivity Measurement
Regulate the temperature of a pure metal film separated by the
H2 gas from a constant T heat-sink
Metal film is both heater and thermometer
Qtotal=QH2 conduction
+Qconvection
+Qradiation
+Qstructural supports
25
Q
H2 gas:
ortho-para
mixture
T
T=constant
20
Design makes the hydrogen conduction dominate
Planar design—linear heat conduction
15
10

5
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Q
T
Pressure (Bar)
Conductivity Cell Cross Section
Expanded View of Actual Cell
Ortho -Para Hydrogen Ratio- Meter
Inlet Needle Valve
l
m
Wires &
Connector
Outlet Needle
Valve
k
S.S. Cap
j
Inner
Wire connec.
n
d
b
a
g
I
Cold Cu Plate
Nylon Screw
c
Second Thermal
Insulation Layer
f
LN
e
In Seal
S.S. Can
2
Mylor Spring
Pure Ni heater & Thermometer
Quartz Spacer (Low Ther. Ex.
& low Thermoconductivity
Thermal Insulation
Electrical Resistance of Nickel
micro-ohm-cm
40
60
80
100 120
A Pure Metal Makes a Good Thermometer and Heater
Use Feed-back Loop
to hold Resistance Constant
Heater
R
R-set
R
Principle of Ratio Measurement
Regulate the temperature of a pure metal film
separated by the H2 gas from a constant T heat-sink
Metal film is both heater and thermometer
Qtotal=QH2 conduction
+Qconvection
+Qradiation
+Qstructural supports
Q
H2 gas:
ortho-para
mixture
T=constant
20
Our design makes the hydrogen conduction dominate
Planar design—linear heat conduction
-100
0
100
T(C)
200
T

Q
T
5
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
Thermometer Time Response Data-again
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 (1dimensional) can be a continuous level detector.
•Ex. 1 Resistive 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
Flow Metering
Differences between wet and dry can be exploited for level detection
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. 495
6