Material Prop. @ Cryogenic 1 Introduction , Cryogenic T-limit, Cooling Processes

2 Production of Cold, Liquefier Vs Refrigerato r

Cryogenics

Temperat ure Measurem ent

Cryogenic properties of Material

3 Heat Transfer1

4 Heat Transfer1

Temperature Measurement/Thermometry Thermal Conductivity of Wire used for T-Sensor/Other Unlike other properties such as volume or length, temperature can Instrumentation not be measured directly. It must be measured in terms of another physical property.

Physical properties that have been used so far include a. Pressure of a gas. b. Equilibrium pressure of a liquid with it’s vapor c. Electrical Resistance d. Junction Voltage of Diode e. Length of a solid f. many more………

Courtesy: Weisend

Temperature Measurement/Thermometry Temperature measurement system components and interaction

Excitation

T- Sensor Physical environment

Measurement instrumentation Data Processing /Analysis//Dispaly

The process of a designing temperature measurement system can be described by the following steps. a. Decide what is to be measured. b. Find a T-sensor that will work in that environment. c. Design measurement system around the sensor Courtesy: Weisend d. Select the sensor and measurement system combination.

Temperature Measurement/Thermometry Selecting a Temperature Sensors The selection of a sensor for a specific application also requires prioritizing the most important factors necessary for the application and compromising to the less important factors. Any one or the several of the following factors. a. b. c. d. e. f. g. h. i. -

Operating temperature range Type of excitation. Sensitivity Interchangeability Package size Thermal and Electrical response time. Power dissipation. Environmental compatibility Magnetic Environment Ionizing Radiation environment Electromagnetic environment. Robustness ( Harsh environment) Courtesy: Weisend

Temperature Sensors Fluid Thermometry

Gas Thermometry Vapor Pressure Thermometry Platinum RTD

Metallic ( or Alloy)Resistance Thermometry.

Rhodium –Iron RTD Carbon Glass

Semiconducting Resisance Themometry

Germanium Cernox

Semiconductor Diode

Silicone Diode

Thermocouple

Thermocouple

Temperature Sensors Recommended sensor Temperature Range 4K-300K

Gas Thermometer Equation of s state for a gas : PV=nRT

If ONE Variable ( Suppose Volume -V) is kept constant SECOND variable ( Suppose Pressure-P) is measured Then THIRD one ( Temperature –T) can be calculated Pressure gauge

Valve

Capillary

Gas Bulb

V

Vapor Pressure Thermometer (VPT) Vapor Pressure? The Pressure exerted by the saturated vapors in equilibrium with its liquid :

Vapor Pressure: Very well defined function of temperature Thus it can be used to measure temperature

Commonly used fluid for VPT: O2, N2,H2, He

Vapor Pressure Thermometer Merits: * The sensitivity is very high in its USEABLE range. - These thermometers are accurate and can be used in between its critical point and triple point

Response time : Good Not affected by ionizing radiation , magnetic field

*** They are used as Standards***

VaporTemperature Pressure Thermometer (VPT) Measurement Demerits These thermometers can Only be used in between its critical point and triple point Therefore several temp. regions are inaccessible to VPT

40K
Above the range of Neon Below the Range of Oxygen and Nitrogen

Metallic Resistance Thermometry Resistivity of Metals ~ f ( temperature)

Positive Temperature Coefficient (PTC)

According of Mattheissen’s Rule The Resistivity (r ) of pure metal r ( total resistivity) = r o ( residual resistivity) + r i ( intrinsic resistivity)

Temperature independent: Caused by scattering of electrons by impurities and imperfection

Temperature dependent: Caused by scattering of electrons by lattice vibration

Below 20K : r = r o ( residual resistivity) Temperature independent: Therefore can not be used below this

Metallic Resistance Thermometry Platinum Resistance Thermometer ( PRT)

* Temp range~ 20K –800K * Platinum is available in purest form: -purity can be maintained batch by batch

Metallic Resistance Thermometry Platinum Resistance Thermometer * Resistivity is almost linear with temperature Resistance Vs. Temp

Sensitivity

Sensitivity goes does down

Best above 50K

Courtesy: Lakeshore

Metallic Resistance Thermometry Platinum Resistance Thermometer

In Magnetic Field Temp > 60K : Smaller ( DT/T % ) Temp < 60K : Larger ( DT/T % )

Thermal response time: ~ 2sec @ 77K ~10sec @298K

In Ionizing Radiation Smaller ( DT/T % )

neutron flux: 3.75E+7 neutrons/cm2/s @ 298K in nuclear pool reactor: Lakeshore Courtesy: Lakeshore

Metallic Resistance Thermometry Rhodium –Iron (Rh-Fe) Temepearture Sensor Temperature Range ~ 1.4K-500K Linear R-T curve: divided in two region

Sensitivity ( dR/dT)

1K-50K

50K-500K

Courtesy: Lakeshore

Metallic Resistance Thermometry Rhodium –Iron (Rh-Fe)Temepearture Sensor

Merits: -Sensitivity high - Stable over thermal cycling -Stable in ionizing radiation -Available in thin film-thermal response time~ms

Sensitivity ( dR/dT)

Demerits: -Not suitable in magnetic environment --Difficult to produce batch-by-batch homogeneity

Courtesy: Lakeshore

Semiconducting Resistance Temperature sensor

Resistivity of Semiconductor ~ f ( temperature)

Negative Temperature Coefficient (NTC)

A low temperature : (Sensitivity)NTC >> (Sensitivity)PTC

Germanium, Carbon, Carbon-glass, Cernox

Semiconducting Resistance Temperature sensor Germanium Temperature sensor * Resistance increases very rapidly with the decrease of temp * Sensitivity also increases with decreasing temp Accurate measurement below 30K @ +/- 0.5mK R vs. T

Sensitivity ( dR/dT)

Courtesy: Lakeshore

Semiconducting Resistance Temperature sensor Germanium Temperature Sensor Small thermal mass:

faster response time ~ 200ms at 4.2K

Small thermal mass: Measuring excitation current has to be chosen Properly as joule heating may create shift in temp 1 mA for T < 2K, 10 mA for 2K < T <15K, 15 mA for 15K< T <40K and 1mA for 40K
Can be used in Ionizing radiation environment But not in magnetic environment because of their high magnetoresistivity

Semiconducting Resistance Temperature sensor Carbon Resistance Thermometer R vs. T

The Resistance temperature characteristics for several commercial resistors : A, thermistor; B, 68  Allen-Bradley; C, 220  Speer (grade1002); D,51  Speer (grade 1002); E’ 10  Speer ( grade1002) [Anderson (1972)] [Courtesy ITS-90 Report]

Sensitivity ( dR/dT)

. Relative sensitivity versus temperature for carbon ( C), carbon-glass(CG), and germanium (Ge) sensors. [Swartz and Swartz (1974)]

Semiconducting Resistance Temperature sensor Carbon Glass RTDs Impregnation of porous glass with high purity carbon

T <100 K : Low Sensitivity ~ 0.01 Ohm/K Limits the usage at higher temperature

R vs. T

Sensitivity ( dR/dT) Good sensitivity

Courtesy: Lakeshore

Semiconducting Resistance Temperature sensor Carbon Glass RTDs It can be used in Ionizing radiation environment And also in magnetic environment with suitable correction

Courtesy: Lakeshore

Semiconducting Resistance Temperature sensor Cernox Thin film Best among the nonmetallic resistive Temp-sensor Temp Range: 300mK to 500K Throughout the temperature range : Sensitivity is HIGH

R vs. T

Sensitivity ( dR/dT)

Courtesy: Lakeshore

Semiconducting Resistance Temperature sensor Cernox The best sensor to be used in magnetic environment. It can also be used in Ionizing radiation environment Stable over thermal cycling +/- 3mK at 4.2K

Thermal Cyling between 300K-77K

Courtesy: Lakeshore

Semiconductor Diode Silicone diode

The forward voltage of a semiconductor junction at a constant current is a well defined function of temperature In a forward biased p-n junction, the forward current ( I ) is related to the junction voltage ( V ) as I = Ii exp(qV/nkT) Where q is the electronic charge, n is a constant, k is Boltzman’s constant, and T is the absolute temperature. Ii is the intrinsic carrier concentration

Most widely used sensors at low temperature: -Stability -Reproducibility -Interchangeability Excitation current~10 mA

Semiconductor Diode Silicone diode V vs. T [Excitation current~10 mA

Courtesy: Lakeshore

]

Semiconductor Diode Silicone diode

Sensitivity ( dR/dT)

It can be used in Ionizing radiation environment But not suitable in magnetic environment

Below 20K Sensitivity is High

Courtesy: Lakeshore

Semiconductor Diode Silicone diode Stable over thermal cycling Shift in temperature versus thermal cycling at 4.2K, 77K and 305K

Temperature measurement: ERROR

Electrical analogy Ohm’s law: DV=I R : R =DV/R For heat flow: DT= q R

Temperature measurement ERROR Contact resistance

DT  Q

RCR •

Temperature discontinuity at the interface:

- spot-like contact points

Features: – Proportional to FORCE, (constant spot area, number of contact points increases with force) – For metals, saturates above 30N @ 300K – Can be reduced by fillers, grease, In, coatings

Temperature measurement ERROR To reduce Contact Resistance: Use some soft filler material in the interface Indium, Apiezon N grease, Copper/Silver paste

grease

Conductance = 1/Contact resistance

1 Q  Conductance = RCR DT

Temperature measurement ERROR

Temperature measurement ERROR Two Probe Vs. Four Probe

Two probe measurement

Example: PT-100 sensor: at 100K Sensor resistance~ 30 Ohm //// Sensitivity~0.4 Ohm/K Suppose Lead Resistance ~ 2 Ohm each Total lead resistance~ 4 Ohm Error in Temperature = (4 /0.4)~10K

Temperature measurement ERROR Two Probe Vs. Four Probe

Lead resistance problem can be resolved by

Four probe measurement

Temperature measurement ERROR Thermo e.m.f Development

Current reversal nullify the thermo e.m.f effect

Temperature measurement ERROR To reduce the heat flow to sensor due to Heat conduction thru sensor leads and joule heating: Provide Thermal Anchoring

Temperature measurement ERROR To reduce the heat flow to sensor due to Heat conduction thru sensor leads : USE low conductive sensor wire

Phosphor Bronze Wire for sensors