Capteurs de la pression atmosphérique au vide primaire
Common Vacuum Gauge Technologies in the range atmospheric pressure to
10-4 mbar
2ème Rencontre du réseau "vide" du CNRS
Dr Andrew Chew
La Rochelle
Tuesday 23rd October 2012
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Contents
Vacuum ranges – gauge operating ranges
- Definitions: Total/partial, Direct/Indirect, absolute/non absolute, accuracy
- Classification of gauges according to measurement principles
•
•
Gauges commonly used in the vacuum range 10-4 to 1013 mbar
•
- U-tube manometer
McLeod
Rotating piston
•
•
•
•
•
•
Mechanical Bourdon
Capsule
- Strain Gauge, Piezo/Piezoresistive
Capacitance Manometer
•
Pirani
Thermocouple
- Convection
•
Spinning Rotor Gauge
•
•
•
Hot and Cold cathode ionisation gauges
Vacuum Gauges
•
The measurement of pressure characterizes the state of a vacuum
environment and/or allows process control
•
Accuracy/uncertainty (the level of systematic error), precision (the level of
random error), resolution level, measuring range, linearity of output,
reproducibility, robustness, process compatability and response time are
key performance criteria
•
Also size, cost and serviceability
Total Pressure Gauges - Classification
Absolute (Primary) gauges
- can be calibrated from their own physical properties in
terms of fundamental units
(measure relative to perfect vacuum)
Non-absolute (Secondary or Transfer) gauges
cannot be so – hence need to be calibrated
Classification
Total pressure gauges measure sum/total pressure (Dalton’s law) of all gas
species and partial pressure gauges* identify and quantify individual gas
species
•
Direct gauges- respond directly to force associated with pressure; the reading is independent
of gas species
- measure the force exerted by the gas on a surface of some sort
•
Indirect gauges – measure pressure dependent physical property of the gas (e.g. thermal
conduction or viscous/molecular drag) or a physical quantity ∝ number density (e.g ions).
-
reading is dependent on gas species:
Ptrue = Pmeasured/f
where f = gauge or gas/sensitivity factor
*partial
pressure gauges will be discussed in a separate lecture
Basics
Pressure = Force per Unit Area = Pascal = N/m2
•
So if we wish to measure pressure directly by measuring the mechanical force
exerted on some sort of transducer of area 1cm2 the forces are:
Pressure
Force (N)
1013.25 mbar
10
1 mbar
10-2
10-4 mbar
10-6
10-9 mbar
10-11
Classification
Elastic
deformation
U-tube manometer
Balance of
hydrostatic
forces
Classification
Kinetic
effects
Charged
particles
Decrement/
Resonance
Rotating
disc
Radiometric
Convection
U-tube manometer
Many other mechanisms
Flourescence
Particle scattering
Brownian motion
Thermal radiation ( e.g. Knudsen radiometer)
Radioactivity
Scattering
UV photons
Radial drift
etc…………
Common Total Pressure Gauges - Hierarchy
Liquid wall
Direct
Gauges
U-tube
McLeod
Strain
Elastic
element
Diaphragm
Bourdon
Capsule dial
Piston
Molecular,
Viscosity
Indirect
Gauges
Thermal
conductivity
(energy
transfer)
Spinning
Rotor
Others: quartz fibre, rotating
disk, quartz oscillator
Pirani, convection
Schulz-Phelps
Thermocouple
Extractor
Hot Cathode
Ionization
Capacitance
Cold Cathode
Others
(radioactive)
Bayard Alpert
Penning
Inverted Magnetron
Gauge measurement ranges
U-tube
liquid
McLeod
Bourdon
Common Total Pressure
Gauges – operating ranges
Gauges which
are subject of
this lecture
Capsule
Dial
Strain
Capacitance Diaphragm
Pirani
Thermocouple
Convection
Spinning Rotor
Hot Cathode Ionisation
SchulzPhelps
Cold Cathode Ionisation
10-11 10-10
10-9 10-8
10-7 10-6 10-5 10-4 10-3 10-2 10-1
Range extended by
special construction
mbar
1
101
102 103
Torricelli 1643
Evangelista (Torr)icelli
(1608-1647)
Gasparo Berti….water manometer ~ 1640
Blaise Pascal 1646 refuted Aristotle’s view horror vacui that nature abhors a vacuum
Liquid Level Manometers
direct and absolute
• U-tube manometer
absolute
•
At equilibrium
P1 – P2 = ρ∆hg
•
If one side is sealed and evacuated to a relatively negligible pressure the
manometer is directly readable. Accuracy and operating range is limited
mainly due to capillary effects at the liquid–wall interface
•
Measurements 103 to about 1 mbar but interferometric methods and high
purity mercury enable very high accuracy readings (<±0.0001%) and
pressure range to 0.01 mbar
•
Still most accurate gauge for pressures >1 mbar
differential
McLeod Gauge 1874
•
McLeod compression gauges extend the
measuring range utilising the Boyle-Mariotte Law:
pressure of gas to be measured is confined to a
fixed volume and compressed by the rising column,
effectively amplifying the measurable height
differential
•
Measures only non-condensables
•
Allowing for the condensation of vapours and using
mercury vapour cold trap (and corrections) allows
measurements at the 10-5 mbar level (<±1%
accuracy)
A = capillary
crosssectional area
p = h2 A / V
Piston gauge
•
•
The measurement range covered is below 103 mbar
A low end threshold component low enough to support the calibration of
typical transfer standards with ranges as low as 1.3 mbar and even 0.13 mbar
Piston-type gauges
counterbalance the
pressure of a gas with a
spring or a solid weight,
in which case it is known
as a dead-weight tester
and may be used for
calibration of other
gauges
Direct and absolute: accuracy ±[0.11 +(11.3 x 10-6p)] Pa = ± 0.11% @100 Pa
Elastic Element Gauges
•
Use a diaphragm or membrane to separate an evacuated and
hermetically sealed chamber (established pressure) from a volume
contiguous with vacuum (effectively replacing the liquid in the liquid
level manometer)
•
The differential pressure across membrane produces force to deflect it
•
The indication is independent of ambient temperature and atmospheric
pressure change
•
These are not absolute but give a direct reading
Bourdon Gauge (1849)
Internal tube is bent into an arc and connected to
vacuum. The external pressure (and hence
differential) causes tube to deflect and move
needle. Changes from 103 to 10 mbar with an
accuracy ±1% (at higher pressures this can be
<±0.1%)
Correction for atmospheric pressure change
needed
Capsule dial
•The capsule dial gauge Uses an hermetically
sealed and evacuated capsule composed of
corrugated membranes – aneroid (no fluid)
•Consequently, it is barometrically compensated
(independent of atmospheric pressure changes).
Gears and
levers
Sealed
diaphragm
Capsule Dial Gauge
•
Typical full scale ranges are 0 to 25, 50, 125 and 1013 mbar measuring
through to 1 mbar
•
Particular ranges can be selected for heightened accuracy by filling a
sensitive capsule element with gas at known pressure e.g. 100-110 mbar
range with ±1% accuracy.
•
Temperature fluctuations are important: from the ideal gas equation ∆T =
3K will give an uncertainty of 1%
Generally quoted to ±1 to 2%
x full scale deflection accuracy
•
Diaphragm Gauge
Non-absolute but give a direct reading
Cavity
sealed
Strain sensor
Change in pressure
causes diaphragm to
move against the pressure
sensitive plate.
Plate voltage will change
with applied pressure.
Diaphragm
Vacuum
Strain Gauge
•
Deflection produces a change in electrical resistance of
piezoelectric transducer
•
Membrane - usually silicone and the compression/extension
conducting layer forms part of a resistive bridge network
•
Rugged and compact - balanced below 1 mbar with range 1 to
2000 mbar (~±1% of full scale deflection)
A modern diaphragm gauge
Grown Piezoresistive Sensor
Bridge Measurement Schematic
Diaphragm = Capacitance Manometer
Capacitor
electrodes
(Inconel)
diaphragm
Getter
Cavity sealed
< 1x10-7 mbar
Vacuum
Capacitance Manometer - Signal Flow
2
Fixed electrodes
The diaphragm will
move with a change
in pressure.
7
1
5
Change in pressure.
3
When the diaphragm
moves, the gap from
the fixed electrodes
to the diaphragm will vary.
0 to10V
DC
Output
Variable
capacitance
signal sent to
electronics.
4
6
Electronics condition
signal to supply
linear output.
When the gap varies, the electrical
capacitance varies as well.
Capacitance Diaphragm Gauge
•
Inductive transducers use ferromagnetic rod to penetrate the core of differential
transformer coil such that deflection of the diaphragm causes movement
•
Capacitance diaphragm gauges are more prevalent – using FM modulated coaxial
capacitor to detect very small deflections
exploit fully +ive and -ive deflection of (25 µm) diaphragm
0.4 nm deflections are measurable!
•
Differential types for lower pressures via an established
pressure (or to measure the pressure differential)
Made with stainless-steel, inconel and
monel, for corrosion resistance
More details in a separate lecture
Capacitance Diaphragm Gauge
•
Thermal fluctuations may cause differential expansion and they can be heated to a
constant temperature
•
Range 10-5 mbar to 104 mbar (104 dynamic range)
•
Resolution usually 1 part in 104
•
High repeatability
Commonly quoted accuracy of ±0.15% of reading
+ 0.005% of full scale
In practice range is ±0.3 to 10%
Thermal conductivity gauges
Thermal gauges utilise thermal transfer as an analogue of pressure.
A filament heated in vacuum loses heat by convection,
conduction
.
and radiation
Kn =λ/d
d = diameter
of filament
convection
conduction
Kn = 10
Kn = 0.01
Radiation/
conduction
losses
through
wire
mbar
Non-absolute and indirect
Thermal Conductivity Gauges
•
These exploit heat transfer between heated wire
(400K) and vacuum wall (at 300K)
•
pressure dependent 0.01 < Kn < 10
Consider
1. In molecular flow energy current equals the conduction
heat loss through gas: φ is sensitivity constant
(dependent on the gas properties) and g is geometrical
factor
P
Q =φ
1 + gP
Thermal Conductivity Gauges
At lowest pressures other effects become predominant:
2. Heat loss (conduction) from ends of wire (most dominant for short
wires)
3. Thermal radiation losses
Q = Aσε (T
High wire-length/diameter ratio and low emissivity material minimises
these losses to extend to lowest pressures. In practice changes in
emissivity and thermal (or energy) accommodation of materials limit
these gauges to accuracy of ±10 to 100%.
4
wire
−T
4
wall
)
Pirani Gauge (1906)
The Pirani gauge operates in the pressure regime where
conduction is predominant.
Heated wire forms one arm of a Wheatstone bridge; as
filament resistance changes bridge becomes out of balance.
There are two modes of operation
• the filament is maintained at a constant temperature
i.e. resistance: sensitivity falls at higher
pressures/temperatures
• a constant voltage is applied to the filament: constant
sensitivity but difficult to operate
Reference compensating filament at atmospheric pressure
and close temperature can minimise thermal effects
The sensitivity of the gauge is both pressure dependent and
gas species dependent, so calibration is essential.
Pirani gauges operate between 103 mbar and 10-4 mbar.
Pirani Gauge – Wheatstone Bridge
Constant
voltage type
Pirani Gauge
Calibration typically at atmosphere and lowest
vacuum points
Heat conductivity varies with gas species..use
calibration chart
The divergence at higher pressures is due to
convection becoming more important
These are not high accuracy gauges and
contamination of the filament can cause serious shifts
in sensitivity, but clean gauges accuracy ± 15%
Any thermal gauge will have a relatively long time
constant
High pressure - heat flow is independent of pressure
except small convection effect which is exploited to
measure to atmospheric pressures
Modern gauges can include electronics to linearise
output
A Solid State Pirani Sensor
Construction of the sensor
Cross section of the sensor
Thermocouple Gauges
•
The filament of a thermocouple gauge is heated by a constant current
and filament temperature is measured by thermoelectric indications
•
Same principle and limitations as Pirani gauge – different measurement
technique
•
The lower limit of these gauges is extendable 10-3 to 10-4 mbar but
dynamic range is 103
•
Accuracy is >±30 to 50%
Thermocouple
Heated
filament
Thermocouple
Supply
Voltage
heated filament and thermocouple
junction thermocouple monitors the
temperature of the wire filament: the
hotter the wire, the lower the pressure
Convection Gauge
Similar to the Pirani gauge, this sensor uses a temperature
compensated, gold-plated tungsten wire to measure heat transfer by
both conduction and convection, and thereby give better
performance at higher pressures. The wire is inside a concentric
insulated cylinder
In the range 10-4 to 10 mbar the gauge operates as per a Pirani ……….
…...from 10 to 103 mbar convection is the heat transfer mechanism
Other features and limitations of this sensor are the same as those of
Pirani and most thermocouple gauges.
Accuracy ±15% of reading ±3 x 10-4 mbar
Repeatability ±5% of reading
Higher pressure reading is susceptible to mounting orientation
–needs horizontal filament?
Typical materials exposed to vacuum
Convection
•
Filament: Tungsten (Gold plated)
•
Sintered filter: Phosphor-bronze
•
Gauge tube: Aluminium or stainless steel (316L)
•
Other: Nickel, fluoro-elastomer, PTFE
Pirani
•
Filament
- Tungsten / Rhenium
- Platinum / Iridium
•
Tube: Stainless Steel 316L & 304L
•
Filter: Stainless Steel 316L
•
Other Glass, Ni, NiFe, PTFE (APG100-XLC only)
Thermocouple
•
Nickel plated steel,
•
alloy 52 (iron nickel) pins,
•
stainless steel,
•
gold, platinum, rhodium and palladium.
Molecular Gauges
•
Momentum transfer or molecular gauges -exploit molecular
drag phenomenon
•
Confusingly also referred to as friction or viscosity gauges
– strictly these exploit viscous drag
•
Several of these span flow regimes
Non-absolute and indirect
The Spinning Rotor Gauge – 1924 Epstein and commercial instrument mid-1980s (Fremerey and Beams)
Deceleration of a high speed (~400Hz) spinning
steel ball due to molecular drag measured.
The rotation of the ball – which has a small
magnetic moment – is sensed by a pickup coil
The sensitivity of the gauge is relatively
independent of gas species and is very stable - the
uncertainty is better that 3% and stability better
than 2% per annum
The gauge can be used as a transfer standard for
calibration
Range 0.1 mbar to 10-7 mbar
Spinning Rotor Gauge
•
ω 10σ eff P
=
= − Fbraking
ω
πρrv
Deceleration rate measured in practice by
considering fixed number of rotations N and
sequence of timing intervals τn which elapse for
N rotations is measured such that
•
ω τ i +1 − τ i
− =
ω τ i .τ i +1
Spinning Rotor Gauge
Other decelerating effects
•
•
ω
= − Fbraking − Fresidual − 2α T − FRN
ω
1K/h drift has an equivalent N2 pressure of 1 x 10-7
mbar, ensuring thermal stability allows equivalent N2
pressures of 4 x 10-9 mbar
•
Reducing vibrations to < 0.03 g means that Fresidual (the pressure
independent decelerating component) due to frequency dependency
of eddy currents is largest ‘offset’ effect
Spinning Rotor Gauge
Low pressures - molecular braking effect becomes of same order as the residual
effect
•
this determines stability and lower measurement limit (Fresidual is of order 10-7
mbar and needs to be measured repeatedly)
σeff means SRG is not absolute but is used widely as a transfer (or secondary)
calibration standard
•
In molecular flow common user operation gives < ±5% accuracy
(intercomparisons by national standards laboratories suggesting accuracy at
the ±1% level)
Spinning Rotor Gauge
For pressures >10-2 mbar a special
linearization procedure is used to
compensate for the diminishing pressure
dependence of the rotor deceleration rate
•
effectively an interpolation procedure to measure to 10 mbar with an
uncertainty of > ±10%
Residual drag limits lowest pressure accuracy - caused e.g. by non-coincident magnetic
centre and mass centre – can be compensated too some degree but varies from ball-to-ball
Quartz friction gauge
-
electrically driven forced oscillation is damped by the gas
-
pressure measured by changes in resonance impedance
± 10% accuracy in range 10-3 to 103 mbar
Indirect
measurement
Rotating disc gauge
Measures molecular torque developed between coaxial discs Dushman 1914
absolute, direct
1989-1995
GD ~ pressure
GI suspension asymmetries
GM molecular drag
GE Eddy current drag
Ionisation Gauges*
Molecules ionized by electron impact and resulting ion-current
measured
•
Hot Cathode/Thermionic
• Bayard Alpert Gauge (BAG)
• Extractor gauge
• Shultz-Phelps
•
Cold Cathode Discharge Gauges
• Penning/Philips Gauge
• Inverted Magnetron Gauge
*These will be discussed in more detail in next lecture
Ionisation Gauge Principle
Vacuum System
Electrode
Electrode
Current flow
Ammeter
Voltage Source
Current flow related to pressure
Bayard-Alpert Gauge (BAG): 1950
Overcame the low pressure X-ray
limit of Triode gauges
thermionic cathode (hot) filament K is a source of electrons I- and is
parallel to positive grid anode A, ion collector C is concentric to this
(greater -ive potential than other electrodes)
Grid A (+ve)
Filament K
Collector C (-ve)
Hot Cathode Ionisation Gauges
I+ = n λ σ In = number density
λ = electron path length
σ = ionisation cross-section/ionisation probability
hence there is sensitivity correction for different gases
Accuracy ±10 to 100%
Ionisation Gauges
>10-4 mbar space charge effects become important: ions orbit around the collector rather
than reach it which reduces the number of ionizing electrons and combined increased
Scattering reduces the apparent sensitivity reading in BAG by 75%
BAG has an upper pressure limit of about 10-3 mbar to avoid filament burn out and
because of space-charge effects
Schulz-Phelps (1957) version extends upper range from 10-3 mbar
to 1 mbar by close-spacing the electrodes: only one ionization per
electron and hence lower sensitivity
Anode
Collector
Cathode
Cold Cathode Gauges
•
Path length of electrons is increased c.f. hot-cathode gauge by using
anode at high voltage (few kV) above cathode and strong magnetic field
- Penning or Philips Gauge
•
In Inverted Magnetron: anode and cathode are parallel and magnetic field
is parallel to Anode (electrons have tight spiral paths = high collision
probability)
•
‘Strike’ by heated filament, field emission, cosmic rays etc. Radioactive emitter
works best at high pressure but no longer commercially available because of EHS restrictions
Penning
Crossed electric and magnetic fields trap electrons.
The high voltage ranges from 2 to 6 kV at the anode
whilst the cathode is grounded and the magnetic
field 1 to2 kGauss
cathode
anode
Ions are collected on the ring anode
Anode current i+ = kps
K and s (~1.1 to 1.2) are gas and gauge
dependent
At low pressures the discharge is unstable
and the calibration can change abruptly
Range ~10-3 to 10-8 mbar
Penning
Range of a standard Penning gauge is ~ 10-3 mbar to 10-8 mbar
The accuracy of Penning gauges is poor: +20 to 100% and -50%,
especially at low pressures and large changes in sensitivity are not
uncommon
They are susceptible to contamination leading to errors in pressure
measurement.
1927
Inverted Magnetron – 1958 Hobson and Redhead
With this orientation of the electric and magnetic fields, electrons
are able to perform relatively long orbits around the anode
increasing ionization event
– allows measurement to lower pressures ~10-9 to 10-10 mbar
Robust, no heat
Magnet
Atom
Gas
N
Electron path
N
Ion
Cathode
Anode
S
Insulator
S
Gauge body
Accuracy -50 to +100%,
Wide Range Gauge
Comines Pirani and Inverted Magnetron : 10-9 to 103 mbar
Pirani
Anode
Striking filament
Cathode cups
Body tube
Magnets
Calibration*
Need calibration especially for process establishment, gauge
retrofit and inter-comparison and for gases other than
calibration gas (N2)
Establish calibration pressure by:
1.
absolute gauge = primary gauge (from 103 to 10-2 mbar)
2. Static expansion ±1% 10 to 10-7 mbar; Dynamic expansion - gas at a known flow
rate through known conductance 10-5 (±1%) to 10-9 mbar (±5%)
3. Molecular beams to determine a pressure with a gauge calibrated as above: 10-8
(±7%) to 10-12 mbar (±30%)
*Will
be discussed in separate lecture
Calibration
Here output voltage or pressure indication
varies in proportion to applied pressure over a
wide range (several decades)
Used for interpolation and extrapolation
between calibration points
Linear: Capsule dial, Strain, HCIG, Capacitance, SRG..
Non linear: Cold-Cathode, McLeod, Pirani, Thermocouple..
Secondary gauges
•
Secondary (transfer) gauges used as traceable reference - not
absolute but calibrated as before (often CM, SRG and HCIG)
•
National standards labs (NIST, BPM, NPL, PTB, LNE, INGC
etc.) for primary calibration of transfer gauges then used on
calibration chambers
•
Commercial calibration labs: typical accuracy of ±26% at 10-6
mbar to ± 2.3% at 103 mbar
Linearity
Here output voltage or pressure indication
varies in proportion to applied pressure over a
wide range (several decades)
Used for interpolation and extrapolation
between calibration points
Linear: Capsule dial, Strain, HCIG, Capacitance, SRG..
Non linear: Cold-Cathode, McLeod, Pirani, Thermocouple..
Vacuum Gauges - Traditional Gauging System
•
•
Gauge lead has special plug for each type of head
Gauge lead is costly shielded or co-axial cable
Vacuum Gauges - Active Gauge Heads
•
•
•
Surface mount technology allows amplification circuits to
be mounted in gauge head rather than the readout
Millivolt signal amplified to 2 to 10 v dc at gauge head
Linearised output
Acknowledgements
Dr Karl Jousten, PTB, Germany
• http://cas.web.cern.ch/cas/Spain-2006/Spain-lectures.htm
Dr Ron Reid, ASTEC, UK
• http://www.cockcroft.ac.uk/education/PG_courses_200910/Spring_2010/Reid%20Lecture%208.pdf
•
Many good texts on vacuum metrology
Capteurs de la pression atmosphérique au vide primaire
Common Vacuum Gauge Technologies in the range atmospheric pressure to
10-4 mbar
Rencontre du réseau "vide" du CNRS
Andrew Chew
Dr Andrew Chew
La Rochelle
Tuesday 23rd October 2012