how accurate are vacuum...
TRANSCRIPT
CAS_2006
Vacuum gauges for the fine and high vacuum
Karl Jousten, PTB, Berlin
1. Measurement of vacuum pressures and the calibration chain
2. Overview of measurement principles and gauge types
3. Direct gauges, indirectly measuring gauges
4. Accuracy of vacuum gauges
2
Measurement of vacuum pressure
The definition of pressure p:
It follows one of the measurement principles.
Fine and high vacuum gauges
AFp = Mass Time
Length
3
Measurement of vacuum pressure
AFp =
Fine and high vacuum gauges
Gas pressure p
p ≈ 0
Balance asforce meter
F
A
This is a traceable instrument usable as primary standard
4
Errors and uncertainties
Fine and high vacuum gauges
Error: A wrong reading of a gauge. Adeviation from a true value defined by the SI units.
Uncertainty: The possible range by which a reading may not reflect the true value defined by the SI units.
True value
Indicated value
Error/deviation
Possible true value Indicated value
Uncertainty
5
Measurement of vacuum pressure
AFp =
Fine and high vacuum gauges
Gas pressure p
p ≈ 0
Balance asforce meter
F
A
6
The calibration chain
ordinary vacuum gauge
working standard
secondary standard
primary standard
Fine and high vacuum gauges
Errors and uncertainties
Reliability increases
Uncertainty increases
7
Traceability and primary standardsFully developed primary standards partly developed standards
Fine and high vacuum gauges
8
Best accuracy available
Relative uncertainties of pressures in primary standards
2E-021E-02
4E-032E-03
3E-04
1E-04
3E-05
7E-063E-06
1E-06
1E-05
1E-04
1E-03
1E-02
1E-01
1E+00
1E-10 1E-08 1E-06 1E-04 1E-02 1E+00 1E+02 1E+04 1E+06
p in Pa
Rel
ativ
e U
ncer
tain
ties
(k=2
)
Mercury ManometerContinuous expansion Series expansion
p atm
Traceability and primary standards
9
Measurement principles and gauges
Fine and high vacuum gauges
1E-04
1E-03
1E-02
1E-01
1E+00
1E-12 1E-10 1E-08 1E-06 1E-04 1E-02 1E+00 1E+02 1E+04pressure in Pa
Mea
sure
men
t unc
erta
inty
AFp =
Heat conductivity
Viscosity
Ionisation
Ionisation and Suppression of X-ray
and ESD
Piezo PiezoresistiveMembraneMechanical Resonance Capacitance
Piston/Cyl U-tube
Gas independent
Total pressure
Gas dependent-D
ifficult t
o inter
pret
10
Measurement principles and gauges
Fine and high vacuum gauges
11
Measurement principles and gauges
Fine and high vacuum gauges
12
Measurement principles and gauges
Fine and high vacuum gauges
Gas inlet
hgAFp Δ== ρ
The mercury U-tube exists since Torricelli (1644). It is still the most accurate vacuum gauge > 100 Pa (1 mbar)!
13
Measurement principles and gauges
AFp =
Fine and high vacuum gauges
Gas
The rotating piston gauge
Gap: 0.2µm
14
Measurement principles and gauges
Fine and high vacuum gauges
M
A
P1 P2
Z
R
Raum 1 Raum 2
Mechanical gauges
3 Groups:
1. Ref.side patm and contains meas.dev.
2. Ref.side p=0, meas.dev. on test side (1)
3. Ref.side p=0 and contain meas.dev.
15
Measurement principles and gauges
Fine and high vacuum gauges
Mechanical gauges: Bourdon gauges
16
Measurement principles and gauges
Fine and high vacuum gauges
Mechanical gauges: Bourdon gauges2 Photo cells
Light source
Amplifier
Precise
Resistor
Mirror
Coils
Quartz
spiral
17
Measurement principles and gauges
Fine and high vacuum gauges
Mechanical gauges: Bourdon gauges
18
Measurement principles and gauges
Fine and high vacuum gauges
Piezoeffect used by membrane
19
Measurement principles and gauges
Fine and high vacuum gauges
Piezoresistive effect
Material: Silicon (MEMS)
20
Measurement principles and gauges
Fine and high vacuum gauges
Capacitance diaphragm gauge
Sensitivity of deflection: 0.4 nm!
Membrane (INVAR, Ceramic): as low as 25 µm.
Two improve zero stability: 2 capacitors plus thermostated housing
21
Measurement principles and gauges
Fine and high vacuum gauges
Electrical block diagram ofcapacitancediaphragm gauge
22
Measurement principles and gauges
Fine and high vacuum gauges
Thermal transpiration effect
1
2
1
2TT
pp
=
2
1
1
2TT
nn
=
Cross section
23
Measurement principles and gauges
Fine and high vacuum gauges
Thermal transpiration effect
036.1296318
=
24
Measurement principles and gauges
Fine and high vacuum gauges
Resonance Silicon Gauges
Designed by MEMS
25Fine and high vacuum gauges
26
Measurement principles and gauges
Fine and high vacuum gauges
Heat conductivity through a gas
27
Measurement principles and gauges
Uncertainties due to the physical principle of measurementExample: Pirani gauge
Fine and high vacuum gauges
Radiation and conduction
Heat transport by gas
convection
28
Measurement principles and gauges
Fine and high vacuum gauges
Electrical circuit for Pirani gauge
Constant temperature
Constant heating voltage, current, or power
29
Measurement principles and gauges
Mikro Pirani (MEMS manufactured) by MKS
Fine and high vacuum gauges
Heated sheet 60°C
MEMS: higher Knudsen number, no convection
30
Measurement principles and gauges
Correction factor for helium for 4 different Piranigauges
Fine and high vacuum gauges
0,00
0,20
0,40
0,60
0,80
1,00
1,20
1,40
1E-03 1E-02 1E-01 1E+00 1E+01 1E+02p in mbar
Cor
rect
ion
fact
or C
FH
e/N
2
VM1 (Pfeiffer)VM2 (Thyracont)VM3 (MKS)VM4 (Leybold)Mean
Helium
MEMS Pirani
31
Measurement principles and gauges
Experimental standard deviations of repeat calibrations for 4 different Pirani gauges at various pressures
Fine and high vacuum gauges
# Gauge s in %0,05 mbar 3 mbar 30 mbar
1 Pfeiffer TPR 280 0,19 0,13 0,092 Thyracont VSP52 0,06 0,35 3,303 MKS 925C 0,10 0,12 0,194 Leybold TTR91 0,03 0,09 0,12
32
Measurement principles and gauges
Thermocouple gauge
Fine and high vacuum gauges
33
Measurement principles
Fine and high vacuum gauges
Viscosity
( )⎟⎟⎠
⎞⎜⎜⎝
⎛−⎟
⎠⎞
⎜⎝⎛⋅= ωωω
σρπ
πRDd
mkTp
&
208
34
Measurement principles and gauges
Sprinningrotor gauge
Fine and high vacuum gauges
35
Measurement principles and gauges
Sprinning rotor gauge
Fine and high vacuum gauges
1,85E-07
1,90E-07
1,95E-07
2,00E-07
2,05E-07
2,10E-07
2,15E-07
404 406 408 410 412 414 416
Rotor frequency f
DC
R-S
igna
l
s-1
s-1
Residual drag vs. frequency of rotor
36
Measurement principles and gauges
Sprinning rotor gauge
•No gas consumption (e.g. by ionization)
•No dissociation (hot cathode)
•Low outgassing rate
•Predictable reading
•High accuracy
•High long-term stability
Fine and high vacuum gauges
37
Measurement principles
Fine and high vacuum gauges
Ionisation
38
Measurement principles
Fine and high vacuum gauges
Fine vacuum IGIonization gauges for different vacuum ranges
39
Measurement principles
Fine and high vacuum gauges
Ionisation gauges for fine vacuum
40
Previous investigations showed that TDLAS is applicable forvacuum measurement:
CO, mid-infrared (5 µm), resolution down to 10-5 Pa, high accuracy.
Partial Absorpion, Reference Predicted from ch2
0.0
0.5
1.0
1.5
2.0
2.5
35 40 45 50 55 60 65 70 75 80
time (ms)
Inte
nsi
ty (
a.u
.)
( ) ( ) LnSeII 10
1 Φ−−− = λλ
Fine and high vacuum gauges
Measurement principles
⎟⎟⎠
⎞⎜⎜⎝
⎛
−Φ= −
−
−− )()(ln
)()( 1
10
10
1 λλ
λλ II
LTSkTp
41
How accurate are vacuum gauges ?
Reasons for inaccuracies of vacuum gauges
General Example
Uncertainties due to calibration chain
Has the vacuum gauge been ever calibrated? Against what standard?
Uncertainties due to installation Pressure at gauge position may not reflect the pressure where the experiment takes place.
Uncertainties due to operation Outgassing of an ion gauge may falsify an outgassing rate measurement.
Inaccuracies caused by the physical principle of measurement
Thermal conductivity or ion gauge is used, but gas mixture is not (accurately) known.
Uncertainties caused by the device itself
See Table 2.
Fine and high vacuum gauges
42
How accurate are vacuum gauges ?
Reasons for inaccuracies
Gas species dependence:
Real total pressure only for force/area measuring gauges and > 100 Pa (1 mbar)! Below 100 Pa consider the thermal transpiration effect.
Spinning rotor gauges: Use a weighted mean mass, if approximate relative composition is known.
Thermal conductivity gauges and ionisation gauges : Scaling factors are available, but do have high uncertainties.
Fine and high vacuum gauges
2
1⎟⎠⎞
⎜⎝⎛∑==
n
iiieff mam ∑ =
=
n
iia
11
∑==
n
iiieff CFaCF
1
43
How accurate are vacuum gauges ?Uncertainties due to the vacuum gauge itself
Fine and high vacuum gauges
General ExamplesOffset measurement residual drag in SRG, zeroing of Pirani gauge,
X-ray- and ESD-effect for ion gauges
Offset instability (drift) Offset drifts with environmental temperature (Piroutte effect in SRG), bridge is no more balanced with time
Resolution Number of digits shown
Influences of environment (mainly temperature) Enclosure temperature of Pirani changes varies, thermal transpiration effect changes in CDG, amplifier changes amplification
Non-Linearity Ion gauge (sensitivity changes with pressure)
Integration time (scatter of data), repeatibility Same signal at repeat measurements? Integration time in SRG, in picoammeter with ion gauge.
Reproducibility (stability of calibration constant) Calibration constants change with time.
Hysteresis Mechanical gauges (up, down measurement)
Prior usage, cleanliness Surfaces change, accommodation coefficients change, secondary yield changes
44
How accurate are vacuum gauges ?
Fine and high vacuum gauges
Table: Relative measurement uncertainty of commercially available vacuum gauges.
Gauge type Measurement range in Pa
Normal uncertainty
Optimum range in Pa
Lowest uncertainty
Piston gauges 10...105 102...105 10-4... 10-5
Quartz-Bourdon-manometer 103...105 103...105 3x10-4... 2x10-4
Resonance silicon gauges 10 ... 105 0.003... 0.0005 100 ... 105 2x10-4... 5x10-5
Mechanical vacuum gauge 102 ... 105 0.1 ...0.01
Membrane vacuum gauge 102 ... 105 0.1 ...0.01
Piezo 102 ... 105 1 ...0.01
Thermocouple gauge 10-1 ... 102 1... 0.3
Pirani gauges 10-1... 104 1 ... 0.1 1 ... 100 0.02 ... 0.01
Capacitance diaphragm gauges 10-4 ...105 0.1... 0.003 10-1 ... 105 0.006... 0.001
Spinning rotor gauges 10-5 ...10 0.1 ... 0.007 10-3 ...10-1 0.006...0.004
Penning gauges 10-7 ... 1 0.5 ... 0.2 10-5 ... 1 0.3...0.1
Magnetron gauges 10-8 ... 1 1 ...0.1 10-6 ... 1 0.1...0.02
Ionisation gauges (Emission cathodes) 10-10 ... 10-2 1...0.05 10-8 ... 10-2 0.2...0.02
45
How accurate are vacuum gauges ?
Fine and high vacuum gauges
Lowest relative uncertainties for vacuum gauges and primary standards
Errors > 100 % (error factor > 1) are possible.
1
0,2
0,05 0,05
0,0060,0040,004 0,003
0,0020,001
0,1
0,010,02
0,010,02
1E-06
1E-05
1E-04
1E-03
1E-02
1E-01
1E+00
1E-10 1E-08 1E-06 1E-04 1E-02 1E+00 1E+02 1E+04 1E+06
p in Pa
Rel
ativ
e U
ncer
tain
ties
(k=2
)
Mercury Manometer
Continuous expansion Series expansion
p atm
Ionisation gaugeMechanical
Piston
QBS
CDGSRG
Pirani
46
Todays commercial gauges
Fine and high vacuum gauges
Pirani gauge
Old classical gauges:
Gauge head + controller
Today: Active gauges or transmitter (all in one)
or
Digital gauges (digital output via interface)
47
Todays commercial gauges
Fine and high vacuum gauges
Profibus ConverterTransmitter gauge plus
48
Todays commercial gauges
Fine and high vacuum gauges
Commercial „active“ Line vacuum gauges
CAS_2006
Fine and high vacuum gauges
We have discussed:
Metrological system - primary standards- calibration chain
Measurement principles and gauges
Direct, indirect measuring gauges
Sources of uncertainties with
values from 0.001% up to 100% or factor