How accurate are vacuum...

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


AFp =


Gas pressure p

p ≈ 0

Balance asforce meter

F

A

This is a traceable instrument usable as primary standard

4

Errors and uncertainties


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


AFp =


Gas pressure p

p ≈ 0

Balance asforce meter

F

A

6

The calibration chain

ordinary vacuum gauge

working standard

secondary standard

primary standard


Errors and uncertainties

Reliability increases

Uncertainty increases

7

Traceability and primary standardsFully developed primary standards partly developed standards


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


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



11



12



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


AFp =


Gas

The rotating piston gauge

Gap: 0.2µm

14



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



Mechanical gauges: Bourdon gauges

16



Mechanical gauges: Bourdon gauges2 Photo cells

Light source

Amplifier

Precise

Resistor

Mirror

Coils

Quartz

spiral

17



Mechanical gauges: Bourdon gauges

18



Piezoeffect used by membrane

19



Piezoresistive effect

Material: Silicon (MEMS)

20



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



Electrical block diagram ofcapacitancediaphragm gauge

22



Thermal transpiration effect

1

2

1

2TT

pp

=

2

1

1

2TT

nn

=

Cross section

23



Thermal transpiration effect

036.1296318

=

24



Resonance Silicon Gauges

Designed by MEMS

25Fine and high vacuum gauges

26



Heat conductivity through a gas

27


Uncertainties due to the physical principle of measurementExample: Pirani gauge


Radiation and conduction

Heat transport by gas

convection

28



Electrical circuit for Pirani gauge

Constant temperature

Constant heating voltage, current, or power

29


Mikro Pirani (MEMS manufactured) by MKS


Heated sheet 60°C

MEMS: higher Knudsen number, no convection

30


Correction factor for helium for 4 different Piranigauges


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


Experimental standard deviations of repeat calibrations for 4 different Pirani gauges at various pressures


# 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


Thermocouple gauge


33

Measurement principles


Viscosity

( )⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟

⎠⎞

⎜⎝⎛⋅= ωωω

σρπ

πRDd

mkTp

&

208

34


Sprinningrotor gauge


35


Sprinning rotor gauge


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


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


37



Ionisation

38



Fine vacuum IGIonization gauges for different vacuum ranges

39



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



⎟⎟⎠

⎞⎜⎜⎝

⎛

−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.


42


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.


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


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



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

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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


Pirani gauge

Old classical gauges:

Gauge head + controller

Today: Active gauges or transmitter (all in one)

or

Digital gauges (digital output via interface)

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Profibus ConverterTransmitter gauge plus

48



Commercial „active“ Line vacuum gauges

CAS_2006


We have discussed:

Metrological system - primary standards- calibration chain


Direct, indirect measuring gauges

Sources of uncertainties with

values from 0.001% up to 100% or factor

How accurate are vacuum...

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