+

Calibration of reference leaks in a wide pressure range Ana L. Fonseca, A. Marta Barreto, A. Sofia Matos, A.M.C.Moutinho and Orlando M.N.D.Teodoro

+ Outline

  Motivation   Reference leaks for sniffers, R134a leaks

  Calibration set-up   Syringe and stepper motor   User interface   Measuring range   Temperature effects   Repeatability   Time needed for calibration

  Typical uncertainty

  Method validation

  Typical results   R134a and He leaks

2

+ Motivation: reference leaks for sniffers 3

  Inside-out He leak detection technique (sniffer)   He reference leaks in the range 10-6 to 10-5 mbar.L/s

  842/2006 regulation and subsequent statements   R134a reference leaks of 5 g/year

+ Motivation 4

  Calibration by comparison using a MS leak detector is not suitable   Calibration pressure should be atmosphere, not vacuum

  EN 13192 method C: calibration of leaks from overpressure to atmosphere   Capillary with a moving slug

  Although vey simple, is of limited application (slug friction, pressure control, etc.)

  However, p∆V method is well suited

+ Calibration method: p∆V 5

  The leak is connected to a closed volume

  A piston is actuated every time a pressure increase is detected to keep the pressure constant   Volume ≈ 1 mL, step < 1µL (resolution)

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1.  M. Bergoglio e D. Mari, “INRIM primary standard for microgas-flow measurements with reference to atmospheric pressure” Measurement 45 (2012) 2459

2.  K. Jousten e U. Becker, “A primary standard for the calibration of sniffer test leak devices,” Metrologia, 46 (2009) 560-568 3.  F. Boineau, “Characterization of the LNE constant pressure flowmeter for the leak flow rates measurements with reference to vacuum

and atmospheric pressure,” PTB-Mitteilungen, 121 (2011) 313

•  Absolute gauge, no further volume is required

•  Differential gauge, reference volume is required

total volume displaced

+ Calibration set-up 6

Computer & Labview software

thermocouples

stepper motor (syringe pump)

leak under calibration

absolute pressure gauge

double wall box (water inside)

Bath, temperature control and

pump syringe

venting valve

+ Calibration set-up

  Piston: 1 mL SGE gas tight syringe with PTFE plunger tip

  Piston actuator: syringe pump NE-500   800 steps/revolution; 1 step≈ 1.7 µm

  Gauge: Druck DPI 142 precision Barometer   Resolution ≈ 3×10-3 mbar; piezo sensor

  Software: Labview platform

  Tubing: 1/8” swagelok fittings and valves

  Stainless stell double wall box with 2 computer fans inside to increase temperature homogeneity

  Water circulator Julabo F25-ED

7

+ User interface 8

Pressure (≈10-2 mbar maximum variation)

Flow rate (integrated measurement), converges with time

Temperature drift (3 thermocouples), critical to achieve low uncertainty

Actual measurement

+ Before calibration

  ∆pV method, for preliminary evaluation

  temperature stability confirmation

9

+

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Syringe characterization

  Gravimetric volume calibration   Step volume Vs=28.0 ± 0.22 nL

  Syringe expansion without leak   Boyle-Mariotte relation

  Full volume range ≈ 32 000 steps

  Step volume Vs and dead volume V0 can be calculated by fitting these parameters in the experimental data

10

p = p0V0nVs +V0

Vs =V0n

p0p!1

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#$

%

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Best fit: Vs = 28.18 nL V0 = 3.862 mL

Step volume as function of position

most used position = larger diameter

fitting parameters

Dark line, average of 2000 steps

+ Measuring range, time needed for calibration

  2000 steps averages thread defects

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total  steps   Q  *me  

s   min   h  

2000  

10-­‐4  mbar.L/s    

560   9.3  

10-­‐5  mbar.L/s   5600   93.3   1.6  

10-­‐6  mbar.L/s   56000   933.3   15.6  

5  g/year  R134  a   1474   24.6  

+ Repeatibility

  Effect of the position of the syringe plunger tip (piston)

12

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average

He leak, nominal leak rate = 2.60 × 10-5 mbar.L/s calibrated to vacuum

+ Repeatability

  4 different leaks were measured 4 times each

  Repeatability depends on:

1.  Number of steps

(acquisition time)

2.  Temperature drift has a

major impact

3.  Leak stability

  On the graph 1000 to

10000 steps were used.

13

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

0 1 2 3 4 5

Lea

k r

ate

(mb

ar.L

/s)

Leak number #

std=14.35%

std=1.13%

std=0.68%

std=0.23%

+ Typical uncertainty

  R134a leak, Q=3.48 g/year

14

Quantity, xi Estimate u(xi) ci u(xi)/d (g/year)

relative contribution %

pressure   1000  mbar   0.015  mbar   1.54E-­‐05   0.027  

volume   22  µL   0.17  µL   7.79E-­‐03   13.7  

temperature   296.9  K   0.06  K   5.38E-­‐04   0.4  

Eme   827  s   9.6  x  10-­‐3  s   1.16E-­‐05   0.02  

repeatability  (∆T≈0.1K)       0.17  g/year   0.049   85.9  

Total  (k=2)  0.098  

100  2.82%  

Q  =  3.48  ±  0.10  g/year  

+ Temperature effects

  The use of an absolute gauge turns this calibration set-up more sensitive to temperature drifts   A drift in the temperature is equivalent to a temporary virtual leak (positive or

negative)

  This effect is slightly cancelled by tubing dilatation

15

1.00E-08

1.00E-07

1.00E-06

1.00E-05

0.01 0.1 1

equ

ival

ent l

eak

rat

e (m

bar

.L/s

)

Temperature drift (K)

Qeq =p0V0t

T0 +!TT0

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Gay-Lussac law

Dead volume V0 ≈ 4 mL

+ Temperature effects

  Total temperature drift is more important than temperature uniformity.   If temperature changes slowly, what matters is the difference between the starting temperature and

the final temperature

  Experiment: induced temperature step   Temperature effects may be cancelled by positive and negative drifts

16

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Temperature step in the PID controller: t=550s, ∆T=-0.5 K t=2200s, ∆T=+0.5 K

Nominal leak rate

+ Method validation

  A series of R134a leaks were built with leak rates ranging 4 to 24 g/year

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Caracterização de microfluxos: medição, geração e calibração

72

Figura 4.5 – Constituintes das fugas de permeação de R134a.

A taxa de fuga, no caso de uma fuga de permeação, é descrita pela equação:

! " " equação 4.1

Onde é a taxa de fuga em mol/s, é o coeficiente de permeabilidade da membrana de permeação

em mol/(Pa.m.s), é a área de permeação em m2, é o gradiente de pressão entre a pressão interna

da fuga e a exterior (pressão atmosférica) em Pa e é a espessura da membrana de permeação em m.

Dezenas de frascos foram testados, constituídos por diferentes tipos de vidros, dimensões e gargalos.

Os ensaios preliminares revelaram que alguns tipos de frascos não suportavam a diferença de pressão

(dentro do frasco a pressão, em condições atmosféricas normais era de aproximadamente 5 bar);

R134a

Elemento de

permeação

Elemento de

vedação

Tampa

Anilha metálica

furada

Área de permeação

Elemento de permeação

Tampa

Frasco de vidro Elemento de vedação

+ Method validation

  Leaks were weighted over several days (U=± 15%) and leak rate calculated.

  Then, all leaks were calibrated in the p∆V set-up and compared

18

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+ R134a reference leaks

  Dependence on the delivering pressure   2000 steps measurements ≈ 25 min

19

5.20

5.25

5.30

5.35

5.40

5.45

5.50

5.55

5.60

940 960 980 1000 1020 1040 1060

Lea

k r

ate

(g/y

ear)

delivering pressure (mbar)

-0.033%/mbar@23°C

+ R134a reference leaks

  Dependence on the temperature

20

0 1 2 3 4 5 6 7 8 9

10

10 15 20 25 30 35 40

Leak

rat

e(g

/yea

r)

Temperature (ºC)

R134a ø1.5 mm #1

0 1 2 3 4 5 6 7 8 9

10

10 15 20 25 30 35 40 Le

ak r

ate(

g/y

ear)

Temperature (ºC)

R134a ø1.8mm

α =3.1%/K α =2.8%/K

+ He reference leaks (permeation)

  Dependence on the delivering pressure   2000 steps measurements ≈ 35 min

21

2.40E-05

2.50E-05

2.60E-05

2.70E-05

2.80E-05

940 960 980 1000 1020 1040 1060

Lea

k r

ate

(mb

ar.L

/s)

delivering pressure (mbar)

-0.027%/mbar@23°C

+ Conclusions

  p∆V calibration method was assembled with a syringe pump and is properly working at Metrovac laboratory in Lisbon.

  One absolute pressure gauge is used   This gauge is suitable to measure both the calibration pressure and the

pressure change

  Temperature stability is critical to achieve good uncertainty

  Uncertainties ≈3% are easily achieved for the range of 10–5 mbar.L/s or 5g/year

  R134a reference leaks were developed and fully characterized.   This leaks were used to validate the method

22

+ 23

Acknowledgements: The financial support of the Portuguese Foundation for Science and

Technology is gratefully acknowledged (PTDC/EME- MFE/098738/2008)

Thanks for your attention

odt@fct.unl.pt