Introductory Lab: Vacuum Methodscourses/p451/experiments/vacuum_exp… · V5 V6 V4 V7 V2 V1 VN VG...

Introductory Lab: Vacuum MethodsExperiments in Modern Physics (P451)

In this lab you will become familiar with the various components of the lab vacuum system. There aremany books on this topic – one of the best is the classic by Roth [1], which is available at the experimentalstation. You will compare theoretical predictions of the conductance of an opening to actual measurementstaken with the vacuum system for two different types of gas. You should also be able to observe the transitionbetween viscous and molecular flow through the orifice.

Introduction

The International System of Units has never become popular among vacuum practitioners. Thus, we will usehere the traditional, non-S.I. units. (For conversion factors between different units, see [1], p.42 and p.65).Unless otherwise indicated, we will use l = liters = 10−3 m3 for volume, and Torr = 133.3 N/m2 (Pascal)for pressure. An amount of gas M is given by M = pV (units: Torr-liters). The gas flow Q measures anamount of gas per time t:

Q = M/t. (1)

Note that Q has units of Torr-l/s.You will be studying the conductance between two volumes. As you might expect conductance C is the

constant of proportionality that turns a pressure differential into a a gas flow Q:

Q = C12(P2 − P1), (2)

where C12 is the conductance between volumes 1 and 2 and P2 and P1 are the pressures in volumes 1 and2. The conductance, measured in l/s, is dependent on the gas type, pressure, and physical properties of theconnection between the volumes.

In Ref. [1] you can find theoretical predictions for the conductance of various openings. One can imaginethis is a critical parameter in the design of vacuum systems. Understand the difference between the viscousflow and molecular flow regimes. How does the conductance depend on the type of gas? Both Helium andair are available for testing.

Setup and Equipment

Pumping station (see Fig. 1) with various types of vacuum pumps, gauges, and valves. Look over the stationcarefully, and be sure you can identify all the indicated components before you start.

Pumps

• Mechanical fore pump (FP1,2). In our case, these are rotating-vane pumps.

• Oil diffusion pump (DP). Why do you think the pump needs to be cooled?

Gauges

• Ionization gauge (GI). How is “ionization involved in measuring pressure?

• Thermocouple gauges (GTC1,2). Read in [1] about the physics behind these devices.

• Convectron gauge (GCV). Contrast this type of gauge with thermocouples.

• Bourdon gauge (GB1). This measures absolute pressure using mechanical deformation.

1

GB1 GCV

FP1FP2

DP

GI

GTC2

GTC1

T 3 2

1V5

V6

V4 V7

V2

V1VN

VG

gas

C C23 12

Figure 1: A diagram of the lab vacuum system.

In addition to understanding the physics principles behind these pumps and gauges, make sure youunderstand the pressure ranges over which each operates. Because some pumps and gauges–like a diffusionpump and an ionization gauge–can be damaged if operated at the “wrong” pressure, it is very important thatyou check with the instructor before turning on pumps or gauges, or opening or closing valves. You alsoshould check before turning these devices off! Shutting down the pumping devices must occur in a certainprescribed order!

Startup Procedure

Check and leave the gate valve, VG, open throughout the experiment. There is no need to ever isolatevolume 1 from the diffusion pump.

1. Close all valves except the gate valve.

2. Turn on gauges GCV and GTC1,2 and pump FP1 using the power strip on the back of the vacuumstation.

3. Open V7 and V2 to begin evacuating volume 1. Opening the valves after starting up the pump ensuresthat any oil ejected from the pump into the lines during pump startup will not go into the vacuumchamber.

4. You may find it interesting to monitor GTC1, GTC2, and GCV as a function of time during thepump-down period. Do NOT turn on the ion gauge GI!

5. Wait until GTC1 is stable. It should be below 100 mTorr.

6. Turn on the diffusion pump cooling water (under sink).

7. Close valve V2 to isolate volume 1 from the fore pump. V7 should remain open as the fore pump willserve as a backing pump for the diffusion pump.

2

8. Engage the heater for the diffusion pump. If it does not stay on, get the instructor!

9. Wait about 1/2 hour, at which time GTC2 should now read near zero and not be visibly decreasing.

10. Turn on the ion gauge GI, record periodically. The pressure in volume 1 will decrease slowly for anhour or so, approaching a base pressure of a few times 10−6 Torr. If this does not happen, there is aleak and one has to search for it (ask for help).

Shutdown Procedure

Note that the shutdown procedure for the diffusion pump takes about 30 minutes – plan your time accordingly!.

1. Close V5, V6, VN if not already closed. V2 should already be closed and V7 should be open.

2. Turn off FP2, the small fore pump for volume 3.

3. Turn off the diffusion pump heater. Leave the cooling water on and FP1 running – the diffusion pumpdoesn’t stop pumping until it cools down.

4. Wait 30 minutes.

5. Close V7.

6. Turn off FP1.

7. Turn off cooling water.

Exercises

Conduct the following experiments. To avoid damage to equipment, carefully follow the prescribed startupand shutdown procedures. Some preliminary or theoretical exercises can be conducted while you are waitingfor the system to reach equilibrium. Writeup your results in a formal lab report. Your report does notneed to provide a valve-by-valve account of the startup and shutdown procedures. Focus on the proceduresrelevant to studying the conductance of the orifice and discuss your results.

• Record your final pressure value as read by GI. Discuss your results with your instructor. Then turnoff the ion gauge for the following investigations in order to avoid damage.

• Develop a scheme for measuring the conductance of the orifice between volumes 2 and 1. This orifice,which is visible through the top window, is a cylindrical hole that has a diameter of 1.10 ± 0.02 mmand a length of 19.20 ± 0.02 mm.

• Based on the information in Ref. [1] develop predictions for the conductance for air and Helium in boththe molecular flow and the viscous flow regime. At what volume 2 pressure do you expect a transitionbetween these two regimes?

• To measure the conductance, you’ll likely want to fill volume 2 with some gas and then examine theflow rate into volume 1. This can be done in a controlled way through the needle valve VN. Be sure theion gauge is off to avoid trips. (Explain why the conductance be measured without precise knowledgeof the pressure in volume 1.) Here is a typical sequence for putting gas in volume 2, which eventuallytransfers to volume 1 and through the diffusion pump.

1. Be sure valves V5, V6, and VN are still closed.

2. Turn on FP2.

3. Open V6 slightly to pump out volume 3 until GB1 reads around 25 Torr. If you pump too muchyou can let more air in by opening V5.

3

4. Close V6 and turn off FP2.

5. Open VN to let air into volume 2. When doing this you want to increase the pressure in volume2 substantially to study the dependence of conductance on pressure. However, avoid letting thepressure on the diffusion pump fore line (measured by GTC1) rise to more than 0.6 Torr.

6. Close VN quickly and observe the pressure decrease in volume 2. To measure the flow rate, utilizeLogger Pro to read out the GCV voltage every 0.1 s.

• Determine the calibration necessary to convert GCV voltage into pressure. This can be found in therelevant pages of the manual (attached). Note the calibration is different for different gasses. Examinethe ROOT script that is used to determine the calibration functions for air. This reproduces the resultsin the manual. A similar script that produces a calibration equation for Helium is also provided.

• Use the ROOT templates to plot the conductance as a function of pressure. Overlay the theoreticalpredictions for the molecular and viscous flow regimes. Comment on any patterns in the data ordeviation from the prediction.

• Repeat the conductance measurements after connecting a Helium source provided by your instructorto the input of volume 3. Flush volume 3 using FP2 several times before taking data with Helium.Again generate plots of the conductance as a function of pressure in volume 2 and compare with theoryand the data for air. Discuss similarities and differences.

References

[1] A. Roth, Vacuum Technology, Elsevier, Amsterdam, 1989.

4

Chapter 3

26 Mini-Convectron Module Instruction Manual - 275512 - Rev. 03

3.2 Nonlinear Analog Output The module contains a convection-enhanced Pirani thermal conductivity gauge. The gauge measures the heat loss from a heated sensing wire that is maintained at a constant temperature.

The analog output produces a nonlinear voltage that corresponds to measured pressure. Output voltage is measured across pins 5 and 8 if the module has one setpoint relay or across pins 5 and 6 if the module has two setpoint relays.

Refer to Table 3-6 on page 27 to calculate pressure (y) as a function of output voltage (x). Figure 3-4 and Figure 3-5 on pages 28–29 are graphs that represent true pressure for N2 or air (y axis) versus voltage (x axis).

• Output impedance is 100 !.

• The output is normalized to 0.375 Vdc at vacuum chamber pressure and to 5.534 Vdc at 1000 Torr (133.3 kPa, 1333 mbar) for N2 or air.

The vacuum chamber pressure indicated by the gauge depends on the gas type, gas density (pressure), and the module orientation. The module is factory calibrated for N2 (air has approximately the same calibration). For gases other than N2 or air, heat loss varies at any given pressure, and you must apply an appropriate conversion factor.

Commonly used Gases Other than N2 or Air

Refer to Table 3-7 on page 30 for pressure versus output voltage for 10 commonly used process gases other than N2 or air.

Refer to Figure 3-6, Figure 3-7, or Figure 3-8 to determine true pressure versus indicated pressure for the gas that is being used.

Other Gases If the gas being used is not included in Table 3-7, or for a gas mixture, you will need to generate a calibration curve using a gas-independent transfer standard such as a capacitance manometer. Use the following equation to determine the maximum usable output voltage:

Output voltage Input voltage 4 Vdc–=

5

Operation

Mini-Convectron Module Instruction Manual - 275512 - Rev. 03 27

Before You BeginInstallation

OperationM

aintenance

Table 3-6 Equations for Calculating N2 or Air Pressure Versus Analog Output Voltage

Segment Output Voltage Equation where y = Pressure and x = Voltage Coefficients

1 0.375 to 2.842 V a –0.02585

b 0.03767

c 0.04563

d 0.1151

e –0.04158

f 0.008737

2 2.842 to 4.945 V a 0.1031

b –0.3986

c –0.02322

d 0.07438

e 0.07229

f –0.006866

3 4.94 to 5.659 V a 100.624

b –0.37679

c –20.5623

d 0.0348656

yTorr a bx cx2 dx3 ex4 fx5+ + + + +=

yPa a bx cx2 dx3 ex4 fx5+ + + + +( ) 133.3"=

ymbar a bx cx2 dx3 ex4 fx5+ + + + +( ) 1.333"=

yTorra cx ex2+ +

1 bx dx2 fx3+ + +--------------------------------------------=

yPaa cx ex2+ +

1 bx dx2 fx3+ + +--------------------------------------------© ¹¨ ¸§ ·

133.3"=

ymbara cx ex2+ +

1 bx dx2 fx3+ + +--------------------------------------------© ¹¨ ¸§ ·

1.333"=

yTorra cx+

1 bx dx2+ +-------------------------------=

yPaa cx+

1 bx dx2+ +-------------------------------© ¹¨ ¸§ ·

133.3"=

ymbara cx+

1 bx dx2+ +-------------------------------© ¹¨ ¸§ ·

1.333"=

6

Chapter 3


Figure 3-4 Analog Output Voltage vs. Indicated N2 or Air Pressure, 1 mTorr to 100 mTorr

7

Operation



OperationM

aintenance

Figure 3-5 Analog Output Voltage vs. Indicated N2 or Air Pressure, 0.1 Torr to 1000 Torr

8

Chapter 3


Table 3-7 Voltages (Vdc) for Commonly used Gases, 0.1 mTorr to 1000 Torr

CH

4

.389

6.4

03.4

38.4

92.5

84.7

961.

053

1.39

22.

014

2.63

23.

313 –

4.69

95/

172

5.58

35.

720

5.86

0 –6.

103 –

6.34

2 – –6.

519 –

6.64

2

Ne

.381

.388

.405

.433

.484

.608

.768

1.00

21.

469

1.97

62.

631

3.71

54.

605

5.40

66.

159

6.48

36.

661

6.72

66.

767

6.80

36.

843

6.89

06.

920

6.94

27.

000

7.05

6

D2

.386

.396

.425

.470

.549

.727

.944

1.26

51.

914

2.60

33.

508

5.05

96.

361 – – – – – – – – – – – – –

Freo

n 22

.388

.400

.432

.480

.566

.764

.990

1.29

11.

805

2.24

72.

666

3.09

03.

330

3.41

43.

509

3.66

03.

883

4.00

54.

088

4.15

14.

203

4.24

74.

271

4.28

64.

321

4.35

4

Freo

n 12

.388

.401

.437

.488

.581

.778

1.00

91.

315

1.82

62.

257

2.64

73.

029

3.20

43.

308

3.43

03.

618

3.82

73.

938

4.01

64.

076

4.12

44.

166

4.19

04.

203

4.23

74.

270

KR

.379

.384

.395

.415

.451

.544

.668

.847

1.19

41.

536

1.92

12.

429

2.73

42.

966

3.07

53.

134

3.26

93.

384

3.46

63.

526

3.57

33.

613

3.63

23.

645

3.67

4 –

CO

2

.385

.395

.412

.462

.536

.705

.900

1.17

91.

668

2.17

22.

695

3.31

63.

670

3.90

34.

071

4.15

44.

336

4.50

24.

621

4.70

84.

775

4.83

04.

860

4.87

74.

919

4.95

5

O2

.384

.392

.417

.453

.521

.679

.868

1.14

11.

664

2.19

52.

814

3.67

24.

225

4.62

04.

916

5.02

65.

106

5.20

05.

315

5.42

25.

515

5.59

25.

633

5.65

85.

713

5.76

2

Hel

ium

.382

.389

.409

.441

.497

.637

.814

1.06

81.

589

2.16

42.

939

4.38

75.

774

7.31

4 – – – – – – – – – – – –

Arg

on

.381

.387

.403

.429

.477

.595

.745

.962

1.38

61.

818

2.33

33.

028

3.48

03.

801

4.03

74.

122

4.19

24.

283

4.38

64.

477

4.55

04.

611

4.64

34.

663

4.70

64.

745

N2

(air

)

.384

.392

.417

.455

.523

.682

.876

1.15

51.

683

2.21

72.

842

3.67

54.

206

4.57

74.

846

4.94

55.

019

5.11

15.

224

5.32

95.

419

5.49

55.

534

5.55

85.

614

5.65

9

True

pre

ssur

e

mba

r

1.3

x 10

–3

2.6

x 10

–3

6.0

x 10

–3

1.3

x 10

–2

2.6

x 10

–2

6.6

x 10

–2

1.3

x 10

–1

2.6

x 10

–1

6.6

x 10

–1

1.3

2.6

6.6

1.33

x 1

01

2.66

x 1

01

6.66

x 1

01

1.33

x 1

02

2.66

x 1

02

3.99

x 1

02

5.33

x 1

02

6.66

x 1

02

7.99

x 1

02

9.33

x 1

02

1.01

x 1

03

1.06

x 1

03

1.19

x 1

03

1.33

x 1

03

kPa

1.3

x 10

–4

2.6

x 10

–4

6.0

x 10

–4

1.3

x 10

–3

2.6

x 10

–3

6.6

x 10

–3

1.3

x 10

–2

2.6

x 10

–2

6.6

x 10

–2

1.3

x 10

–1

2.6

x 10

–1

6.6

x 10

–1

1.33

2.66

6.66

1.33

x 1

01

2.66

x 1

01

3.99

x 1

01

5.33

x 1

01

6.66

x 1

01

7.99

x 1

01

9.33

x 1

01

1.01

x 1

02

1.06

x 1

02

1.19

x 1

02

1.33

x 1

02

Torr

or

mTo

rr1

mTo

rr2

mTo

rr5

mTo

rr10

mTo

rr20

mTo

rr50

mTo

rr10

0 m

Torr

0.2

Torr

0.5

Torr

1 To

rr2

Torr

5 To

rr10

Tor

r20

Tor

r50

Tor

r10

0 To

rr20

0 To

rr30

0 To

rr40

0 To

rr50

0 To

rr60

0 To

rr70

0 To

rr76

0 To

rr80

0 To

rr90

0 To

rr10

00 T

orr

9

Operation



OperationM

aintenance

Figure 3-6 True Pressure versus Indicated Pressure for Commonly used Gases, 10–4 to 10–1 Torr

Indicated pressure in Torr (N2 equivalent)To convert Torr to pascal, multiply by 133.3To convert Torr to mbar, multiply by 1.333

True

pre

ssur

e in

Tor

r

Convectron gauge axismust be horizontalPressure unit equivalents:1 µm Hg = 1 mTorr = 1 x 10–3 Torr1000 µm Hg = 1 Torr1 mbar = 100 pascal

10

Chapter 3


Figure 3-7 True Pressure versus Indicated Pressure for Commonly used Gases, 10–1 to 1000 Torr

True

pre

ssur

e in

Tor

r



11

Operation



OperationM

aintenance

Figure 3-8 True Pressure versus Indicated Pressure for Commonly used Gases, 10–1 to 1000 Torr

True

pre

ssur

e in

Tor

r



12

Introductory Lab: Vacuum Methodscourses/p451/experiments/vacuum_exp… · V5 V6 V4 V7 V2 V1 VN VG...

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Transcript of Introductory Lab: Vacuum Methodscourses/p451/experiments/vacuum_exp… · V5 V6 V4 V7 V2 V1 VN VG...