Introductory Lab: Vacuum Methods

Transcription

1 Introductory Lab: Vacuum Methods Experiments in Modern Physics (P451) In this lab you will become familiar with the various components of the lab vacuum system. There are many books on this topic one of the best is the classic by Roth [1], which is available at the experimental station. You will compare theoretical predictions of the conductance of an opening to actual measurements taken with the vacuum system for two different types of gas. You should also be able to observe the transition between viscous and molecular flow through the orifice. Introduction The International System of Units has never become popular among vacuum practitioners. Thus, we will use here 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 m 3 for volume, and Torr = N/m 2 (Pascal) for pressure. An amount of gas M is given by M = pv (units: Torr-liters). The gas flow Q measures an amount 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 = C 12 (P 2 P 1 ), (2) where C 12 is the conductance between volumes 1 and 2 and P 2 and P 1 are the pressures in volumes 1 and 2. The conductance, measured in l/s, is dependent on the gas type, pressure, and physical properties of the connection between the volumes. In Ref. [1] you can find theoretical predictions for the conductance of various openings. One can imagine this is a critical parameter in the design of vacuum systems. Understand the difference between the viscous flow and molecular flow regimes. How does the conductance depend on the type of gas? Both Helium and air are available for testing. Setup and Equipment Pumping station (see Fig. 1) with various types of vacuum pumps, gauges, and valves. Look over the station carefully, 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

2 GB1 C GCV 23 C 12 T 3 2 V5 VN V1 1 GI GTC2 gas V6 GTC1 V4 V7 V2 DP VG FP2 FP1 Figure 1: A diagram of the lab vacuum system. In addition to understanding the physics principles behind these pumps and gauges, make sure you understand the pressure ranges over which each operates. Because some pumps and gaugeslike a diffusion pump and an ionization gaugecan be damaged if operated at the wrong pressure, it is very important that you check with the instructor before turning on pumps or gauges, or opening or closing valves. You also should check before turning these devices off! Shutting down the pumping devices must occur in a certain prescribed order! Startup Procedure Check and leave the gate valve, VG, open throughout the experiment. volume 1 from the diffusion pump. There is no need to ever isolate 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 vacuum station. 3. Open V7 and V2 to begin evacuating volume 1. Opening the valves after starting up the pump ensures that any oil ejected from the pump into the lines during pump startup will not go into the vacuum chamber. 4. You may find it interesting to monitor GTC1, GTC2, and GCV as a function of time during the pump-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 will serve as a backing pump for the diffusion pump. 2

3 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 an hour or so, approaching a base pressure of a few times 10 6 Torr. If this does not happen, there is a leak 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 Turn off the diffusion pump heater. Leave the cooling water on and FP1 running the diffusion pump doesn 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 startup and shutdown procedures. Some preliminary or theoretical exercises can be conducted while you are waiting for the system to reach equilibrium. Writeup your results in a formal lab report. Your report does not need to provide a valve-by-valve account of the startup and shutdown procedures. Focus on the procedures relevant 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 turn off 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 mm and a length of ± 0.02 mm. Based on the information in Ref. [1] develop predictions for the conductance for air and Helium in both the molecular flow and the viscous flow regime. At what volume 2 pressure do you expect a transition between these two regimes? To measure the conductance, you ll likely want to fill volume 2 with some gas and then examine the flow rate into volume 1. This can be done in a controlled way through the needle valve VN. Be sure the ion gauge is off to avoid trips. (Explain why the conductance be measured without precise knowledge of the pressure in volume 1.) Here is a typical sequence for putting gas in volume 2, which eventually transfers 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 much you can let more air in by opening V5. 3

4 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 volume 2 substantially to study the dependence of conductance on pressure. However, avoid letting the pressure 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, utilize Logger 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 the relevant pages of the manual (attached). Note the calibration is different for different gasses. Examine the ROOT script that is used to determine the calibration functions for air. This reproduces the results in 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 theoretical predictions for the molecular and viscous flow regimes. Comment on any patterns in the data or deviation from the prediction. Repeat the conductance measurements after connecting a Helium source provided by your instructor to 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 theory and the data for air. Discuss similarities and differences. References [1] A. Roth, Vacuum Technology, Elsevier, Amsterdam,

5 Chapter 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 2829 are graphs that represent true pressure for N 2 or air (y axis) versus voltage (x axis). Output impedance is 100 Ω. The output is normalized to 0.375Vdc at vacuum chamber pressure and to Vdc at 1000 Torr (133.3 kpa, 1333 mbar) for N 2 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 N 2 (air has approximately the same calibration). For gases other than N 2 or air, heat loss varies at any given pressure, and you must apply an appropriate conversion factor. Commonly used Gases Other than N 2 or Air Other Gases Refer to Table 3-7 on page 30 for pressure versus output voltage for 10 commonly used process gases other than N 2 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. 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 26 Mini-Convectron Module Instruction Manual Rev. 03 5

6 Operation Table 3-6 Equations for Calculating N 2 or Air Pressure Versus Analog Output Voltage Segment Output Voltage Equation where y = Pressure and x = Voltage Coefficients to V y Torr = a + bx + cx 2 + dx 3 + ex 4 + fx 5 a b y Pa = ( a + bx + cx 2 + dx 3 + ex 4 + fx 5 ) c y mbar = ( a+ bx+ cx 2 + dx 3 + ex 4 + fx 5 ) d e f to V a a + cx + ex 2 y Torr = bx + dx 2 + fx 3 b y Pa c a + cx + ex 2 = d bx dx 2 fx e y mbar = a + cx + ex bx dx 2 fx f to V a a + cx y Torr = bx + dx 2 b y Pa c a + cx = bx dx d y mbar = a + cx bx dx Mini-Convectron Module Instruction Manual Rev Before You Begin Installation Operation Maintenance 6

7 Chapter 3 Figure 3-4 Analog Output Voltage vs. Indicated N 2 or Air Pressure, 1 mtorr to 100 mtorr 28 Mini-Convectron Module Instruction Manual Rev. 03 7

8 Operation Figure 3-5 Analog Output Voltage vs. Indicated N 2 or Air Pressure, 0.1 Torr to 1000 Torr Mini-Convectron Module Instruction Manual Rev Before You Begin Installation Operation Maintenance 8

9 Chapter 3 Table 3-7 Voltages (Vdc) for Commonly used Gases, 0.1 mtorr to 1000 Torr Torr or mtorr 1 mtorr 2 mtorr 5 mtorr 10 mtorr 20 mtorr 50 mtorr 100 mtorr 0.2 Torr 0.5 Torr 1 Torr 2 Torr 5 Torr 10 Torr 20 Torr 50 Torr 100 Torr 200 Torr 300 Torr 400 Torr 500 Torr 600 Torr 700 Torr 760 Torr 800 Torr 900 Torr 1000 Torr True pressure kpa 1.3 x x x x x x x x x x x x x x x x x x x x x x x 10 2 mbar 1.3 x x x x x x x x x x x x x x x x x x x x x x x 10 3 N 2 (air) Argon Helium O CO KR Freon Freon D Ne CH / Mini-Convectron Module Instruction Manual Rev. 03 9

10 Operation Figure 3-6 True Pressure versus Indicated Pressure for Commonly used Gases, 10 4 to 10 1 Torr True pressure in Torr Convectron gauge axis must be horizontal Pressure unit equivalents: 1 µm Hg = 1 mtorr = 1 x 10 3 Torr 1000 µm Hg = 1 Torr 1 mbar = 100 pascal Indicated pressure in Torr (N 2 equivalent) To convert Torr to pascal, multiply by To convert Torr to mbar, multiply by Mini-Convectron Module Instruction Manual Rev Before You Begin Installation Operation Maintenance 10

11 Chapter 3 Figure 3-7 True Pressure versus Indicated Pressure for Commonly used Gases, 10 1 to 1000 Torr True pressure in Torr Convectron gauge axis must be horizontal Pressure unit equivalents: 1 µm Hg = 1 mtorr = 1 x 10 3 Torr 1000 µm Hg = 1 Torr 1 mbar = 100 pascal Indicated pressure in Torr (N 2 equivalent) To convert Torr to pascal, multiply by To convert Torr to mbar, multiply by Mini-Convectron Module Instruction Manual Rev

12 Operation Figure 3-8 True Pressure versus Indicated Pressure for Commonly used Gases, 10 1 to 1000 Torr True pressure in Torr Convectron gauge axis must be horizontal Pressure unit equivalents: 1 µm Hg = 1 mtorr = 1 x 10 3 Torr 1000 µm Hg = 1 Torr 1 mbar = 100 pascal Indicated pressure in Torr (N 2 equivalent) To convert Torr to pascal, multiply by To convert Torr to mbar, multiply by Mini-Convectron Module Instruction Manual Rev Before You Begin Installation Operation Maintenance 12