Experiment 4

High Vacuum Pumpdown from Atmospheric Pressure

Nitrogen Environment

 

Objective: Experimentally obtain the high-vacuum, pumpdown curve from an nitrogen atmosphere. Demonstrate correct operation of a high vacuum system.

Reference: Modules 1, 3, 10 and 11

Required Materials:

Varian Vacuum Trainer

Stop watch

Latex gloves

Nitrogen source with regulator

 

Experimental Procedure:

Students must wear latex gloves when handling vacuum components.

If the system is not at rough vacuum, perform Steps 1 through 6. If the system is under vacuum, proceed to Step 7.

1. Insure that the system has achieved equilibrium at atmospheric pressure. All valves should be closed. Remove the bell jar and let stand for at least 15 minutes.

2. Visually inspect the base plate and gasket for cleanliness. If the gasket needs cleaning, wipe with alcohol. Dry thoroughly with a lint-free cloth. Lubricate with a thin film of vacuum grease.

3. Place the bell jar and shield on the baseplate.

4. Close all valves.

5. Plug in the vacuum trainer.

6. Turn the Mechanical Pump Switch to the ON position. Turn the Turbo Pump Switch to the OFF position.

7. Purge the nitrogen system before attaching it to the vacuum trainer. Turn the nitrogen source ON and obtain a moderate flow of gas (maximum 2 psi output). While the gas is still flowing, attach the nitrogen source to vent the port on the trainer.

8. Isolate the bell jar from the high vacuum.

Close the High Vacuum Valve.

Turn OFF the Turbo Pump Toggle Switch.

Press STOP on the Control Panel.

Close the foreline valve.

Press START to restart the mechanical pump.

9. Slowly OPEN the vent valve completely, until the system is at atmospheric pressure.

10. Turn OFF the vent valve when the system is at atmospheric pressure and verify by momentarily lifting the bell jar off the base plate.

11. OPEN the roughing valve in one continuous motion. Start the stopwatch as you begin opening the roughing valve.

12. Record the pressure at 30 second intervals until crossover.

13. At 3 Torr, begin crossover operation by

- closing the roughing valve

- opening the foreline valve and waiting 10 seconds

- opening the high vacuum valve

14. Press START on the Turbo Controller Panel. Allow the Turbo Controller to come up to speed ( 75 K rpm) and indicate NORMAL OPERATION.

15. Switch the gauge to BA (ION gauge). The gauge will begin operation when in range (<1 mtorr).

16. Continue taking readings until reaching base pressure. Record the pressure at 30 second intervals for 5 minutes, at 1 minute intervals for the next 5 minutes, and 2 minute intervals until completion. (Minimum length of the experiment is 20 minutes.)

17. Plot pressure versus time on seven-cycle x 70 division semilog graph paper.

18. Compare the graph to the data obtained in Experiment 3. Note the differences due to nitrogen preventing the adsorption of water.

19. In your lab report, discuss the difference between the pumpdown curves for this experiment and Experiment 3.