Experiment 2

Rough Pumpdown from Atmospheric Pressure

Nitrogen Environment


Objective: Experimentally obtain the pumpdown curve from a nitrogen atmosphere and compare the results to the to the pumpdown curve for the air fill experiment. Demonstrate correct venting procedures.

Reference: Modules 1, 3, 8 and 9

Required Materials:

Varian Vacuum Trainer

Stop watch

Latex gloves

Nitrogen source with regulator


Experimental Procedure:

If the system is not at rough vacuum, repeat steps 1 through 6 of Experiment 1. If possible, leave at vacuum overnight.

Students must wear latex gloves when handling vacuum components.

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

2. 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.

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

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

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

6. 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.)

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

8. Compare the graph to the data obtained in Experiment 1. Note the differences due to nitrogen preventing the adsorption of water.

9. In your lab report, discuss the difference between the pumpdown curves for Experiment 1 and Experiment 2.