Lab Vacuum Equipment

The reason for using a turbomolecular dry pump is to minimize the possibility that oil and other material will backstream into the detector dewar and contaminate the detector. This is also the reason for using a liquid nitrogen cold trap. Actually the cold trap protects both the detector dewar from material coming from the pump and protects the pump from contaminating material coming from the detector dewar. Never use the pumping station without using the cold trap.

Personal Protective Equipment:

A cold mitten is needed for handling the cold trap and liquid nigrogen funnel. Make sure a mitten is available before beginning pumping operations.

1. Make sure value 3, the air inlet and the compressed gas inlets are all closed.
2. Make sure the LN tank cold tap is clean, dried, installed, and secured with screws with the red knurled caps.
3. Make sure valve 1 and valve 2 are open.
4. Make sure the vacuum hose attached to the high vacuum flange is secure and properly attached to the dewar being pumped. Most of our dewars use a standard vacuum hose attachment fixture. The dewar vacuum valve may be closed or open, depending on whether or not the dewar has a vacuum or not.

Pumping Procedures:

1. Turn on the vacuum pump by pressing the large green button on the side of the pump. The pump is the black metal box at the bottom of the pumping station. The whirring sound of the pump should be heard. The pitch of the pump sound increases as the speed of the pump increases. You should begin to see the pressure drop, as measured by the low pressure gauge. The meter for the low pressure gauge is mounted at the left-front edge of the pumping station.
2. Fill the cold trap with liquid nitrogen. A funnel to aid in filling the cold trap is kept on the pumping station table top. Use caution when handling the funnel after the cold trap is filled since the funnel will be very cold. Use a cold mitten or pliers to handle the funnel. Fill the cold trap only until LN begins to bubble out the top of the trap. Overfilling the trap may cause excess LN to spill onto some of the vacuum joints which could freeze and result in loss of vacuum.
3. Monitor the vacuum with the low pressure gauge. The pressure typically drops to about 10 mTorr or less on this gauge when pumping our lab dewars. If the dewar being pumped has been open for some time you may want to pump for 15 minutes to 1 hour to allow the dewar to outgas. Otherwise just a few minutes of pumping at 10 mTorr should be sufficient.

1. Close the vacuum valve on the dewar.
2. Close valve 1.
3. Turn off the vacuum pump by pressing the green button on the front of the pump.
4. Loosen, but don't yet remove the red knurled screws on the top of the cold trap.
5. Close valve 2.
6. Open valve 3 and the air inlet valves. This brings the vacuum hose back up to atmospheric pressure. This will be indicated on the high pressure gauge.
7. Open valve 2. This brings the LN cold trap up to atmospheric pressure.
8. Remove the red knurled screws holding the LN cold trap cover in place and remove the trap from the outer jacket. Caution: the trap is filled with liquid nitrogen. Use a cold mitten when handling the trap.
9. Carefully pour the LN from the cold trap back into the LN holding flask.
10. Set the cold trap aside to warm up to room temperature. Do not put the trap back into the outer jacket since this will cause water to accumulate inside the pump.