2 questions regarding vacuum pump

[nimbus8]

First im having an issue with my manometer. I have a controlled leak valve on my rotary vacuum pump setup to controll how much vacuum it pulls. By logic as i close the valve my manometer should read lower torr, but instead it goes up. My manometer has a positive and negative pressure connection like the one in this picture

http://cfnewsads.thomasnet.com/images/large/583/583883.jpg

. But the manual that came with mine did not say which was positive and which was negative so i tried to get readings with both sides but each side had the same issue, and the vacuum increased the torr increased which shouldnt happen.

The second question is a quick one, can i leave clean vacuum oil in the pump?

 

[Compositepro]

Your manometer is reading gage pressure not absolute pressure. Gage pressure is the difference from atmospheric reference pressure. If you lessen the air leak into the vacuum you will get a lower absolute pressure but the gage pressure reading will go up. Take the time to understand how your instrument works.

 

[nimbus8]

Ive tried can seem to find any manuals online for one like mine.If its reading gage pressure could i not just subtract the reading from atmospheric to get the absolute pressure?

 

[Compositepro]

Yes, you could. But, you can't know the actual atmospheric pressure without a barometer, which is itself an absolute pressure gage. Closed-end mercury manometers used to be the most common instrument for reading vacuum. With one end closed and filled completely with mercury, it created a zero absolute pressure reference.