Bringing a McLeod Gauge to Vacuum Conditions

To bring a McLeod gauge to vacuum conditions and before turning the vacuum pumps on, be sure that the two-way and three-way stopcocks are rotated to their off positions. Then, turn on the vacuum pump and also do the following  

Slowly open the two-way stopcock a small amount, allowing the mercury to slowly rise from the storage bulb up to the bottom of the second bulb, then close the stopcock. 

Slowly open the three-way stopcock to evacuate the McLeod gauge, drawing the mercury back into the storage bulb. 

Repeat steps 1 and 2 several more times until the mercury enters the upper bulb very slowly and evenly. The number of repetitions is directly related to the volume of your system. Thus, the larger your system, the more repetitions of steps 1 and 2 will be required. 

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