The acceptance criteria for a vacuum chamber leak rate test will vary with the application of the vacuum chamber or the operations to be performed in the chamber and/or the materials being placed in the chamber.  In the heat treating industry the purpose of vacuum is to reduce the impact of oxygen on the materials to be heat treated.  Typical acceptable leak rate of a new clean, dry cold and out-gassed furnace with mechanical pumps would be less than 10 microns per hour.  For a diffusion pumped furnace it would be lower (maybe 5 microns per hour) as the ultimate vacuum capability will be expected to be lower. Pumping becomes more difficult as the pressure is reduced, thus one needs lower leak rates.

Pump the cold furnace down as low as possible, close the vacuum valve and time the rise in pressure from that point to another.  Calculate the rise in pressure from point A to B to get a millitorr (micron) per hour amount.  This will give you what is called the rate of rise.  If the rate of rise is to high then do the following:

Connect a helium mass spectrometer into the pumping line in front of the vacuum pumps. Reduce the pressure as low as possible and begin to spray helium around all penetrations of the vacuum vessel starting at the top, as helium rises.  Care must be given to the maximum operating pressure of the spectrometer relative to the pressure of the furnace.

A real leak would be defined as air or gas leaking through a vessel from the outside of the vessel into the evacuated space.  A virtual leak is not a through the vessel situation, it is gas being slowly released into the evacuated space from a place where it is trapped, e.g. a bolt in a blind hole.