It is important to consider the relationship between atmospheric pressure and altitude as it affects vacuum pump performance. Basically, the higher you are, the less vacuum you can attain.
Because free air is less dense at higher altitudes (i.e. lower atmospheric pressure) operation at these higher altitudes has the effect of reducing the capacity and maximum vacuum levels attainable. In general, flow is not affected, only the maximum vacuum level attainable.
The basic formula is:
Current Atmospheric Pressure x Max. Rated Level of a Vacuum Pump
29.92″ Hg (absolute vacuum)
Refer to the following table to correct for vacuum pump performance at various altitudes.