This is a measurement of the ultimate power of the vacuum.  This can be compared to a car’s horsepower.  You’ll see this term on high performance 1/4 Charging Hoses connected to vacuum chucks where holding power is the most important factor regardless of how long it takes to reach the full vacuum level.  Knowing a pump’s maximum Inches of Mercury will tell you how secure of a hold you can achieve.

The majority of all mid-range electric style vacuum pumps will list their CFM and omit their max “Hg.  This is because they assume the pump is being purchased as a replacement or the user knows that a high vacuum rating is not an issue.  Pumps for vacuum chucks that draw a high CFM are often used in the woodworking industry where porous wood and particle board creates a constant leak.  High end 1/4″ standard colored hose that are used in machining plastics and metals require higher holding forces.  The maximum Inches of Mercury is key to these pumps.

This is a measurement of the speed of the vacuum flow.  This can be equated to a car’s top speed.  You’ll see this term most often on electric rotary vane pumps or piston multi-purpose charging set for A/C system (reverse air compressors).  Knowing a pump’s CFM is key to knowing how fast you can remove the air from a tank or refrigeration system.