Medela Vacuum Gauge fits most standard breastpump flanges (27mm -24mm) and measures vaccum of pumps in mmHg.

Breast pump effectiveness is evaluated by measuring the vacuum (also
called suction) of the pump with a pressure gauge, an instrument that
measures negative pressure. The gauge needle points to a number from 0
to 450 mmHg (the abbreviation for millimeters of mercury Hg is the
chemical symbol for mercury). The reading on the gauge is then
compared to a standard range for the specific breast pump that is being
tested, to determine whether the pump is performing adequately or not.

May need to adjust readings for high altitudes – see chart below.

Sea level to 1650 ft (500 meters) 255 mmHg (235 275 mmHg)
1650 to 3300 ft (500-1000 meters) 240 mmHg (220 260 mmHg)
3300 to 6600 ft (1000-2000 meters) 225 mmHg (205 245 mmHg)
Above 6600 ft (2000 meters) 199 mmHg (179 219 mmHg)

Please Note: The pressure gauge is very fragile andneeds to be handled with care. Dropping a gauge can result in an incorrect reading.