A&D Weighing AD-1690 - Manual (Page 8)

6
After confirming, return the inside pressure of the Leak Tester to atmospheric
pressure by removing either the right or left air plug. (If an air plug is not removed,
the pressure inside the instrument remains in a state of vacuum.) When not starting
another measurement immediately, reconnect the air plugs and close the cover on
the AC adapter jack, to avoid dust from invading the main unit.