Gauge : Vent the transducer that you are zeroing. Perform a single point "Zero" calibration for that sensor by following the instructions for the specified product. Absolute : Zeroing an absolute sensor requires a vacuum standard such as a Hastings gauge or Baratron. To properly set zero on an absolute sensor a vacuum must be pulled down to a pressure close to 600 mtorr. When the vacuum pressure has stabilized perform a single point "Zero" calibration for that sensor by following the instructions for the specified product.