As you repeatedly press [-], the selection moves from left to right, and
you see these screens in sequence:
9.2 Calibration
Two types of calibration are available: Zero (fresh air) and Span.
Note: Complete calibration procedures are detailed on page 39.
Press [+] to select Calibration. Otherwise, press [MODE] to go back,
or press [-] to advance to the next menu.
The Calibration screen is shown:
Select Zero or Span Calibration by pressing [-]. Once your choice is
highlighted, press [+].