Post-1993 vs. Pre-1993 Evacuation Standards
The evacuation requirement for post-November 15, 1993 equipment is 25 mm Hg absolute, which is a very deep vacuum measured on an absolute scale (0 = perfect vacuum). For pre-1993 equipment, the requirement is 25 inches Hg vacuum, which is a gauge reading relative to atmospheric pressure and represents a much shallower vacuum. Confusing these two measurements is one of the most common exam errors. Remember: mm Hg absolute (deep vacuum) for newer equipment; inches Hg vacuum (gauge) for older equipment.
Post-1993: 25 mm Hg absolute (very deep vacuum). Pre-1993: 25 inches Hg vacuum (gauge). Do not confuse these.
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