In vacuum service
In vacuum service means that equipment is operating at an internal pressure which is at least 5 kilopascals below ambient pressure.
40 CFR § 98.128
Except as provided in this section, all of the terms used in this subpart have the same meaning given in the Clean Air Act and subpart A of this part. If a conflict exists between a definition provided in this subpart and a definition provided in subpart A, the definition in this subpart shall take precedence for the reporting requirements in this subpart.