The easiest tool to use is OSHA's Confined Space Advisor. It's free to download and easy to use. It will clearly indicate to you whether a space is a legally defined Confined Space, if such a space requires a Permit to Enter, and if alternate procedures are available to make a Permit Required Space into a a Non-Permit one. Such alternate procedures could be purging a tank with pneumatically operated (compressed air not electric powered) fans to make it free of a hazardous atmosphere and to provide plenty of fresh air. Entry with a SCBA (Self-Contained Breathing Apparatus) requires an Entry Permit, specific team training and specialized equipment such as an emergency plan, communications, extraction hoist for underground manholes, specialized monitoring equipment for oxygen and any hazardous airborne contaminant (Draegger's Chip Reader is the easiest to use and self calibrates), and a SCAS (Self Contained (emergency) Air Supply) except for those locations where the entrant can be easily seen from a surface opening. In a Marine setting (Marine Terminal or Shipboard), only an Industrial Hygienist or Gas Free Engineer is considered legally competent to issue an Entry Permit for Confined Space Entry.