When conducting confined space entry training classes, I always tell the learners to take away one piece of information from my class, and it should be that NO confined space EVER be entered unless the atmosphere had been examined with a proper gas monitor or LEL.
The OSHA Permit Required Confined Space standard 1910.146 indicates that confined areas must be examined, with a calibrated direct-reading device, for oxygen concentrations, for flammable fumes and vapors, and for potential toxic air contaminants prior to entry. For areas where air flow is used to control the atmospheric hazard, then regular monitoring is also required to guarantee that a safe atmosphere is managed.
The 1910.146 standard, like most OSHA standards, is efficiency centered and not particularly prescriptive about gas monitoring requirements. The standard represents Appendix B which provides primary details about order of gas tracking and indicates that monitoring should take place every four feet towards direction of travel and side to side. The OSHA Standard does not provide information on selection or calibration as each piece of monitoring equipment is different. Employers and users of equipment must ensure that the gas meter is functioning effectively. There are a number of best methods for monitoring that is not particularly resolved in the OSHA standards.
NFPA is creating a best practice for confined spaces that will consist of more prescriptive details about gas monitoring. Items that may be involved in the best practices are the kind of calibration gas to be used, the suggested regularity of calibration, device reaction times and bump examination and verification.