We have a SOG that air monitoring is required after the fire is knocked down prior to any interior entry without SCBA. We currently have air monitors on our truck company and one of our quints, with new air monitors on the way for all front-line engines and the other quint.
No one - firefighters engaged in overhaul, investigators, residents, or anyone else - who is not in SCBA is allowed to enter a post-fire building until air monitoring is complete.
For most multi-gas meters the limits for the gases detected are programmed in. So if a level is higher than the exposure limits the unit will alarm. So if the CO is to the level to alarm, then the SCBAs stay on and you keep venting, keep doing so until the levels are less than the set point.
For instance our CO level is set for 35 ppm. While one can be exposed to such a CO level for a short time without an SCBA, that is where the alarm is set. So if the alarm goes off, you keep the SCBA on, at least for us.