It's not only gas detectors that need to be calibrated and checked regularly. Just about all safety equipment does. If you're going draining, are you taking an escape set as well?
In an ideal world or more like a controlled working environment you'd each have a gas monitor, escape sets, attached to a rescue/pulley system, radio's, a watchman and even a rescue team on stand-by, plus a confined space entry board to monitor you in and out. Never mind the pile of paper - risk assessments, method statements, permit to work, rescue plan, confined space entry procedures, certificates for all the equipment being used, etc. As well as certificates for any lockouts or isolations put in place.
I'm not sure if most of the above makes it any safer if the shit did hit the proverbial fan. An escape set would give you the chance to get out, but they are cumbersome to carry with you all the time and usually just 15 minutes for a portable one.
I tend to check both ends of culverts before entering, you can usually feel a good airflow when in them. Sewers are a different matter and not something I tend to dabble in, I'd have a minimum of a 4gas and bail if I had any doubts.