the sensors do burn out eventually though, IIRC the O2 is the shortest lived sensor -and a pretty useful one to have as explorers a number of years back found when exploring 'kinder surprise' a straight forward culvert that had been done before, only unknowingly one of the exits had been blocked and O2 levels had dropped horrendously, thankfully their 4gas warned them .
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.
There have only been a couple of occasions in sewers where the air quality became a serious risk, both were times before I had a gas sensor and I was lucky enough to realise I needed to get out. Once was in the HF which is usually fine.
I got a Microclip XT new off eBay for around £180. A 6 month calibration costs £60 and it needed another £100 worth of parts last time.
Not cheap but it's reassuring knowing you've reduced the chance of being caught off guard. Having said that I know people that rarely use them and have been fine, just depends if the drain gods decide to shit on you at any given time.