His company also carries a full line of atmospheric gas safety monitors which can check for things such as oxygen and carbon monoxide. Here is a link to those
If you are working on combustion equipment you really should have an atmospheric CO detector with you to insure the space you are working in does not have a dangerous buildup of CO. I would not trust the inexpensive detectors you buy in big box stores. They often have a long delay before alarming, even at levels that can be dangerous. You need something that displays the CO level so you can test the space when you first enter without relying on an alarm.
A combustible gas detector is also advisable. You don’t want to enter a space that has a buildup of combustible gas. In the old days of halide torches I narrowly avoided being a statistic of an explosion caused by a gas leak. I was going to enter a crawl space to check for a refrigerant leak with a halide torch. I waited to light the torch until I actually got under the house, which probably saved my life. When I approached the crawl space door I smelled gas, so I did not light the torch. The odd thing was that there was no gas equipment under that part of the house. I followed my nose across the yard to a large LP tank with a bad leak where the line came out of the regulator. The LP being heavier than air had drifted downhill 50 feet across the yard and collected in the crawl space. I fixed the leaking flare and told the home owner what I had found. They said they had just received a delivery the previous day. Had I not paid attention to my combustible gas detector (my nose) there might not have been a “Fundamentals of HVACR” because one of the authors would have perished decades before. It is always a good idea to test the spaces you plan to enter. To be in a position to help anybody else you must first insure your own safety.