Mark,
You will note all over this site that people spend hundreds if not thousands of dollars on overland gear and kit for related hobbies. Arclight and Fyreman speak the truth. If your serious about getting into unseen places, you should drop the coin on a good monitor. We use AreaRAE monitors, among several other types, in the US&R system. Our "canary" is a hazmat technician or specialist with the monitor attached to their body. Their whole purpose is to make sure the space is safe for us to work in or search. Even then, God blinks. Our people have been burned, gotten sick, and our K-9's have gotten sick from the crap found in confined spaces. Despite having the hazmat folks there, they even teach us lowly medical specialists how to use the monitors; it's that important. Stay safe and enjoy the hobby.