How can one find whether or not there is enough oxygen for breathing present inside a (dry) well of diameter 1 m at a depth of 50 m?
Many people in Nepal are killed while cleaning wells because of a lack of oxygen.
The ideal solution would be a single gas personal detector, such as this one (it's only the first example I found, I don't know this brand, but there are many equivalent apparatuses from other vendors).
While digital personal detectors are expensive ($200+ range), there are simpler systems which also provide some basic safety (better than nothing). The Davy lamp is perhaps the best known example of these, and it was a huge breakthrough in mine safety when it was popularized after 1815.