Which instruments should be used in a flammable atmosphere?

In a flammable atmosphere, it is essential to use instruments that are specifically designed to prevent sparks or ignition that could lead to an explosion. The option indicating instruments ranked "UL" or "FM" approved as "intrinsically safe" is correct, as these certifications signify that the instruments have been tested and are deemed safe for use in hazardous environments.

"Intrinsically safe" devices are engineered to function safely under conditions where flammable gases or vapors are present, ensuring that energy levels are limited to a point where ignition cannot occur. This includes protection against potential heat generated by an instrument during operation and any sources of ignition, making them suitable for environments classified as hazardous due to the possibility of explosions.

In contrast, using instruments without explosion-proof ratings, common household thermometers, or standard electrical devices poses a safety risk, as these items are not designed to operate safely in flammable atmospheres and can easily create sparks or heat sufficient to ignite flammable substances. Thus, in environments where flammability is a concern, using intrinsically safe instruments is crucial for safety and operational integrity.