Bombay barometer 

That miraculous sensor situated in the anal tract that enables the operator to determine whether the chamber is loaded with gas or solid matter, before risking a fart. The Bombay barometer may give faulty readings when, for example, swamped by large quantities of beer and curry, with disastrous results.

"Who dropped the dark blockbuster? Smells like someone's Bombay barometer failed!"

Bombay barometer by bromp January 20, 2009