A young lady of my acquaintance kept failing the chemical test at airport security. This was inconvenient---she wanted to fly---and inaccurate: she had no recent traffic with explosives, weapons, etc. 

She asked them to “clean the machine” and try again.  They did, and she passed.

I am not sure that it would occur to me to ask them to clean the machine. In any case, this is a useful thing to know.