Fram used to supply shops with an "air filter tester"- - - -a round housing with a light bulb inside it. Place the used filter on the fixture, put the lid on, and push a button. The amount of light visible through the filter media would indicate if it needed replacing. Hint- - - -it had a VERY low-wattage bulb! Just a gimmick to help sell a lot of filters!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!