The most accurate indicators of all were probably those fitted to the exhaust air duct on dirty-fan cleaners, like the Hoover Turbopower, Moulinex, and Hotpoint Universal. They required air to be blown onto the sensor part of the gauge (be it a switch or whatever) to activate the indicator. This would only occur when the exhaust air was under high pressure, like when the bag was full, and never when suction was restricted, like when using tools etc. The tubes for the sensors or the sensors themselves were located in such a place that a blockage was unlikely to occur after it, therefore unlikely to activate due to a blockage.