How did bag-check indicators work on direct air vacuums?

[ilovehoovers]

Sorry if this is in the wrong place, but I was just wondering how bag-check indicators worked on direct air vacuums? I understand how they work on clean-air machines, but I imagine the dirty fan system would've made it more complex. Any ideas?

 

[eurekaprince]

My fan-first Hoover Platinum Lightweight Bagged upright has a “check bag” light on the handle. Not sure how it works, but I imagine it senses higher pressure in the soft bag meaning it no longer filters the air without stress.

 

[blackheart]

depends

It could be using the motors speed. as the bag clogs the motor would be spinning faster due to lower air resistance. Or if we're going old school it could be like the dial a matic (yes I know it's clean air) where the bag check indicator is mechanical. the flowing exhaust air keeps the indicator out of sight, when it's turned off or becomes clogged it's visible

 

[rugsucker]

"-bag check indicators-"

2 of the early ones were Hoover Mo 150 with a red flag that appeared in small opening & post WWII Universal tank that had platform under bag to weigh dirt and turn on light.Others will know and can add more.