Dad’s short answer: They do, and this is one of the few smart features in the category I'd call genuinely useful. A sensor reads how dirty the water coming off your floor is and ramps the suction and water up automatically, then eases off when the floor is clean.

How it works and what it's called

The sensor sits in the dirty water pathway and measures turbidity — essentially how cloudy the water is — using an infrared emitter and receiver. Cloudy water means the floor is still dirty, so the machine works harder. Clear water means it's done.

Every brand names it differently, which makes shopping confusing. Roborock calls it Intelligent Dirt Detection or DirTect. Narwal calls it DirtSense. Dreame and MOVA both call it a dirty water sensor. Same underlying idea in each case.

The most useful version of it also drives a light or a display ring that tells you when a spot is actually clean. That sounds like a gimmick until you use it — it stops you doing four passes on a floor that was clean after two, and it stops you walking away from a spot that wasn't.

Where it earns its keep

Auto mode with a working dirt sensor is how I run these machines most of the time. You stop thinking about settings, it ramps up in front of the sink and the dog bowl and eases off in the open floor, and the run takes less time because you're not over-cleaning the clean parts.

It also saves water and battery, which matters on a machine with a small tank.

Dad’s Take: This is the rare smart feature that does something. Auto mode with a dirt sensor is the setting I actually leave it on.