Fountain drink machine stops when full. How does it know?

[Santo_Rugger]

Drum God, check out groman’s drawings. When the cup is tilted and begins to overflow, the small amount that overflows will flow down the lever, instead of down the outside of the cup. I forget exactly what it’s called (something having to do with adhesion and cohesion), but the liquid has an easier path down the lever, for two reasons. One, it is more vertical, and two, it has a lower relative roughness, allowing for easier flow. You might get one or two drips going down the cup, but hardly enough to notice.

[Rigamarole]

@Pygmy Rugger

Drum God, check out groman’s drawings. When the cup is tilted and begins to overflow, the small amount that overflows will flow down the lever, instead of down the outside of the cup. I forget exactly what it’s called (something having to do with adhesion and cohesion), but the liquid has an easier path down the lever, for two reasons. One, it is more vertical, and two, it has a lower relative roughness, allowing for easier flow. You might get one or two drips going down the cup, but hardly enough to notice.

Yes, that’s correct.

@Charger

The next time you fill a cup, loudly announce the following. “I am so baffled by this machine! How does it know when the cup is full?” Then quickly press your ear to the side of the machine. If you hear snickering, you’ve got your answer.

Very funny.

[Drum_God]

@Pygmy Rugger

Drum God, check out groman’s drawings. When the cup is tilted and begins to overflow, the small amount that overflows will flow down the lever, instead of down the outside of the cup. I forget exactly what it’s called (something having to do with adhesion and cohesion), but the liquid has an easier path down the lever, for two reasons. One, it is more vertical, and two, it has a lower relative roughness, allowing for easier flow. You might get one or two drips going down the cup, but hardly enough to notice.

Got it. So, then, very little product is lost, too.

[StinkyBurrito]

@Pygmy Rugger

Stinky Burrito, the sensor being wet or dry is irrelevant. What is relevant is how much electrical energy can be stored by it. If the lever is touching the liquid in the cup (because it has overflowed), it creates a bigger “tank” for the capacitance. It would be pretty easy to measure this change in capacitance. For all intents and purposes, we can pretty much assume the leve is always wet (except for the first cup poured that day). What is important is weather or not there is an electrical circuit that can be completed, that flows through the liquid in the cup.

Ah Ha! Now I see. I think this would explain it then. Thanks.

Although I was hoping to get rigamarole to do all sorts of wacky experiments.

[cmosdes]

@Pygmy Rugger

Once the liquid stops overflowing, a meniscus is formed above the rim. The surface tension of the liquid allows it to touch the bar, the rim of the cup, and the liquid, all at the same time. Think about pouring a glass of milk, you can actually pour it higher than the brim of the glass without it spilling out. Cohesion is a wonderful thing.

I’m familiar with surface tension/cohesion, but I was unaware you could have the liquid in contact with the bar and NOT have it dribbling down bar until it pulls back to the other side of the rim of the cup and loses contact with the bar. I’ll have to tilt some cups at home and see if I can get it to be in contact with an external bar after it has overflowed and settled back down.

[Q.E.D]

I’ve actually had a chance to play with one of these machines–one of the local Burger Kings has one in the lobby for customers to draw their own drinks which I’d never paid attention to before this thread. It seems to work exactly as described by Rigamarole and it is, indeed, capacitively controlled. Further, if you depress the metal filling lever with your finger, you get a just short squirt of beverage before it shuts off due to your body capacitance. The fill lever simply being wet makes no difference; there must be a sufficient quantity of liquid to create a large enough circuit path between the beverage in the cup and the fill lever. In fact, if you leave the cup in place, you get a continuous cycle of squirts to top off the cup which stops until the level of liquid no longer sufficient to make contact with the lever, then it tops it off again, ad infinitum. Very clever design, for the most part.