Ops, I am sorry, I was not clear enough. The provided link was a very quick search just to show the existence of such a sensor/technology. There are many variations of these sensors and some of them are tunable.
Oh don't worry! I did a bunch of searching and the one you linked as an example happened to be a very reasonable looking candidate. It included what appeared to be some kind of depth sensing. For $10 it was worth a try, though unfortunately the information it came with was sparse.
For such a weird application, there's probably no way around trying a couple of things.
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Ok that’s fine I didn’t notice your preamble on this mentioning this has to cost $50 or less. When I saw the sight glass I assumed a more elaborate operation.
For the record switching a high current load isn’t complicated or expensive. The 24VAC contactor in household ac units is only $12-15 and will switch 30-40A. Just get one with a 120vac coil and run it off a Zwave or zigbee lamp module. $30-35 tops.
I mentioned current measurement is easy because you can passively measure it with an inductive current sensor.
For your budget a rare earth magnet attached to a float inside the tube and household alarm reed switches attached to side of tube will work and meet your budget target.
A magnetic switch does look like the next thing to try. I am not sure I can get a strong enough magnet to float in the small diameter of the tube, we'll see.
(An additional factor with the current sensing method is that even if you shut off the element before it is damaged, if any of the sugary liquid scorches on the element before it shuts off -- and it happens fast -- that imparts a burned taste to the beer and ruins the batch. So it is definitely better to not expose the element at all.)
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Funny you should mention this technology.
I was JUST looking at the stuff this small shop up in Minnesota has come up with along these lines. Not your typical cheap rain sensors but they are a pretty novel way to get around some of the problems that plague your standard Irrigation System Rain Sensor / Switch that use those problematic hygroscopic discs.
The RG-15 even looks ripe for those that can't help but get under-the-hood and do more than just use this as a switch like the RG-11 is designed to be. That RG-11 hacked into a Zigbee Contact Switch might just do.
EDIT ADD: OP might contact this company with the subject use case. If he has the need there are likely others that have the need. The adaptation of the technology they currently employ to something as simple as indicating the passage of a meniscus is likely "child's play" for them. I bet the biggest challenge would be designing the lens & optic package which could work/fit/couple on a wide array of such fluid "sight glass tubes". Who knows, they might make a prototype if there's a big enough market for this.
That is a NEAT sensor.
I tried that one, but it is not sufficiently sensitive. It "detects" liquid if it is placed at any point on the sight glass. I'm going to contact the manufacturer to see if there are any undocumented settings or tricks they can suggest.
