What kind of sensors are suitable for measuring liquid level accurately? minimum level 2 or 3 millimeter. Ultra Sound sensors are not such precise.
*There are various types of liquids with different densities.
Electronic – Precise liquid level measurement sensor
sensor
Related Solutions
They probably use strain gauges - see this wiki page.
There are thousands if not millions of variants and they work by altering their electrical resistance when subjected to mechanical force.
You are probably looking for Reflective Object Sensors
You often see a red filter glass on the automatic tap, indicating there is an IR reflective sensor behind it, containing from an IR LED and an IR photodiode. The LED and photodiode look in the same direction, so in normal circumstances the phototransistor won't receive any of the LED's light.
When however you wave your hand in front of the combination, the IR light will reflect from your skin onto the phototransistor and the tap will activate.
The trick is that the LED is flashing, and the receiving photodiode is trying to synchronously detect these flashes. If it detects IR light that is not flashing in sync with the LED, it knows that the source of the light is coming from somewhere else and thus the tap must not be opened.
I suspect the electronics in a tap is powered by a small 'generator' in the water line, that will spin from the water flowing through the tap. I have never seen a battery compartment or power adapter, but then again I am not responsible for these facilities.
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Best Answer
I'm going to assume the liquid in question has a dielectric constant that is very much greater than the dielectric constant of air. This is the case for many liquids, including all liquids whose major component is water. Many "medical liquids" fall in this category, as people are mostly water too.
I am am going to recommend a capacitive measuring system based on the concept described Scott Edwards' article "Measure Water Level Without Getting Wet" (Stamp Applications no 27, 1997):
Basically, you put the liquid in a small narrow tube, either as a container or through communicating vessels from a larger reservoir. You apply conductive tape to both sides of the tube. You now have made a capacitor with the liquid as dielectric. Because the dielectric constant of the liquid is much higher than the dielectric constant of air, the capacitance of the system increases with increasing water level. Because of the finite dielectric constant of air, the relation is linear but not strictly speaking proportional.
The good thing about this system is that it's relatively accurate, and the electronics never get anywhere near the liquid.
For your specific case, you are measuring relatively small quantities. If it's acceptable in your application, I would get rid of the reservoir altogether and replace it with a single tube about \$10 cm^2\$ in cross section and one meter long. This should enable you to store 1 liter of fluid while giving you plenty of swing in your capacitor value.
For actually measuring the capacitance, there are many solutions. Here's a few:
You can find more circuits to perform the capacitance measurement in the comments of this blog post I made on the subject of capacitive measurement.