Need water surface proximity detection

Hello friends,

I need some guidance to figure out how can I detect proximity of water surface? Basically what i wanted do is to implement tiny unit which will going to fix on my water tank such that whenever water surface is in the proximity of unit (1-2 cm away from it), unit will start giving notification via buzzer. I do not want to use any electrode method since I wanted to keep this unit as handheld unit, so that i can install it anywhere within few second. Also tank will contain pure water and the accuracy level of unit should be more that 90%.

Can you give me some guidance ?

Thanks in advance

This may help you.
https://www.instructables.com/id/Automatic-Water-level-controller/

The best way to make the sensor with zero battery current is a float switch. Simple and rugged.

I don't understand the reservations against sensor electrodes, may be a matter of taste.

A capacitive proximity switch isn't bad.

Ultrasonic has been suggested, you'll want a hermetic transducer to make it survive possible flooding.

Is this intended to be an external non-contact low water sensor for a glass-lined metal tank ?

If not then what, where , how tiny and must have restrictions.

After having dealt with a well system for the past thirty years, I'm inclined to agree with FvM, a float switch is certainly, "Simple and Rugged," as I have four large tanks each equipped with a float switch and had only one failure during the thirty year duration.

A simple relatively short metered reel of low current waterproof rated cable with a float switch at the end is probably the most cost effective and reliable technique, by the way, I and many others have designed and constructed similar instruments with either float or electrode contact switch for deep well measurements, in fact the U.S. Geological Survey (USGS) use this technique to routinely measure water well depth across this country. These instruments typically employ a audible (buzzer) and/or visual (LED) indicator once the sensor has contacted the water level, at which point the operator simply reads the depth from the cable meter marks.

Yes, "Simple and Rugged."

BigDog