OBJECTIVE: A device that floats on the water's surface and measures the depth. If any changes occur in the depth, it sends a report to a remote receiver.

MY RESEARCH: I have considered the following sensors but they don't seem to be effective:

  1. IR won't work because water absorbs red light.
  2. Ultrasonic: I can't find one that works underwater.
  3. I also read a thread: Underwater distance measurement/Sensing however, I don't know if I can implement that.


What sensor can I use for this purpose?

  • 1
    \$\begingroup\$ Fish finders also make good rock finders for boat hulls \$\endgroup\$ Apr 13, 2021 at 13:21
  • \$\begingroup\$ Only ultrasonic can work in water. Check fish finder, it is same principals. But you may consider stationary positioned ultrasonic sensor, like HC-04, measure distance to water surface. It is easy and cheap. \$\endgroup\$
    – user263983
    Apr 13, 2021 at 13:22
  • \$\begingroup\$ Searching for "ultrasonic depth sensor" brings up many results. In addition to that, ultrasonic sensors like those used in boats (either for fish finding or as a simple depth gauge) would also work. Look for AirMar transducers for example. \$\endgroup\$
    – StarCat
    Apr 13, 2021 at 13:25
  • \$\begingroup\$ Water absorption of red light is negligible over the depth of a pool, so you could use optical sensors. 850nm and below are good choices. \$\endgroup\$ Apr 13, 2021 at 14:06
  • 3
    \$\begingroup\$ Ultrasonic: I can't find one that works underwater. - virtually every submarine and boat use sonar so you can't have researched this very well. \$\endgroup\$
    – Andy aka
    Apr 13, 2021 at 15:24

2 Answers 2


A pretty standard fluid level sensor (optical, capacitive, ultrasonic, IR) will mount to a fixed point and look down at the fluid.

If you must have the sensor float, consider measuring distance to some fixed point above the water (since your floating platform will be on the water, it is at water level).

Laser rangefinders should work, although depending on the range and precision you need the result may end up large. You can also make up for it by slower sampling and a proper filter.

The real simple solution however would be to drop an anchor. If the tension slacks or angle of the anchor line goes beyond some point, your level has dropped. If you used a torsion spring to maintain tension you could even count it as how much this spring has turned.

  • \$\begingroup\$ @user263983 Apparently it depends a lot on the laser, especially (but not only) the wavelength: blog.rpmclasers.com/bathymetry \$\endgroup\$
    – jcaron
    Apr 13, 2021 at 16:01
  • \$\begingroup\$ It must float, that is the main obstacle. I'm looking into laser rangefinders, as many people have suggested it. \$\endgroup\$
    – AB4595
    Apr 13, 2021 at 17:11
  • \$\begingroup\$ @AB4595 Is it safe to sine a laser down into a pool people will be swimming in? Water reflects, scatters and lenses \$\endgroup\$
    – DKNguyen
    Apr 15, 2021 at 19:20
  • \$\begingroup\$ @DKNguyen It will be used in empty pools only. \$\endgroup\$
    – AB4595
    Apr 15, 2021 at 22:18

Does it have to float? A pressure sensor on the bottom would do the job and be simpler and more reliable, even if you need to run a tether to a surface antenna, whether that antenna is fixed or floats on a buoy.

  • \$\begingroup\$ Yes, it needs to float. \$\endgroup\$
    – AB4595
    Apr 15, 2021 at 22:19

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