I have a barrel of clear liquid which has sediment build up at the bottom of a barrel. Over time I want to measure the build up of this sediment. Nothing can touch the liquid and I can't place anything in the barrel. The measurement must be taken from the top of the barrel. Also the liquid in the barrel is not still. There are ripples and bubbles within the liquid. The barrel is quite small at around 24-36 inches max.

I have been trying to find a way of measuring distance through water but everything I have found uses infrared to measure distance. Why are they all infrared? Obviously infrared will bound off the top of the liquid. The only thing I can see of getting through is a visible light laser such as 650nm red laser.

Is there anyway of doing this fairly reliably. It doesn't need to be doown to the nearest mm. Nearest cm I think would be enough.

  • \$\begingroup\$ Dolphins use ultrasound for this purpose... \$\endgroup\$
    – Eugene Sh.
    Feb 22, 2017 at 23:27
  • 1
    \$\begingroup\$ You didn't mention your budget, so how about a emerson.com/catalog/en-us/… \$\endgroup\$
    – Tyler
    Feb 22, 2017 at 23:27
  • \$\begingroup\$ @Tyler I doubt that the radar will penetrate far enough into the liquid to reach the sediment. \$\endgroup\$ Feb 22, 2017 at 23:34
  • \$\begingroup\$ The resonant frequency if your barrel might change with sediment buildup, so you might try to generate a sweep with a piezoelectric element and measure the response. It will be impossible to derive the sediment height directly with a formula, because a longitudinal standing wave between sediment and surface will not be the only excitation mode. With a series of measurements, you would be able to find the relation between resonance and sediment thickness. \$\endgroup\$
    – Andreas
    Feb 23, 2017 at 0:26
  • \$\begingroup\$ The "dambusters", British bombers of WW2, used searchlights on bottom of wings, aimed to intercept below the plane, if it flew at correct height above the lake behind the dam they were visiting that night. \$\endgroup\$ Feb 23, 2017 at 3:10

1 Answer 1


What is the nature of the sediment? If it has a big density difference from the liquid ultrasounds should work pretty well. If it is too similar to the liquid phase, you won't get much reflection. Laser range finding should also work with any wavelength for which the liquid is transparent. IR scatters less from fine particles, so if your liquid is slightly cloudy that might be better than visible, but you say the liquid is "clear", so I assume you can see the bottom with your eyes, in which case a red laser should be just fine.

Both ultrasound and lasers will have a large reflection from the top surface. You will need to exclude that signal. For ultrasound, the easiest way to do this is by time gating -- just ignore any signal at the receiver until after the initial reflection is detected. You will have to watch out for multiple bounces as well, but with a little trial-and-error this should work OK. This means you may not be able to use a plug-and-play module, since they usually sample only the first reflection.

One potential problem with ultrasound is that the speed of sound is temperature dependent. You may have to temperature correct your readings to get good enough accuracy.

With laser range finding, excluding the first reflection is potentially easier: just mount the sensor at a slight angle so that the top surface reflection misses the receiver, but this depends on the surface being flat enough for specular reflection. If you shine a laser pointer at the liquid and you see a lot of scatter from the liquid surface, you are going to have a problem. If you primarily see the spot on the sediment, an off-the-shelf laser range finder might work fine. In principle you can do time gating on a laser system too, but it is a lot more difficult since the round trim time is so small (a few ns).

One other approach to consider is to use a camera and do this with software image processing. If you can point a camera at the bottom corner of the barrel, you can just watch the sediment accumulate. This trades hardware simplicity for a lot more software work and calibration. If you could paint indicators on the inside wall of the barrel that would make this a lot easier, but I am assuming you can't do that. The problem here is that without reference marks, you are going to have a hard time compensating for the refraction of the liquid.

  • \$\begingroup\$ Thank you very much for your reply. I hadn't thought about ultrasound actually working through water by excluding the initial reflection. That's something I will definitely look into. I figured using a laser would be difficult due to the nature of the liquid moving. I will also need to look at the potential for marking the inside of the barrel. That might not be impossible. :-) Just one more thing, when you say an off the shelf ultrasound won't work, what would I need to look out for to get something working. Thanks!! \$\endgroup\$ Feb 23, 2017 at 8:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.