Water level sensing with some challenges

[Zandr]

I'm working on a project that requires liquid level sensing with a couple of complications I'm trying to address. The application is measuring levels in a vessel that is about 1.4m tall, so the ranges of interest are about 100mm to 1.5m. The target liquid is water-based, though not necessarily colorless. I've been testing with clear water as that's a worst case.

The challenges relate to packaging. I need to install the sensor in a fitting attached to the lid of the vessel. For various reason, this means I'm in the end of about a 40mm tube, with a ~20 degree field of view. The other problem is that the fitting is at about a 6 degree angle to the surface of the water. I've been testing with a VL53L1X (set to a central 4x4 ROI) and a VL53L4CX, and things are tantalizingly close to working, but not very reliable. This of course works flawlessly if I have the sensor perpendicular to the surface, even at the end of this tube.

Any tips on sensor selection or configuration/calibration/technique would be greatly appreciated. I know the angle is problematic, but the packaging constraints have put us here.

Thanks,
-Zandr

[John E KVAM]

You have the right approach. The VL53L4 has an 18-degree field of view. So, if the diameter of your tube is 1/3 of it is length, it should work. 

but in practice, the laser spreads a bit farther than the 18 degrees, so if there is an obstruction nearby, the sensor can see the obstruction. 

Can you polish the down-tube? A mirror finish on the tube will assure no photons reflect back without hitting the water. 

The L1X and its 4x4 ROI is a good try as the effective FoV will be narrow. But you might need more SPADS than that to see 1.4 meters. The 27-degree laser output also has issues. The polished down tube would help here as well.

The ROI narrows the area from where photons reflect, but the light still goes out at 27 degrees. 

But by far your biggest issue is that "6-degree angle to the surface of the water". The physics of light hitting water means that the light that is perpendicular to the water is what reflects most. And it is those photons that have travelled the shortest. All the others penetrate, bounce and return. They screw your measurements. I think your only try here is to polish that downtube so as much light as possible will hit the perpendicular surface fo the water and reflect.

This is a tough problem. 

Good luck,

- john

 

[Zandr]

Yeah, I figured I was up against the laws of physics here.

I'm actually working with 3D printed mockups of the fitting right now, and crosstalk calibration seems to do an OK job of keeping the sensor from telling me about the fitting. I hadn't thought about polishing the down-tube, my instincts were to create a matte black 'snoot' instead.

So, if I'm understanding your suggestion, we're sort-of creating a fat multi-mode fiber from the sensor to the end of the down-tube. That's really interesting, and I'll try mocking something up.

Thanks for the suggestions!

-Zandr