Hello @kurtm  and welcome to the ST community,

AN5851 "Water and liquid level monitoring using VL53L4CD Time-of-Flight high accuracy proximity sensor"

For other sensor part numbers:

AN5843 "Water and liquid level measurement using VL53L5CX Time-of-Flight 8x8 multizone sensor with wide field of view"

Liquid Level Monitoring using ST Time-of-Flight technology

Liquid Level Monitoring using ST’s Time-of-Flight: VL53L5CX

I have checked the suggested documents but not found what I am looking for. The problem I see is that you will get some reflected light from the pipe walls that will increase with distance. What I would like to know is a little more what I can expect. In my case, pipe is plastic type and water is waste water which can vary from none to almost clean to water with foam and movements. The important thing is to detect if water level is rising above 200 - 300 mm from bottom. 

Hi:

Thank you for the additional application details. Based on your geometry and the principles AN5851 described in for liquid level sensing.

A VL53L4CD can be evaluated for this application, but the main challenge is indeed pipe wall reflections, especially in a narrow pipe with a long top-down measurement path.

In the chapter 2.1 of AN5851 , the ToF sensor FoV is conical, so the container geometry has a direct impact on the measurement. FoV is approximately 18°, So the beam expands with distance. The footprint diameter can be roughly estimated as D≈2h*tan (9°​​).

If your decision point is about 200~300mm above the bottom, with a total pipe height of about 800mm, then the sensor-to-surface distance is roughly 500~600mm. we can get the diameter D is 158.5~190.2mm. 

At that distance, the beam footprint becomes much larger than a 50~75mm pipe diameter, so part of the light will inevitably hit the pipe walls. Therefore, the optical situation is intrinsically one of *mixed returns*, not direct clean liquid-surface ranging.

your requirement is only to detect whether the level has crossed a fixed threshold, for example around 200~300mm from the bottom, so this application is better positioned as threshold detection rather than continuous level gauge.

Based on AN5851 and your use case, you can follow the setup of chapter4:  

1. Validate the setup using the real pipe, real wastewater, and real flow conditions；

2. choosing one clear threshold first, for example a liquid level at 250mm from the bottom, this corresponds to a theoretical sensor-to-surface distance of about 550mm.

• testing at least the following conditions:

- Empty pipe

- Clean water, static

- Clean water, moving

- Wastewater, static

- Wastewater, moving

- Foam condition

• testing around the threshold region, for example at:

- 100mm

- 200mm

- 250mm

- 300mm

- 400mm

from the bottom. This covers below-threshold, near-threshold, and above-threshold cases.

For each condition and each level point, collecting 100 samples. 

• Recommended data to record:

- Raw distance values

- Valid / invalid status

- Current condition and level point

• After testing, the main points to check by using commonly used mathematical methods to make judgments:

- Whether the outputs below and above the threshold are separable

- Whether the empty pipe causes false detection

- Whether foam and motion create significant instability

3. If the results show that the two sides of the threshold can be distinguished, the next step would be to optimize the solution with filtering, hysteresis, and simple decision logic. or if the outputs overlap too much across different conditions, then the structure or sensing approach may need to be reconsidered.

Hope this will help for you.

Best Regards,

Bin FAN