VL53L1 Water level measurement incorrect values

[LDMigotto]

Hello.

I’m utilizing the VL53L1 sensor to measure water level inside of a closed water bucket.

While using a 30cm tall white bucket half full, with the VL53L1 positioned at the top aiming down at the air-water interface, the sensor sometimes outputs incorrect distance values for long periods of time. During one test run, the sensor spent almost half an hour outputting distance values oscillating within a 1,5cm range around the real distance value, later stabilizing on the correct distance value. This kind of issue has happened multiple times already on similar tests.

Given the water level and surface is completely static, I was wondering whether there are any minimum range limitations, given I’m measuring distance to a liquid and also utilizing a small recipient. The only minimum distance information I was able to find on the sensor’s datasheet is its 4cm minimum guaranteed ranging system.

Thanks in advance.

[John E KVAM]

The easy way to fix this is to switch to the  VL53L4CD and try again.

So why?

The VL53L1 has a dual mode laser to get that wider field of view. And the outer area of the laser comes on just a touch after the main beam. Generally, this does not matter, but water is special.

When the photons hit the water dead on, they tend to bounce back. But if they hit as a slight angle, they will penetrate the liquid, hit the side or bottom of the bucket and reflect.  

this issue is made worse by the fact that the photons are slowed down by the water. 

The VL53L4CD avoids a lot of this issue by using a single-mode laser with an 18 degree FoV. So fewer photons hit at those wide angles.

If you are dead set on using the L1, you can reduce the Region of Interest. Specify only the 4x4 group of SPADS (Photon detectors) in the middle and you can avoid a lot of the stray photons. 

If you want to be accurate to the bottom of the bucket, I suggest that you somehow blacken the bottom. It reduces the number of photons that reflect off the bottom. 

- john