The article describes how to detect objects below the water surface using fiber optics with teflon sleeve.

Dear all,

we received the following application description:
customer tries to detect objects in a bucket below water with a through beam fiber optic sensor.
How does that work and which fiber is suitable?

The use of a standard fiber with plastic or stainless steel sheet is no good idea, because in general our fibers aren't gas- and waterproof, therefore the water will go between the fiber core and the sheet. After that the direct reflection inside the fiber is destroyed and the fiber won't work anymore. 

The only possibility is the use of a fiber with teflon (Polytetrafluorethylen=PTFE) coating over the complete length of the fiber that's in contact with the water:

5308066 LL3-TY01         https://www.sick.com/de/de/5308066
5308067 LL3-TY02         https://www.sick.com/de/de/5308067
5325982 LL3-TY03         https://www.sick.com/de/de/5325982
5336504 LL3-TY036000 https://www.sick.com/de/de/5336504
5325981 LL3-TY04         https://www.sick.com/de/de/5325981

I made a quick test how clear water influences the actual value of the fiber amplifier.

setup: WLL180T-F434 and LL3-TY01 (sender receiver distance 200mm) in a SICK box

• empty box actual value 398digit at responsetime HiGh (at all other response times the receiver where in saturation 9999digits)

• box with water actual value 756digit 

• glass object between sender and receiver 443digit

• cup between sender and receiver zero digits
  

This quick test showed how important it is to check the detection intensively in the real application environment.
Because the actual value with water is higher than without, this has to be considered if the water isn't always in the bucket.
Then a partly transparent object can lead to the same actual value as missing water.

If
-the water is always present, 
-or the detection is only necessary if the water is there, 
-or the detection of objects that blocks the light completely,
-and the thresholds of the fiber amplifier are set accordingly
the detection should work well.