Power Loss in Light Intensity in Water

I'll be making a comment without researching first. I'll just be making a guess as to how difficult it would be. I'd like to make a guess as to how it can be accomplished but it will have to wait until I think it through.

First off, flowing water has turbulence and areas of fast flow and slow flow. In our old science classes, we use a plexiglass tank with a light above and we can see the waves and turbulence on the white surface under the tank.

If you shine a laser through the tank, the turbulence will cause the light to refract or bend or be diminished unevenly. This will happen while the beam is on the way down and again after it is reflected back up. Your detector will interpret this as noise and you won't get a reliable reading.

Then there's the problem of uneven velocities in flow through a pipe or tank. The walls of the tank tends to slow down the water. What happens is that the flow in the middle of the tank is faster than the flow that is close to the walls. This can be compensated for however.

There is a way to measure flow by measuring height. You can use the laser to measure the height of a weir's overflow. This would be a workable idea for your laser flowmeter.

Split the beam into three parts. Direct one beam into a photodetector to track the intensity of the laser itself, as it's light intensity will vary over time, both in the short term and in the long term as the laser ages.

Direct the second beam into a known depth of the same bolus of water whose depth you want to measure. Measure the intensity of the the light reflected from the bottom (presumably you're using some kind of mirror) using the same kind of photodetector. This is your reference depth, which will be affected by the opacity of the water. But as the depth is known and constant, it is the opacity of the water you're measuring.

Direct the third beam into the unknown depth of water and measure the intensity of the reflected beam, again using the same kind of photodetector as the first two. The intensity is again a function of the opacity which, in turn, is a linear function of the depth. This assumes, of course, that the water's opacity is the same throughout.

Normalize the intensity of the second and third measurements against the first to eliminate variations in the laser itself.

If the water is very, very clear, you're screwed.

Think about using ultrasound to measure the water depth, since the speed of ultrasound in water is pretty much independent of the opacity/turbidity of the water. For precise measurements (you didn't say) you will need to measure the water temperature and compensate for variations of the speed of sound in water due to temperature variations. Overall, I'd say using ultrasound in this application would be quite a bit cheaper, not to mention being much simpler. You might google for an off-the-shelf solution.

Is there a particular reason why you're using a laser for this application?

-e

We manufacture photon detectors which are used for laser range finding. The usual method employed is to use a very fast gating detector, typically they will be in the 'on' state for about 10ns, a laser pulse is fired at the object being measured & the detector rapidly cycles on and off and captures the reflection. The time difference between the two allows you to calculate the distance. A similar method is used to measure flow. Try http://www.ghcc.msfc.nasa.gov/sparcle/sparcle_tutorial.html for more information.

Water severely absorbs any transversal wave and light is one of them. At depths of 60 meters or so, sunlight is completely absorbed. So light is a very bad choice to do your measurements.

As Europium suggested, choose sound. For depth measurement you may use something similar to the ping-sonar and for flow (in duct) a Doppler one. Or just measure the depth and time and deduct flow.

You didn't say whether you were designing a method or could use an off the shelf design. You can measure water depth with a pressure gage. Water flow is normally measured mechanically with turbine or paddle wheel flow meters, but there are electronic ways too, such as magnetic or venturi tubes. see omega.com

Before you go any further are you SURE the turbibity of the water is constant??????