In my opinion this is virtually impossible as the light penetration will be affected by many factors other than turbidity. In particular the sun angle above the horizon would affect the path length even if you corrected for light intensity at the surface.

In my opinion this is virtually impossible as the light penetration will be affected by many factors other than turbidity. In particular the sun angle above the horizon would affect the path length even if you corrected for light intensity at the surface.

Thanks for the answer Dr. Newman, it is very useful. And what if i take a water sample and i measure the penetration of light with an artificial lamp in for example a dark box?

Using full range of light would not give accurate reading. In case of turbidity measurments, common practice is use of 750 nm (upper end of visible light) wave leangth. when you use just "light", influance of other colors in the sample will come to action.

To some extent I agree with Amila but this is probably a matter of avoiding, as much as possible, the natural colour of some samples. The old turbidity units, Jackson Turbidity Units, were actually based on obscuration of light from a candle. The best nephelometers measure light scattering at several angles to correct for colour. However if you are looking at the effects on plants on the river bed perhaps the total obscuration of light is a good thing to measure.

I assume that this question arose because of lack of resources to purchase the right instrument. If it is not then I suggest that the effort involved in trying to find a way round the problem is probably not worth the money you will save.

If you really do not have the resources I can make a few suggestions.

You could get acceptable results (depending on the samples), with a simple colorimeter to measure obscuration. Several commercially available colorimeters and portable spectrophotometers (e.g. HACH) come pre-calibrated for this application. If you could get access to a fluorimeter, you might get a good measure of the light scattered and calibrate it using commercially available standards. If the water is very turbid but not coloured you could get an acceptable measurement of relative turbidity using some of the old visual methods for optical density measurements that were used in the early 20th century or, if you incorporated a coloured filter, you could rig something up using your light box idea but this is not going to be easy and you need to put a lot of effort into checking that it is working reliably. Probably not worth the effort. Good luck

Hello

Yes, if you could correlate the intensity of passing light (Lux) and turbidity. It can be done firstly by using known concentrations.

Best Wishes

Thanks for the answers and help. It is part of the project of the lab buy a field turbidimeter soon. The original idea arise from a question of a partner. We going to a field survey tomorrow and we don´t have the device yet. Thanks again!

Thanks Newman for directing me on new insights.

In a nutshell, if you use lux meter you may face some difficulties of interpretations and in scientific communications. For example: correlation analyses up to what extent? In case the water quality change (or source change), you need many samples for correlating lux meter readings. Expressing unit also another concern (NTU or JTU). As Newman explained, it is a time consuming and need much more efforts. In case of using the lux meter observations for publications, it may need more efforts (not so easy).

(By the way sorry for typos: I comment using my phone).

Thanks Amila. You are right, that could increase too much the number of samples. The dynamic and complexity of these tropical rivers is too high. We need to use the turbidimeter for quality data. For the survey of tomorrow we are going to try with a Turbidity tube (Myre & Ryan, 2006).

I have tried using lux meter for measuring the bacterial turbidity (in afabricated box with led 1w lamp and a tube 10mm dia similat to a calorimeter.The lux units decrease proportional to the turbidity.The lux meter has to be calibrated with probably with a standard turbidimeter or spectrophotmeter .The advantage of lux meter is that it is cheap .

Hello - what brand of LUX meter have people used? I am not looking to relate LUX values to turbidity. I simply want to determine light penetration. I have used a rugged, waterproof LUX meter in the past but do not know the brand. Thanks in advance.