Turbidity sensors: Smart phone version https://docs.google.com/drawings/d/1Z8tmVe19cu3z0X4PzvXQA7228orwcwIWU8alSdFUt20/edit This version would need some sort of a container to hold the water sample. A clear container with a backing to reflect the flash from the camera. Maybe a pattern is printed on the backing to help the camera focus and determine the clarity of the water. Take one photo with the empty container to calibrate the flash power, then another with the water. Exposure will have to be fixed for both photos so that the light doesnt change. Simple version https://docs.google.com/drawings/d/182cRy51lv6cNB0RsvtAA62wYaFOjK9BNsgbsKE57bt0/edit This version will use a light meter and a LED. Simple PVC construction. Water is collected in one part of the tube with a LED at one end and the light sensor set at 90 degrees.