More about the PIFcamp http://wiki.ljudmila.org/PIFcamp_log#Nitrit_Test_with_DIY_Photometer
And loaaads of info about turbidity meters and other photespectrometers on the hackteria wiki http://hackteria.org/wiki/DIY_turbidity_meters
Inspired by this paper on Amateur Scientist's device for Nitrate analysis from
http://pubs.acs.org/doi/abs/10.1021/ed072p57