|Sample cell variations||High or low results. The impact of this issue is most severe when measuring low-level turbidity.|
|Stray light||High results|
|Contamination||High results. Results from the build up of particles/scale within the instrument, or from microbiological fouling.|
|Instrument optical aspects||High or low results. Degradation of the optical components within the instrument and the effect of calibration|
|Absorbing coloured particles||Low results|
|Sample Colour||Low results (if using a wavelength in the visible region)|
|Particle Size||High or low results (wavelength dependent) Large particles scatter longer wavelengths of light more readily than small particles Small particles scatter shorter wavelengths more efficiently than longer wavelengths|
|Particle settling||High or low results. Tends to be an issue with portable instrumentation rather than online.|
|Particle Density||Low results|
A new way of measuring turbidity has been developed, the PTV1000 from Lovibond, which incorporates the expertise of a team with over 100 years of experience in turbidity measurement.
Instrument optical aspects
Turbidity calibration is all based around the response of the instrument to formazin. Formazin was specially developed for the calibration of turbidity instruments and is a polymer which has relatively consistent light scattering properties. It and is the ONLY real primary calibration standard available, every other standard you can get for a turbidity instrument is a secondary standard which relates back to formazin and should only really be used for verification. Unfortunately, formazin has two major drawbacks in that it it’s not very nice to handle and secondly, it isn’t terribly stable in dilute solution. Given that drinking water works are working below 1NTU then the solutions you would use to calibrate would have to be made up fresh before use.
Sample colour/coloured absorbing particles