NTU stands for Nephelometric Turbidity Unit and signifies that the instrument is measuring scattered light from the sample at a 90-degree angle from the incident light.
FNU stands for Formazin Nephelometric Units and also signifies that the instrument is measuring scattered light from the sample at a 90-degree angle from the incident light. FNU is most often used when referencing the ISO 7027 (European) turbidity method.
NTU is most often used when referencing the USEPA Method 180.1 or Standard Methods For the Examination of Water and Wastewater.
When formazin was initially adopted as the primary reference standard for turbidity, units of FTU or Formazin Turbidity Units were used. These units, however, do not specify how the instrument measures the sample.
FAU or Formazin Attenuation Units signify that the instrument is measuring the decrease in transmitted light through the sample at an angle of 180 degrees to the incident light. This type of measurement is often made in a spectrophotometer or colorimeter and is not considered a valid turbidity measurement by most regulatory agencies.
A JTU or Jackson Turbidity Unit is a historical unit used when measurements were made visually using a Jackson Candle Turbidimeter. Water was poured into a tube until a flame underneath the tube could no longer be distinguished.
The turbidity units NTU, FNU, FTU, AND FAU are all based on calibrations using the same formazin primary standards. Therefore when a formazin standard is measured, the value for each of these units will be the same, however the value on samples may differ significantly.