The units used in turbidity measurements can be confusing…
Here is a guide:
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 in accordance with USEPA Method 180.1 (USA).
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 but in accordance with the European standard ISO 7027.
FTU stands for “Formazin Turbidity Units” and is the generic term that does not specify how the instrument measures the sample.
So what to make of this? Well, the good news is you don’t have to remember them all, because:
1 NTU = 1 FNU = 1 FTU = 1 ppm formazine
Occasionally, you might encounter other terms that relate to the angle of measurement:
FAU stands for “Formazin Attenuation Unit”. It signifies that the instrument is measuring the decrease in transmitted light through the sample at an angle of 0 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.
FBU stand for “Formazine Backscatter Unit”. It signifies that the instrument is measuring the decrease in backscattered light through the sample at an angle of 180 degrees to the incident light.
Happy analyzing 🙂
Kemtrak, the leader in LED-based photometry.