Re: What does a NTU (Nephelometric turbidity unit ) equal in other terms?

Hi Dolph:

According to this dictionary of units of measurement, an NTU is:

   abbreviation for nephelometric turbidity unit, a unit
   used in measuring water quality. Turbidity is an optical property:
   the scattering and absorption of light by solids suspended in
   water. In other words, water is turbid if you can't see through
   it. An instrument called a nephelometer (from a Greek word meaning
   "cloudy") measures turbidity directly by comparing the amount of
   light transmitted straight through a water sample with the amount
   scattered at an angle of 90 degrees; to one side; the ratio
   determines the turbidity in NTU's. The instrument is calibrated
   using samples of a standard solution such as formazin, a synthetic
   polymer. Drinking water should not have a turbidity above 1 NTU,
   although values up to 5 NTU are usually considered safe. Outside
   the U.S., this unit is usually called the FNU
   (formazin nephelometric unit).

So, NTUs are a ratio of two measurements of absorbance. Absorbance is a measurement of how much light can get through a substance, and is often given as a proportion, hence unitless (e.g. an absorbance of 50% means that only 50% of the light shined one one side of a solution makes it through to the other side).
Furthermore, whenever you take the ratio of two things with the same units, the ratio is unitless (the units drop out when you divide them).
So, to bring that all together, NTUs are a ratio of two unitless measurements. NTUs are therefore unitless as well.

Hope that helps!
Rob Campbell, MAD Scientist