TM 5-6630-215-12
h.
Turbidity, 0-500 FTU Range (fig. 2-11 and 2-12 para 2-18h).
(1) Turbidity occurs in most surface waters as the result of
suspended clay, silt, finely divided organic and inorganic matter,
plankton and other micro-organisms.
Turbidity measurement of water is
important to those industries where the product is destined for human
consumption,
such as the food and beverage industries, and municipal
water treatment plants.
(a) The turbidity test measures an optical property of the
water sample which results from the scattering and absorbing of light
by the particulate matter present.
The amount of turbidity registered
is dependent on such variables as the size, shape, and refractive
indices of the particles.
No direct relationship exists between the
turbidity of a water sample and the weight concentration of the
matter present, as is determined in the suspended solids test.
(b) Turbidity calibrations were originally based on the Jack-
son candle turbidimeter with results expressed in Jackson Turbidity
Units (JTU).
As the Jackson equipment lacks sensitivity below 25 JTU
(most treated water ranges from O to 5 JTU, the meter scale calibr-
ations have been based on a uniform milky polymer called formazin,
which allows accurate calibrations over a wide range.
The results are
expressed as Formazin Turbidity Units (FTU) and are equivalent to the
Jackson units.
(2) Turbidity Test Procedure.
NOTE
Filtering (para 2-22) is recommended for highly
colored samples.
The filtered water is then used
in place of the clear, colorless water called for
in step (b).
(a) Take a water sample by filling a clean sample cell to the
25 ml mark.
(b) Fill another sample cell to the 25 ml mark with clear,
colorless water and place it into the cell holder.
Insert the Turbid-
ity (Absorptometric Method) Meter Scale into the meter and adjust the
wavelength dial to 450 nm.
Adjust the light control for a meter
reading of of zero FTU.
((c)
Replace the blank sample
(step (c)) with the sample
(step (a) in the cell holder and read the formazin turbidity
units (FTU).
2-51
