Hanna Instruments HI 96822 - Manual (Page 4)

4
Salinity determinations are made by measuring the refractive index of seawater. Refractive Index is
an optical characteristic of a substance and the number of dissolved particles in it. Refractive Index
is defined as the ratio of the speed of light in empty space to the speed of light in the substance. A
result of this property is that light will “bend”, or change direction, when it travels through a
substance of different refractive index. This is called refraction.
When passing from a material with a higher to lower refractive index, there is a critical angle at
which an incoming beam of light can no longer refract, but will instead be reflected off the interface.
The critical angle can be used to easily calculate the refractive index according to the equation:
sin (θ
critical
) = n
2
/ n
1
Where n
2
is the refractive index of the lower-density medium; n
1
is the refractive index of the
higher-density medium.
In the HI 96822 refractometer, light from an LED passes through a prism in contact with the sample.
An image sensor determines the critical angle at which the light is no longer refracted through the
sample.
Specialized algorithms then apply temperature compensation to the measurement and convert the
refractive index to: PSU, PPT or S.G. (20/20).
PSU is defined as a definitive salinity-conductivity ratio relationship. It is based upon the work of
the UNESCO, ICES, SCOR and IAPSO. This information is published in The Joint Panal of Oceanographic
Tables and Standards. PPT (10
-3 ) is an older used salinity scale where salinity is defined by “the
salt content is the weight of the inorganic salts contained in 1 kg of seawater if all bromide and
iodide are replaced by an equivalent amount of oxides” (Knudsen, 1901).
PRINCIPLE OF OPERATION