YSI 6000UPG - Manual (Page 114)

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6.10 TURBIDITY
Turbidity is the measurement of the content of suspended solids (cloudiness) in water and is
typically determined by shining a light beam into the sample solution and then measuring the
light that is scattered off of the particles which are present. For turbidity systems capable of field
deployment, the usual light source is a light emitting diode (LED) which produces radiation in
the near infrared region of the spectrum. The detector is usually a photodiode of high sensitivity.
The angle between the emitted and detected light varies (usually between 90 and 180 degrees)
depending on the probe used. The International Standards Organization (ISO) recommends the
use of a light source with a wavelength between 830 and 890 nm and an angle of 90 degrees
between the emitted and detected radiation ( ISO 7027).
The turbidity system available as an option for use with the 6000 UPG consists of a probe which
conforms to the above ISO recommendations and an interface printed circuit board which must
be factory installed within the 6000
UPG sonde. The output of 6000 UPG turbidity sensor is
processed via the sonde software to provide readings in nephelometric turbidity units (NTUs).
If the option is installed, the 6000 UPG can easily measure turbidity in short term sampling
applications, but it has also been designed with long term deployments in mind. Optical
measurements are particularly susceptible to fouling, not only from long term build up of
biological debris, but also to shorter term formation of bubbles from outgassing of the
environmental water. For this reason, the 6000
UPG turbidity probe is equipped with a mechanical
wiper which rotates on the probe face, keeping the optics as free as possible from bubbles and
fouling build-up. The wiper can be activated in real-time during discrete sampling and
calibration operations or will function automatically during long term unattended sampling
studies. The number of wiper movements for the unattended mode can be set up in the sonde
software. Generally one rotation is sufficient, but in media with particularly heavy fouling,
additional cleaning will be required even though battery life will be reduced.
CALIBRATION AND EFFECT OF TEMPERATURE
The 6000 UPG sonde software offers the option of 1 point, 2 point, or 3 point calibrations
procedures. For most applications, a 2 point calibration at 0 and 100 NTU is sufficient.
However, because the calibration curve of the system exhibits a small nonlinearity below 10
NTU, a 3 point calibration at values of 0, 10, and 100 NTU can be carried out to provide
maximum accuracy over the normally encountered environmental turbidity range (0-1000 NTU).
Although a 1000 NTU calibration would be ideal, the sensor is extremely linear between 100 and
1000 NTU and, because of the cost of the reagents, the benefit of use of the higher standard is
not significant. If the range of turbidity in the environmental sample is well known, standards of
other turbidity values can be utilized (in either 3 point or 2 point routines). However, one of the
standards must be 0 NTU. Note that, for all calibration procedures, it is very important that the
standards used in the calibration should be based on either formazin or AMCO-AEPA styrene
divinylbenzene beads as described in Standard Methods for the Examination of Water and
Wastewater. Use of standards prepared from other materials made result in spurious and inaccurate