YSI 600DW-B Sonde - Manual (Page 227)

Principles of Operation Section 5
(3) Ion selective electrodes generally drift more than pH sensors. Be sure to check for this drift by placing
the sonde in a standard at the end of each deployment.
(4) Remember that the chloride sensor will take longer to stabilize after exposure to pH buffers. To
accelerate this process, soak the sensor in 1000 mg/L standard for a few minutes after performing a pH
calibration. In addition, be particularly careful that readings are stable during chloride calibration after
exposure to buffers.
5.11 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 YSI sondes consists of a probe which conforms
to the above ISO recommendations. The output of the sonde turbidity sensor is processed via the sonde
software to provide readings in nephelometric turbidity units (NTUs).
The 6136 turbidity probe is available for use with YSI sondes. The probe is equipped with a mechanical
wiper to periodically clean the sensor either by manual or automatic activation. These wiper systems make
the probe ideal for long term monitoring,
While drinking water is very clean relative to surface water, it is important to remember that optical
measurements drinking water can still be susceptible to fouling, not only from long term build up of
impurities or those flushed from older distribution pipes, but also to shorter term formation of bubbles from
outgassing of the water. These bubbles can generally be removed in short term sampling application by
simply agitating the sonde manually. However, for studies longer than a few hours where the user is not
present at the site, the quality of the turbidity data obtained with a turbidity sensor that has no capability of
mechanical cleaning is likely to be poor. However, as noted above, both the 6136 probe is equipped with a
mechanical wiper that makes them ideal for unattended applications. The wiper can be activated in real-
time during discrete sampling operations or will function automatically during long term unattended
sampling studies. The number of wiper movements and the frequency of the cleaning cycle for the
unattended mode can be set in the sonde software. Generally one movement is sufficient for most
environmental applications, but in media with particularly heavy fouling, additional cleaning cycles may be
necessary.
CALIBRATION AND EFFECT OF TEMPERATURE
The 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 approximately 100 NTU is sufficient for the 6136 sensor to meet
the quoted accuracy specification of +/- 2% of the reading or 0.3 NTU, whichever is greater. (Note that the
specification is relative to measurements in calibration standards.) However, a user might wish to carry
out a 3-point calibration at values of approximately 0, 100 and 1000 NTU to provide maximum accuracy
over the entire normally encountered environmental turbidity range (0-1000 NTU). 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, in all calibration procedures, one of the standards must be 0
NTU and this should be the first calibration point.
YSI Incorporated Drinking Water Monitoring Systems Operation Manual 5-14