YSI ADV6600 - Manual (Page 125)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 115
and the quality of the water. For all monitoring studies using ion selective electrodes, the user
should acquire a few “grab samples” during the course of the deployment for analysis in the
laboratory by chemical means or with another chloride sensor which has been recently calibrated.
Remember that the typical accuracy specification for the sensor (+/- 15 % of the reading or 5 mg/L,
whichever is larger) refers to sampling applications where only minimal time has elapsed between
calibration and field use.
9-15. Turbidity
Procedure from Standard Methods for the Examination of Water and Wastewater: 2130 B.
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 the ADV6600 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).
Two turbidity probes are available for use with the ADV6600. The probes are detailed briefly
below; both are equipped with a mechanical wiper to periodically clean the sensor. These wiper
systems make the probe ideal for long term monitoring, but they also work well for spot sampling
applications.
• The Model 6026 probe has been offered by YSI since 1995 and is characterized by relatively
small optics, a factor that results in minimal penetration of the light beam into the sample and
thus allows the use of shorter probe guards on the sonde in which it is installed. If a 6026
sensor is selected in the Sensor menu of the ADV6600, the turbidity will automatically be
reported in the units “turbid NTU”.
• The Model 6136 is a new (2002) turbidity probe from YSI that is characterized by relatively
large optics. Since these larger optics result in deeper penetration of the light beam into the
sample, the use of a longer probe guard is required, a slight disadvantage. However, the larger
optical cell volume of the sensor has two significant advantages:
(1) There is less background noise associated with the turbidity readings