YSI ADV6600 - Manual (Page 126)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 116
(2) The absolute values of the readings are significantly closer to those from
typical laboratory turbidimeters (such as the Hach 2100AN) which use a large
cell volume.
If a 6136 sensor is selected in the Sensor menu of the ADV6600, the turbidity will
automatically be reported in the units “turbid+ NTU”.
In environmental water that contains suspended particles of varying size and density, the readings
for the two probes will differ with the 6026 almost always showing higher readings and the extent of
the difference generally being proportional to the overall turbidity. For example, if both probes are
calibrated and then placed in a turbid river, the 6026 might read 400 NTU while the 6136 would
read 300 NTU. The absolute difference in the sensor readings would shrink as the turbidity
dropped. Thus, if the 6026 read 30 NTU, the 6136 would likely read approximately 21 NTU.
9-15.1. Effect of Fouling
It is important to remember that field optical measurements are particularly susceptible to fouling.
Fouling may be caused not only from long term build up of biological and chemical debris, but also
by short term formation of bubbles from outgassing of the environmental water. These bubbles can
generally be removed in short term sampling application by simply agitating the sonde manually.
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. All YSI turbidity probes for the ADV6600 are equipped with a mechanical wiper that
makes them ideal for unattended applications. The wiper functions automatically during sampling
studies. The number of wiper movements 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.
9-15.2. Effect of Temperature
While the effect of temperature on the turbidity sensor is small, this factor is automatically taken
into account by the ADV6600 software providing temperature compensated readings. Temperature
coefficients of 0.3%/degree C and 0.6%/degree C are automatically activated for the 6026 and 6136,
respectively, when these sensors are activated in the Sensor menu.
9-15.3. Effect of Particle Size
The output of turbidity sensors is susceptible not only to the overall cloudiness of the environmental
medium, but also to the particle size of the suspended solids which pass across the optics on the
probe face. Thus, although the turbidity of an environmental sample may appear to the eye to be
relatively stable, the displayed turbidity can vary significantly depending on the nature of the