Xylem 6-Series Multiparameter - Manual (Page 258)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-20
example, the label of the YSI 6073 turbidity standard bottle indicates that the value of the standard is
100 NTU when used for calibration of the 6026 sensor, but 126 NTU when used to calibrate the
6136. Note that the phenomenon of a sensor-specific formazin/AEPA-AMCO ratio is well known for
sensors other than the 6026 and 6136.
YSI and its approved vendors offer easy-to use AMCO-AEPA turbidity standards which can also be
quantitatively diluted with turbidity-free water to provide calibrant suspensions of lower values. Hach also
offers relatively inexpensive formazin suspensions at various NTU values up to 1000 NTU under the
Stablcal TM designation which will provide accurate calibration of either the 6026 or 6136 sensors as long as
the user is willing to exercise the proper safety precautions as outlined in the MSDS associated with formazin.
The primary advantage of the Hach formazin standards is their cost; their primary disadvantage is that the
suspended matter settles out fairly rapidly. Although the AMCO-AEPA standards are somewhat more
expensive, they do not settle out, making them significantly easier to use.
While the effect of temperature on the turbidity sensor is small, this factor is automatically taken into
account by the sonde 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.
MEASUREMENT AND CALIBRATION PRECAUTIONS
(1) For best results, use only freshly prepared or purchased turbidity standards. Degradation of standards
can occur on standing, particularly formazin prepared from dilution of concentrated suspensions such as
Hach 4000 NTU standard.
(2) If unusually high or jumpy readings are observed during the calibration protocol, it is likely that there
are bubbles on the optics. Manually activating the wiper of the 6026 or 6136 from a computer or 650
MDS keypad removes these bubbles.
(3) When calibrating the 6136 sensor, be aware of the fact that precautions must be taken to avoid
interference of the bottom of the calibration vessel. Instructions for two methods of calibrating the 6136
sensor are provided in Section 2.6.1 of this manual. Unless these precautions are taken, field turbidity
readings can exhibit an offset of approximately 1.5 NTU. For example, a field reading of 2.5 NTU would
really be 4.0 NTU. The offset is not magnified at higher turbidity (for example, a reading of 100 NTU
would really be 101.5 NTU) and thus, the effect is much more important in water of low turbidity.
(4) 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 particles in the optical path at the
instant of measurement. For example, if individual readings are taken every 4 seconds in a discrete sample
study of environmental water, variations of 0.5-1.0 NTU are common between readings. In long term,
unattended studies this effect can be even more exaggerated with spikes of up to 10 NTU sometimes
observed. This apparent jumpiness is not observed for freshly prepared turbidity standards, since the
particle size in these suspensions is homogeneous.
The sonde turbidity system allows the user to either observe these real turbidity events (while obtaining
somewhat jumpy readings) or to apply a mathematical filter to the raw data so that the NTU output may be
more reflective of the overall cloudiness of the environmental sample. From the Advanced|Data Filter
menu of the sonde software, the user can activate the data filter that is specific to turbidity and “fine tune”
its performance. For typical sampling and monitoring applications, YSI recommends that the Data Filter
settings be selected as follows: Enabled -- On; Wait for Filter -- Off; Time Constant = 12; Threshold =
0.010.