YSI ADV6600 - Manual (Page 134)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 124
9-17.1. 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 100 ug/L is sufficient. However, for maximum
accuracy over the entire 0-200 ug/L range of the sensor, a 3-point calibration procedure can slightly
enhance the accuracy of the sensor. Note that YSI does not offer rhodamine WT standards, but does
suggest a vendor for user-production of standards and, later in this section, provides instructions for
preparing a solution which is 100 ug/L in Rhodamine WT.
While the effect of temperature on the rhodamine sensor and electronics is small, the fluorescence of
rhodamine WT changes significantly with temperature. The combination of the two factors is
automatically taken into account by the sonde software providing temperature compensated
readings.
9-17.2. Effect of Turbidity
As described above, the filters in front of the photodiode in the YSI 6130 rhodamine probe prevent
most of the green light which is used to excite the rhodamine molecules from reaching the detector
after being backscattered off of non-fluorescent particles (turbidity) in environmental water.
However, the filter system is not perfect and a minor interference on rhodamine WT readings from
suspended solids may result. Laboratory experiments indicate that a suspension of typical soil
measured with a YSI 6026 sensor will have turbidity interference characterized by a factor of about
0.03 µg/L per NTU. For example, the turbidity of the water must be above 100 NTU to produce an
apparent rhodamine WT reading equal to 3 µg/L. In very cloudy water, the user may wish to use the
independently-determined turbidity value and the above compensation factor to correct measured
chlorophyll values using, for example, a spreadsheet.
9-17.3. Effect of Chlorophyll
While the green LED used in the rhodamine WT sensor is not ideal for excitation of the chlorophyll
in phytoplankton, some fluorescence of environmental chlorophyll will always be induced by the
rhodamine sensor. Because the filter system for the rhodamine photodiode is not perfect in
excluding chlorophyll fluorescence, a minor interference on rhodamine WT readings from
phytoplankton may result. Laboratory experiments indicate that a suspension of phytoplankton
measured with a YSI 6025 sensor will have chlorophyll interference characterized by a factor of
about 0.10 µg/L Rhodamine WT per ug/L of chlorophyll. For example, the chlorophyll content of
the water must be above 30 ug/L chlorophyll to produce an apparent rhodamine WT reading equal
to 3 µg/L. In water with a high algal content, the user may wish to use the independently-
determined chlorophyll value and the above compensation factor to correct measured rhodamine
values using, for example, a spreadsheet.