YSI ADV6600 - Manual (Page 130)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 120
From our studies, it appears that entry of a value of 1 to 2 % per degree C for “Chl tempco” is
appropriate to partially account for changes in the fluorescence of environmental phytoplankton
with temperature. This value can be estimated in the above example as follows:
Change in Temperature = 21–1 = 20 °C
Change in Fluorescence = 226-185 = 41 µg/L
% Change in Fluorescence = (41/185) x 100 = 22.1
Chl Tempco Factor = 22.1/20 = 1.11 % per degree °C
Note that the use of this empirically derived compensation does not guarantee accurate field
readings since each species of phytoplankton is likely to be unique with regard to the temperature
dependence of its fluorescence. Changes in fluorescence with temperature are a key limitation of
the in vivo fluorometric method (see below) which can only be reduced, not eliminated, by this
compensation. In general, the best way to minimize errors is to calibrate with phytoplankton
standards of known chlorophyll content that are as close as possible in temperature to that of the
environmental water under investigation.
9-16.4. Effect of Particle Size
The output of the YSI fluorescence sensor is susceptible not only to the overall phytoplankton
concentration in the environmental medium, but also to the size and rate of movement of the
suspended particles that pass across the optics on the probe face. Thus, although the phytoplankton
content of an environmental sample may appear to the eye to be relatively stable, the displayed
chlorophyll reading can vary significantly depending on the nature of the particles in the optical path
at the instant of measurement. In a discrete sample study of environmental water, for example, the
variability of the output can be significant. This apparent jumpiness is not observed in dye
standards, since these are homogeneous solutions containing no suspended matter.
9-16.5. Effect of Turbidity
As described above, the filters in front of the photodiode in the YSI 6025 chlorophyll probe prevent
most of the 470 nm light which is used to excite the chlorophyll 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 chlorophyll readings from
suspended solids may result. Laboratory experiments indicate that a suspension of typical soil
measured with a YSI 6026 sensor will have a 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 chlorophyll 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.
9-16.6. Limitations of In Vivo Measurement