YSI 600DW-B Sonde - Manual (Page 233)

Principles of Operation Section 5
REMEMBER: The use of rhodamine WT for “calibration is only an approximation. To assure
accurate readings from the 6025 sensor, the user must relate the field fluorescence readings to data
from extractive analysis samples as described above. YSI does not provide an accuracy specification for
chlorophyll due to these limitations.
EFFECT OF TEMPERATURE ON READINGS
While the effect of temperature on the chlorophyll sensor itself is very small, YSI experiments have
indicated that the fluorescence of phytoplankton suspensions can show significant temperature dependence.
For example, the apparent chlorophyll content of our laboratory test samples of algae increased from 185 to
226 µg/L when the temperature was dropped from 21 °C to 1 °C even though no change in phytoplankton
content took place. In the absence of compensation, this effect would obviously result in errors in field
chlorophyll readings if the site temperature were significantly different from the calibration temperature.
This temperature error can be reduced by employing a chlorophyll temperature compensation routine (“Chl
tempco”) resident in the sonde software under the Advanced|Sensor menu.
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.
EFFECT OF FOULING ON OPTICAL MEASUREMENTS
Field optical measurements are particularly susceptible to fouling, not only from long term build up of
biological and chemical debris, but also to shorter term formation of bubbles from outgassing of the
environmental water. These bubbles can sometimes be removed in short term sampling applications 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 chlorophyll data obtained with a fluorescence sensor that has no capability of
mechanical cleaning is likely to be compromised. The YSI 6025 probe is equipped with a mechanical
wiper that makes it ideal for unattended applications. The wiper can be activated in real-time during
discrete sampling operations or will function automatically just before each sample is taken during long
term unattended monitoring studies. The number of wiper movements and the frequency of the cleaning
cycle for the unattended mode can be set in the sonde software. Generally, one wiper movement is
sufficient for most environmental applications, but in media with particularly heavy fouling, additional
cleaning cycles may be necessary.
EFFECT OF TURBIDITY ON CHLOROPHYLL READINGS
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
YSI Incorporated Drinking Water Monitoring Systems Operation Manual 5-20