Xylem 6-Series Multiparameter - Manual (Page 266)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-28
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.
The sonde chlorophyll system allows the user to apply a mathematical filter to the raw data so that the
sensor output may be more representative of the average phytoplankton content of the environmental
sample. From the Advanced|Sensor menu of the sonde software, the user can activate the data filter
and adjust its performance. For typical sampling and monitoring applications, YSI recommends that
the Data Filter settings for chlorophyll be selected as follows: Enabled -- On; Wait for Filter -- Off;
Chlorophyll Time Constant = 12; Chlorophyll Threshold = 1. The advantage of the filter is a more
stable display of chlorophyll readings.
5.15 RHODAMINE WT
INTRODUCTION AND PRINCIPLE OF OPERATION
Rhodamine WT is a red dye that is commonly used in stream flow studies. The amount of the species at
various points (horizontal and vertical) in the water under examination is determined by utilizing the fact
that Rhodamine WT fluoresces when irradiated with the proper wavelength of light and thus the YSI 6130
Rhodamine WT sensor works on the same principles as described above for the 6025 chlorophyll sensor.
As for chlorophyll, Rhodamine WT fluoresces, that is, when irradiated with light of a particular
wavelength, it emits light of a higher wavelength (or lower energy). The ability of Rhodamine WT to
fluoresce is the basis for all commercial fluorometers capable of measuring the analyte in situ.
Fluorometers of this type have been in use for some time. These instruments induce Rhodamine WT to
fluoresce by shining a beam of light of the proper wavelength into the sample, and then measuring the
higher wavelength light which is emitted as a result of the fluorescence process. Most Rhodamine systems
use a light emitting diode (LED) as the source of the irradiating light that has a peak wavelength of
approximately 540 nm. LEDs with this specification produce radiation in the visible region of the spectrum
with the light appearing green to the eye. On irradiation with this green light, Rhodamine WT in the water
emits visible light with a higher wavelength than that of the exciting beam, i.e. the Rhodamine fluoresces.
To quantify this fluorescence, the system detector is usually a photodiode of high sensitivity that is
screened by an optical filter that restricts the detected light. The filter minimizes (a) the exciting light being
detected when it is backscattered off of particles in the water and (b) the interference from other fluorescent
species such the chlorophyll in phytoplankton. Without the filter, turbid (cloudy) water or water with high
levels of phytoplankton would appear to contain Rhodamine WT, even though none were present. The
following diagram can be used to better understand the principles of the YSI Rhodamine WT system.