Xylem 6-Series Multiparameter - Manual (Page 274)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-36
capability of mechanical cleaning is likely to be compromised. The YSI 6131 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 firmware. Generally, one wiper movement
is sufficient for most environmental applications, but in media with particularly heavy fouling, additional
cleaning cycles may be necessary.
LIMITATIONS OF ACCURACY FOR THE BGA-PC SENSOR
As mentioned above, the measurement of PC-containing BGA from in vivo fluorescence measurements
will almost always be less accurate than determinations made using either cell counting or
spectrofluorometric quantitation of molecular phycocyanin after its extraction from cells. Some of the
reasons for this accuracy limitation with in vivo BGA-PC measurement include the following:
 Interferences from other microbiological species such as chlorophyll-containing phytoplankton
 Interference from sample turbidity.
 Differences in the general fluorescence intensity of different PC-containing BGA species
 Differences in the effect of temperature on the fluorescence intensity of different PC-containing
BGA species
 Effect of the variation in ambient light conditions on BGA fluorescence and differences in this
effect between different PC-containing BGA species.
In addition, when present in high concentrations, colonies of BGA can often be seen with the naked eye
and may resemble fine grass cutting or take the form of small irregular clumps or pinhead-sized spheres.
When BGA colonize into these forms, the sensitivity of the YSI sensor in terms of the fluorescence per cell
of BGA is reduced because it has been designed to detect microscopic, free-floating cells and not large,
macroscopic floating particles. Thus, the sensor is likely to underestimate the total amount of BGA
present in the water when clumps are present.
Users should take careful note that these limitations mean that any in vivo sensors such as BGA-PC and
chlorophyll will be significantly less quantitative than any of the other sensors offered for use with YSI 6-
series sondes and make it impossible for YSI to provide an actual accuracy specification in cells/mL for the
6131 Phycocyanin Probe.
ESTIMATION OF THE BGA-PC RANGE IN CELLS/ML
As noted in the above section, the use of in vivo phycocyanin fluorescence to estimate the cell content of
PC-containing algae has significant limitations. These limitations also make the designation of a range (or
full scale sensor reading) for any PC-BGA sensor less than quantitative. The range estimate for the YSI
6131 sensor is based on the fact that its reading in an empirical sample of PC-containing algae is about 40%
less than that of the industry standard fluorometer from Turner Designs which is configured for PC-BGA.
In the estimation experiment, a Turner Cyclops sensor was fixed on its middle range and then its voltage
reading in a PC-BGA culture (Microcystis aeruginosa) was determined. The YSI 6131 sensor was placed
in the same culture and its sensitivity found to be about 40% less in terms of the percent of full scale
deflection relative to the Turner sensor on its middle range. Since Turner Designs has designated the
middle range of its sensor as 0-200,000 cells/mL, the YSI sensor is estimated to have a range of about 40%
more or 280,000 cells/mL. Naturally, this range is only an estimation for both the YSI and Turner sensors
because of the general limitations of in vivo fluorescence measurements described above.