71
Fouling Precaution
Fluorescence-based measurements are vulnerable to both biofouling and non-biological fouling. This is because of the high
sensitivity and resolution of measurements, which can be affected by any changes to the sensor face that light must pass through.
Any obstruction of that light path will affect measurements, and even bubbles on the sensor’s face can affect measurements.
Without any active anti-fouling, users may find limited applications where Total Algae monitoring with the ProSwap Logger can be
effective. YSI recommends short-term deployments in low-fouling conditions. Even then, interference will always be a risk.
TAL Units
The TAL sensors report data in RFU and μg/L of pigment (Chl, PC or PE) units. YSI recommends reporting in Relative Fluorescence
Units (RFU).
RFU is used to set sensor output relative to a stable secondary standard, Rhodamine WT dye. This allows users to calibrate sensors
identically so that results from sensor to sensor can be compared. Calibration with Rhodamine WT also enables users to monitor for
sensor drift and external factors such as biofouling or declining sensor optical performance over time as the LEDs age.
The excellent linearity of RFU, once the channels are calibrated with Rhodamine WT, translates to the best accuracy of
measurements. For example, a chlorophyll reading of 100 units will represent twice the pigment detected by the sensor than with a
chlorophyll reading of 50 units. This high linearity (R2>0.9999) doesn’t always hold for μg/L of pigment since that unit was derived
from laboratory monocultures, and an environmental algal population can behave quite differently. This is also why the TAL sensors
and in situ monitoring should not be regarded as a perfect replacement for other methods such as pigment extractions and cell
counting.
The μg/L output generates an estimate of pigment concentration that is based upon correlations built with sensor outputs and
extractions of pigments from laboratory-grown blue-green algae. Synonymous with parts per billion (ppb), μg/L is still commonly
used by regulatory agencies, but has the drawback that it is very dependent upon the composition of the algal population, the time
of day, the physiological health of the algae, and a number of other environmental factors. Thus, users are advised to do their own
check of our correlation with a population of algae relevant to their own sites, as described below.
A 2-point RFU calibration is advised to be performed first. Next, with samples collected from the site of interest, measure both RFU
and μg/L with the sensor(s). Observing careful handling and preservation of the samples, as soon as possible extract the pigments
from the samples, using standardized methods to determine the μg/L in each sample. The extraction data may be used to assess
how RFU and μg/L delivered by the sensor compare with the μg/L of pigment that would be predicted by RFU from the sensor.
The user’s requirements can guide the decision as to whether RFU or μg/L is the best unit to read from the sensor for any specific
application.
TAL Raw values can only be seen under
in the System menu and are unaffected by user calibrations. These values range
from 0-100, representing the percent of full scale that the sensor detects in a sample, and are used for diagnostic purposes.
Sensors, Calibration, and Maintenance