YSI 6 series User Manual page 261

Principles of Operation
6025 chlorophyll probe is equipped with a mechanical wiper to periodically clean the optical face either by
manual or automatic activation. With these features, the YSI chlorophyll sensor provides the same level of
performance as the sonde fluorometers, but is much easier to use and can be deployed in environmental
water for several weeks without the need for service. In addition, the probe will be a component in sondes
that can acquire up to ten other parameters simultaneously with chlorophyll, rather just providing the single
parameter.
CALIBRATION METHODS -- GENERAL
As described in Section 2 of this manual, the sonde software offers the option of performing 1-point, 2-
point, or 3-point calibration procedures in µg/L of chlorophyll. The 1-point calibration is designed only to
zero the sensor in chlorophyll-free water and is sufficient for most field applications as described below.
Performing a 1-point calibration also zeroes the relative fluorescence unit (RFU) parameter. The 2-point
calibration will normally be used for checking the stability of the sensor during deployment by using a dye
solution to set the relative sensitivity of the sensor as described below. The 3-point calibration routine is
only for special applications and seldom improves the accuracy of the sensor.
For calibration of the YSI chlorophyll system, only one type of standard will assure accurate field
readings: a suspension of phytoplankton of known chlorophyll content. The chlorophyll content of this
suspension should be determined by the extractive analysis procedure described in Standard Methods. Most
users will not have this analyzed plankton suspension available prior to field studies for use in the 2-point
calibration protocol resident in the sonde software. Thus, the best "calibration" method usually comprises the
following steps:
1. Prior to use in the field, place the sensor in clean water and perform a 1-point calibration at 0 ug/L.
2. Immerse the sonde in a dye standard (see below) and record the reading. Note that you are not
"calibrating" to the dye reading, only checking its value relative to the default sensitivity of the sensor.
3. While making your field readings (sampling or unattended studies), collect a few grab samples and record
the date/time and location where they were acquired.
4. On return to the laboratory, perform extractive analyses for chlorophyll on the grab samples and record
the results for later use.
5. After the study is complete, place your YSI chlorophyll data in the column of a spreadsheet and place
your laboratory data in an adjacent column for comparison. Calculate ratios of field to laboratory results
for each grab sample point and then average the results to produce a ratio for adjustment (or
„postcalibration") of your field results into accurate chlorophyll readings (relative to extractive analysis).
6. Use the calculating power of the spreadsheet to multiply all of your field readings by the correction ratio
to obtain the best possible accuracy for your 6025 data.
A variation of this method is to perform a 2-point calibration prior to deployment using clear water and a dye
standard, setting apparent chlorophyll equivalent of the dye standard to the value shown in the tables below.
However, it must be emphasized that this technique does not increase the accuracy of the chlorophyll sensor
significantly over the simple 1-point calibration recommended above – the user still must collect grab sample
and perform laboratory analysis to be assured of meaningful chlorophyll values. The primary utility of the
dye is to check for sensor drift during deployment by reanalyzing the dye solution after sonde recovery. The
use of the 2-point dye calibration procedure may make it easier to quantify the predeployment dye value and,
if so, may be preferable. Remember, however, that no real enhancement of sensor accuracy is attained by
using the dye as a calibrant.
YSI Incorporated Environmental Monitoring Systems Manual
Section 5
5-23