Xylem 6-Series Multiparameter - Manual (Page 257)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-19
turbidity standards supplied by YSI, the value of the standard is 100 NTU when used for calibration of
the 6026 sensor, but 126 NTU when used to calibrate the 6136.
In environmental water that contains suspended particles of varying size and density, the readings for the
two probes will also differ after calibration with either formazin or polymer beads, with the 6026 almost
always showing higher readings and the extent of the difference generally being proportional to the overall
turbidity. For example, if both probes are calibrated at 0 and 100 NTU and then placed in a turbid river, the
6026 might read 400 NTU while the 6136 would read 300 NTU. The absolute difference in the sensor
readings would shrink as the turbidity dropped. Thus, if the 6026 read 30 NTU, the 6136 would likely read
approximately 21 NTU.
Note that the sonde software labels the output of the two sensors with slightly different units of turbidity
(“turbid NTU” for the 6026 and “turbid+ NTU” for the 6136) so that the user will readily be able to
determine which sensor was used in a particular study during later data analysis.
No matter whether the 6026 or 6136 is installed in your sonde, it is important to remember that 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 generally be removed in short term sampling applications by simply agitating the sonde
manually. However, for studies longer than a few hours where the user is not present at the site, the quality
of the turbidity data obtained with a turbidity sensor that has no capability of mechanical cleaning is likely
to be poor. However, as noted above, both the 6026 and the 6136 probes are equipped with a mechanical
wiper that makes them ideal for unattended applications. The wiper can be activated in real-time during
discrete sampling operations or will function automatically during long term unattended sampling 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 movement is sufficient for most environmental applications, but
in media with particularly heavy fouling, additional cleaning cycles may be necessary.
CALIBRATION AND EFFECT OF TEMPERATURE
The sonde software offers the option of 1-point, 2-point, or 3-point calibrations procedures. For most
applications, a 2-point calibration at 0 and approximately 100 NTU is sufficient for either the 6026 or the
6136 sensor. However, a user might wish to carry out a 3-point calibration at values of approximately 0,
100 and 1000 NTU to provide maximum accuracy over the entire normally encountered environmental
turbidity range (0-1000 NTU). If the range of turbidity in the environmental sample is well known,
standards of other turbidity values can be utilized (in either 3 point or 2 point routines). However, in all
calibration procedures, one of the standards must be 0 NTU and this should be the first calibration point.
NOTE: Before calibrating your 6026 or 6136 turbidity sensor, pay particular attention to the following
cautions:
 For all calibration procedures, you MUST use standards that are based on either formazin or AMCO-
AEPA styrene divinylbenzene beads as described in Standard Methods for the Examination of Water
and Wastewater
and have been prepared either by Hach (formazin based) or AMCO-AEPA based
standards prepared by YSI or an approved YSI vendor listed on the YSI website (www.ysi.com). THE
USE OF STANDARDS FROM OTHER VENDORS AND/OR THOSE PREPARDED FROM
MATERIALS OTHER THAN FORMAZIN OR AMCO-AEPA POLYMER BEADS WILL RESULT
IN BOTH CALIBRATION ERRORS AND INACCURATE FIELD READINGS.
 For AMCO-AEPA standards, the value entered by the user during the calibration protocol is
DIFFERENT depending on which sensor (6026 or 6136) is being calibrated. This reflects the
empirically determined fact that the 6026 and 6136 sensors which have been calibrated to the same
value in the primary standard formazin will have different responses in the suspensions of the AEPA-
AMCO beads. This effect is likely due to the larger optical cell volume of the 6136. Thus, for