YSI 6 series User Manual page 357

ROX Optical DO Sensor Appendix M
guard. It is also important to note that if the membrane is exposed to bright sunlight for extended periods,
i.e., more than 60 minutes, the life of the membrane may be reduced due to photo-bleaching of the dye.
Thus, make certain that you protect the membrane from bright sunlight during any storage and/or
transportation to your deployment site.
Is there any effect of wiping the sensor membrane on the ROX DO readings?
Yes, there is a minor effect. Typically the optical DO reading will drop by about 1.5 % immediately after
a wipe which is activated during Discrete Sample studies. After 12 seconds, the reading has typically
returned to within 0.7 % of the final reading. It takes about 30 seconds for complete recovery of the ROX
optical DO reading after the sensor membrane is wiped. This effect is due to the physical contact of the
wiper pad with the membrane since it is not observed when the wiper assembly is removed and only the
wiper shaft turns when a wipe command is issued.
Will this wiping effect cause inaccuracy in my Unattended study readings with the ROX DO sensor
since I will be calibrating without wiping, but logging readings after the ROX sensor has been
wiped?
The error will be minor in any case, but will vary depending on what other optical probes you have
installed in your sonde. Since the effect is physical as described in the previous question, the longer it
takes to log a reading after the ROX sensor has wiped, the less error there will be relative to the calibration.
Under the worst case scenario, the user has ONLY a ROX sensor present with an ODO Time Constant
setting of 12 seconds as set in Advanced|Data Filter. This set-up means that readings will be logged
internally 12 seconds after the wipe has terminated and, under these conditions, there will be a typical error
of 0.7 % of the reading – within the sensor specification of 1 % of the reading. This error will be further
reduced if other optical probes are present since the time for them to be wiped plus their time constants will
further increase the time between the wiping of the ROX sensor and the time a point is logged to internal
memory. (Note that the ROX sensor is ALWAYS WIPED FIRST, no matter what optical port it is
installed in as long as you have Version 3.04 or later of firmware installed in your sonde.) The extent of
the error would thus be greatest with a 6920V2-1 running ROX DO only, less with a 6600-type or 6920V2-
2 sonde running ROX DO and turbidity, still less with a 6600-type sonde running ROX DO and
chlorophyll or BGA (since the time constant these other optical sensors is 24 seconds) and least for a
6600V2-4 sonde containing ROX DO and 3 other optical sensors. Typical errors in Unattended readings
from this wiping effect are 0.7% for a sonde with only a ROX sensor and 0.3% for a sonde with a ROX and
one other optical sensor. There is no discernible error for a 6600V2-4 sonde with a ROX and three other
optical sensors.
Can I reduce the small error from the wiping effect?
Yes. By increasing the Time Constant value for ODO in the Advanced|Data Filter menu, the error will
definitely be reduced. For example, with a ROX DO probe as its only optical sensor, typically your error
would be reduced from 0.7% to 0.3% by increasing the ODO Time Constant from 12 to 24 seconds.
Naturally, however, this will have an adverse effect on the battery life of your sonde for deployments so
you should balance this factor relative to the small DO error which will be observed if you leave the Time
Constant at 12 seconds.
I will be installing a ROX sensor in my 6600EDS V2-2 sonde along with another optical probe. Does
it matter in which optical port I place the ROX sensor?
Yes. You should always install the ROX probe in the center (T) port of the 6600EDS V2-2 sonde and
substitute the special EDS wiper for the black wiper that came with the ROX probe. The other optical
probe should be installed in the outer optical port (c) with a standard wiper assembly. This installation
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