Xylem 6-Series Multiparameter - Manual (Page 353)

ROX Optical DO Sensor Appendix M
YSI Incorporated Environmental Monitoring Systems Operations Manual M- 4
Should I calibrate my ROX sensor in water-saturated air or air-saturated water?
Studies at YSI have shown that the sensor shows effectively the same reading in air-saturated water and
water-saturated air. Thus, if the calibration is carried out properly, either medium can be used with
confidence. The advantage of air-saturated water is the quick equilibration of the sonde sensors relative to
thermal and humidity factors; the disadvantages are that you will require an aquarium pump, an air-stone, a
large vessel to immerse the sonde in the air-saturated water, and you must sparge the water for at least 1
hour prior to calibration to assure that it is air-saturated. The advantage of using water-saturated air is that
the calibration can be carried out in the cup supplied with the sonde and it can be done somewhat faster
than the air-saturated water method. Note, however, that if you use water-saturated air as the calibration
method, you should still wait at least 15 minutes after placing the sensors in the calibration vessel to assure
thermal equilibration between the temperature and ROX DO sensors.
The bottom line is that either method will give good results as long as the considerations above are
followed. The air-saturated water method may be slightly preferable if you have the equipment because of
the assured equilibration, but the advantage is slight.
If I want to perform a 2-point calibration, how do I generate a zero-oxygen medium?
Two methods are generally used to provide a zero-oxygen environment:
1.
Place the ROX sensor in a vessel which is filled with a flowing inert gas such as nitrogen gas
2.
Place the ROX sensor in an aqueous solution of sodium sulfite at a concentration of approximately
2 g/L)
The following qualifiers apply to the zero point calibration methods:
• If you use nitrogen gas for the zero point calibration, you should make certain that the vessel you
use has a SMALL exit port to prevent back diffusion of air and that you have completely purged
the vessel before confirming the calibration.
• If you use sodium sulfite solution for the zero point calibration, you should make up the solution
at least 2 hours prior to use and keep it sealed in a bottle which does not allow diffusion of oxygen
through the sides of the container. You should also transfer the sodium sulfite solution rapidly
from its container to the sonde calibration cup, fill the cup as full as possible with solution to
minimize head space, and seal the calibration cup to the sonde to prevent diffusion of air into the
vessel.
Whichever method you use, it is very important that you wait at least 10-12 minutes and until the readings
are stable for at least 2 minutes before confirming the zero point calibration entry.
If I make a mistake in a 2-point calibration, will my sensor always be inaccurate?
No. You can either perform a new 2-point calibration with better control of the conditions or you can
return to the factory default calibration by using the “uncal” command as described below.
• Select Calibrate from the Main menu, then Optic T Dissolved Oxy, and then run any of the
ODOsat % or ODO mg/L options.