CALIBRATION PROCEDURES : General Steps
To ensure more accurate results, you can rinse the calibration cup with water, and then rinse with a small amount of the calibration
solution for the sensor that you are going to calibrate. Discard the rinse solution and add fresh calibrator solution. Use tables 1 to find
the correct amount of calibrator solution.
Table 1: Calibration solution volumes for 6920V2-2 multi-parameter sonde.
1. Carefully immerse the probes into the solution and rotate the calibration cup to engage several threads. YSI recommends supporting
the sonde with a ring stand and clamp to prevent the sonde from falling over.
2. With the proper cable, connect the sonde to YSI 650MDS data logger. From the sonde Main menu, select 2-Calibrate (Figure 1).
Figure 1: Calibration menu for 6920V2-2 multi-parameter sonde as seen on 650MDS data logger.
3. Note that the exact appearance of this menu will vary depending upon the sensors that are available and enabled on your sonde. To
select any of the parameters from the Calibrate menu, input the number that is next to the parameter. Once you have chosen a
parameter, some of the parameters will have a number that appears in parentheses. These are the default values and will be
used during calibration if you press Enter without inputting another value. Be sure not to accept default values unless you
have assured that they are correct. If no default value appears, you must type a numerical value and press Enter.
4. After you input the calibration value, or accept the default, press Enter. A real-time display will appear on the screen. Carefully
observe the stabilization of the readings of the parameter that is being calibrated. When the readings have been stable for
approximately 30 seconds, press Enter to accept the calibration. The calibrated value is bolded on the example screen on the
following page.
5. Press Enter to return to the Calibrate menu, and proceed to the next calibration.
ROX OPTICAL DISSOLVED OXYGEN
Place the sensor either (a) into a calibration cup containing about 1/8 inch of water which is vented by loosening the threads or (b) into
a container of water which is being continuously sparged with an aquarium pump and air stone.
Wait approximately 10 minutes before proceeding to allow the temperature and oxygen pressure to equilibrate.
Select ODOsat % and then 1-Point to access the DO calibration procedure. Calibration of your Optical dissolved oxygen sensor in
the DO % procedure also results in calibration of the DO mg/L mode and vice versa.
Enter the current barometric pressure in mm of Hg. (Inches of Hg x 25.4 = mm Hg).
Note: Laboratory barometer readings are usually “true” (uncorrected) values of air pressure and can be used “as is” for
oxygen calibration. Weather service readings are usually not “true”, i.e., they are corrected to sea level, and therefore cannot be used
until they are “uncorrected”. An approximate formula for this “uncorrection” (where the BP readings MUST be in mm Hg) is:
True BP = [Corrected BP] – [2.5 * (Local Altitude in ft above sea level/100)]
Press Enter and the current values of all enabled sensors will appear on the screen and change with time as they stabilize. Observe the
readings under ODOsat %. When they show no significant change for approximately 30 seconds, press Enter. The screen will indicate
that the calibration has been accepted and prompt you to press Enter again to return to the Calibrate menu.
The minor advantages and disadvantages of calibration in air-saturated water versus water-saturated air are outlined in Appendix M,
ROX Optical DO Sensor
NOTE CAREFULLY: As opposed to the 6562 Rapid Pulse Polarographic DO sensor described above, there is no difference
between the calibration routine for sensors which will be used for sampling or monitoring applications. Usually the Autosleep RS-232
feature in the Advanced|Setup menu will be activated for ROX calibrations, but there is no problem if it is not active.
Rinse the sonde in water and dry the sonde.
6150 ROX OPTICAL DO PROBES
CAUTION: The sensor membrane of the 6150 probe should be cleaned ONLY with a lens tissue which has been moistened with
WATER. ALCOHOL should NOT be used in the cleaning process as it will dissolve the outer paint layer of the membrane assembly;
other organic solvents will likely dissolve the dye itself. Under NO circumstances should you use organic solvents to clean your
sensor membrane.
How does the ROX Optical DO probe work?
In general, optical dissolved oxygen sensors from a variety of manufacturers are based on the well-documented principle that
dissolved oxygen quenches both the intensity and the lifetime of the luminescence associated with carefully-chosen chemical dyes.
The 6150 sensor operates by shining a blue light of the proper wavelength on this luminescent dye which is immobilized in a matrix
and formed into a disk about 0.5 inches in diameter. This dye-containing disk will be evident on inspection of the sensor face. The
blue light causes the immobilized dye to luminesce and the lifetime of this dye luminescence is measured via a photodiode in the
probe. To increase the accuracy and stability of the technique, the dye is also irradiated with red light during part of the measurement
cycle to act as a reference in the determination of the luminescence lifetime.
When there is no oxygen present, the lifetime of the signal is maximal; as oxygen is introduced to the membrane surface of the sensor,
the lifetime becomes shorter. Thus, the lifetime of the luminescence is inversely proportional to the amount of oxygen present and the
relationship between the oxygen pressure outside the sensor and the lifetime can be quantified by the Stern-Volmer equation. For most
lifetime-based optical DO sensors (including the YSI 6150), this Stern-Volmer relationship (((Tzero/T) – 1)) versus O pressure) is not
2
strictly linear (particularly at higher oxygen pressures) and the data must be processed using analysis by polynomial non-linear
regression rather than the simple linear regression used for most polarographic oxygen sensors. Fortunately, the non-linearity does not
change significantly with time so that, as long as each sensor is characterized with regard to its response to changing oxygen pressure,
the curvature in the relationship does not affect the ability of the sensor to accurately measure oxygen for an extended period of time.
The 6150 probe is warranted for 2 years and sensor modules are warranted for 1 year by YSI.
Each YSI sensor module (the assembly which is attached to the face of the probe by three screws) is factory-calibrated over a range of
0-100 percent oxygen to quantify the relationship of its luminescence lifetime as a function of oxygen pressure. The Stern-Vollmer
3
2
parameters from this data are then fit to a third order regression equation (ax + bx + cx) and values of a, b, and c determined. These
coefficients, along with the luminescence lifetime at zero oxygen pressure (Tzero), are provided to the user in coded form with each
sensor membrane module or probe/sensor module combination. If you install a replacement sensor membrane assembly (YSI 6155) on
your existing probe, you will be required to enter these coded constants into the sonde as described in the instructions which come
with the 6155 prior to the use of the sensor. If you have purchased a probe/membrane combination, i.e. a new 6150 Optical DO sensor,
the constants are already stored in your probe and will automatically be transferred to your sonde when the sensor is installed.
A schematic representation of the 6150 ROX Optical DO sensor is shown in the picture below:
The ROX membrane differs from the traditional electrochemical sensor membrane, requiring fewer steps for installation and
less maintenance. ROX membranes will last for one year and are made of a durable material that is unlikely to be damaged
in the field. The ROX sensor eliminates stirring dependency that was required of most traditional polarographic sensors.
Acknowledgements:
The entire content of this calibration guide was taken directly from the YSI 6-Series Multiparameter
Water Quality Sondes User Manual. All information and images are explicit property of YSI and were
used solely for informational purposes. For more detailed information on all YSI products and
software, visit their website at https://www.ysi.com/