Note: the sonde will only read 100% saturation if the barometric pressure is 760 mmHg. To
determine the actual DO in %, divide the barometer reading by 760 and then multiply that
number by 100.
Example: 750 ÷ 760 = 0.9868 x100 = 98.68%
Check the ODO Gain in the Advanced Menu. An acceptable range is 0.85 to 1.15.
CALIBRATION METHOD – TWO-POINT “AIR or WATER”
Note: YSI does not recommend the two-point calibration unless (A) you are certain that the sensor does
not meet your accuracy requirements at low DO values and (B) you are operating under conditions
where you are certain you can generate a medium that is truly free of oxygen!
1. REMOVE THE ROX WIPER! Sodium sulfite will be absorbed into the pad and will be
applied to the ROX membrane each time the wiper is activated. This will contaminate the ROX
membrane and create a problem with the second point of the calibration.
2. Mix 2 grams of sodium sulfite to 1000 milliliters of tap water 2 hours prior to use and keep it
in a sealed bottle. Rapidly transfer this mixture into the sonde’s calibration cup and fill to
overflow. Install the sonde and completely seal the cup to prevent diffusion of air into the
vessel.
3. o calibrate, select ODO % Two-Point.
4. The first entry for the two-point calibration will be in mg/L! Wait at least 10 to 12 minutes for
the solution to re-equilibrate and then type 0 and press Enter. Remove the sonde from the
sodium sulfite and thoroughly rinse all exposed surfaces, especially the ROX membrane, with
tap water. Rinse at least 4 times if using a filled calibration cup.
5. Enter the barometric pressure in mmHg for the second calibration point and press the Enter key
when the reading is stable. Remember that you must wait 15 minutes for the air to saturate in
the cal cup or 5 minutes if the sonde is in saturated water.
6. Verify that the ODO Gain found in the Advanced Cal Constants menu is between 0.85 and
1.15.
Note: If you believe that you have calibrated a ROX probe in error (especially the zero), then you can
return the probe back to its original factory calibration by using the “uncal” command. With a PC, type
the word ‘uncal” instead of the barometric pressure and press Enter. If using a 650, at the point where
you would type in the barometer, press the Esc and Enter keys simultaneously.
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