Dissolved Oxygen meters must be calibrated to true, or uncorrected, barometric pressure, which means the value has not been corrected to represent sea level.
When you calibrate, calculate what your DO meter should be reading according to the current barometric pressure. You can do this by dividing your true barometric pressure by 760 and then multiplying it by 100%. This is the number you should use to calibrate your meter. After you have tested your water sample, test the calibration solution again and check to see that the reading varies by only 2% (it should vary by only 1% for an optical sensor). When you turn your instrument off and then on again, you can confirm that is it holding its calibration by remembering this value. Turn on, let the instrument stabilize, and then look to see whether the reading is still within 2% of the calibration value that you determined.
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