If dissolved oxygen is a critical parameter, EPA New England recommends a second calibration point using the zero DO solution or a check of low DO accuracy with a zero DO solution.
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QA Bulletin: Calibration of Dissolved Oxygen Meters: February 2006 (PDF)(3 pp, 10 K)
If dissolved oxygen is a critical parameter, EPA New England recommends a second calibration point using the zero DO solution or a check of low DO accuracy with a zero DO solution.