To create a zero-oxygen calibration solution
1) Add approximately 1500 mL of distilled water to a 2 L beaker.
2) Using a steady stream of clean compressed nitrogen gas (approximately 10-40 mL per minute flow rate), aerate the water for a minimum of 30 minutes.
3) Keep nitrogen flowing while taking the reading.
Reference: ASTM International, Designation: D888 – 09: Standard Test Methods for Dissolved Oxygen in Water