What variables impact an oDO calibration?
Optical dissolved oxygen (oDO) sensors still require calibration, though less frequently than the polarographic versions. New sensors (or new caps on existing sensors) require a two point calibration in air and an air free environment, ideally high purity nitrogen (N5 purity 99.999%). Best results from the zero point calibration will be obtained using a calibration fixture with a chamber to hold the sensor and calibration gas, which can purge all oxygen from the chamber after 3 minutes of flow time. The fixture must allow free flow of the calibration gas at a constant flow rate with no back pressure or leaks.
The calibration process must compensate for external influences like temperature, pressure, and humidity. Internal temperature compensation should account for slow changes, but errors up to 3% can be present if rapid changes in temperature occur. To prevent these temperature related errors, ensure that the environment temperature is stable before calibration. The temperature element is embedded in the metal body of the sensor, which can lead to errors up to 3% if the sensor is handled excessively prior to calibration. Not compensating for atmospheric pressure and humidity changes at the measurement location may lead to a maximum potential error of up to 13.2% for the air calibration point. The partial pressure of oxygen will vary depending on the atmospheric pressure, so the local atmospheric or barometric pressure must be accounted for during the air point calibration to reduce potential error. Learn more on the Atmospheric Pressure and Calibration knowledge base page. Higher humidity will lead to higher oxygen concentrations, especially at higher ambient temperatures. To prevent this compounding error, ensure that the air point calibration is performed in a water saturated environment (by mounting the sensor over a beaker of water or wrapping the tip with a wet cloth). The zero point calibration will not have the pressure and humidity fluctuations (since no oxygen is present). More information about the zero point calibration can be found in the Best Practices: Zero Point Oxygen Calibration knowledge base page.
To account for these interferences due to temperature, pressure, and humidity, Hamilton’s ArcAir software has a built-in calibration walkthrough for oDO sensors which includes inputs for atmospheric pressure and humidity. The software will automatically compensate for the local laboratory conditions that are entered into the calibration walkthrough. If the local pressure and humidity are not being measured, they can often be estimated from outdoor conditions.
More information can be found in the White Paper: Measurement Challenges with Optical Dissolved Oxygen Sensors.