Comparisons of dissolved-oxygen calibration methods at various elevations, Oregon, 2023

Two dissolved-oxygen experiments were completed using internally logging water-quality monitors. Dissolved-oxygen sensors on the water-quality monitors were calibrated using various settings and at various elevations in Oregon, USA. Data from these experiments can be used 1) to assess how elevation affects dissolved-oxygen values recorded by continuous water-quality monitors, and 2) to inform best practices for calibrating and recording dissolved oxygen at various elevations. 
        Experiment #1 started by calibrating six YSI 6-series optical dissolved-oxygen sensors in Bend, Oregon, at an elevation of approximately 3,700 feet above sea level: 3 sensors were calibrated to 100% saturation, and 3 sensors were calibrated to 100% solubility (in mg/L) based on USGS DO Tables. During the experiment, sensors always remained in a 100% saturated environment. All calibrations and calibration checks were performed in a 100% air-saturated water environment (air-purged bucket of water). Sensors were kept in a water-saturated air environment (referred to as the wet-towel method) when in transport; the wet-towel method allowed the sensors to remain in a 100% saturated environment while the elevation and barometric pressure changed. Monitors were set to log internally at 15-minute intervals, and the monitors were transported over a pass in the Cascade Range and then to Salem, Oregon, which is at approximately 200 feet elevation. In Salem, sensor calibrations were checked, and then dissolved-oxygen sensors were calibrated again using the same settings as previously described. Monitors were transported back to Bend, Oregon, following the same route, where calibrations were checked.
        Experiment #2 started by calibrating six YSI optical dissolved-oxygen sensors in Bend, Oregon: 2 YSI 6-series sensors were calibrated to 100% saturation, 2 YSI 6-series sensors were calibrated to 100% solubility (in mg/L) based on USGS DO Tables, and 2 YSI EXO sensors were calibrated to 100% local saturation. Similar to experiment #1, sensors always remained in 100% saturated environments; calibration and calibration checks were performed in air-saturated water, and sensors were kept in a water-saturated air environment when in travel status. Monitors were transported over a pass in the Cascade Range and then to downtown Portland, Oregon, which is approximately 20 feet above sea level. Sensor calibrations were not checked or changed in Portland. Monitors were transported back to Bend, Oregon, following the same route, where calibrations were checked.