Effects of pressure on oxygen sensors

Abstract

To measure the effects of pressure on the output of a membrane oxygen sensor and a nonmembrane oxygen sensor, the authors pressure cycled a CTD sensor package in a laboratory pressure facility. The CTD sensor package was cycled from 30 to 6800 db over a range of temperatures from 2° to 38°C. Pressure decreased the output of the membrane sensor and increased the output of the microhole sensor. The pressure terms for both types of oxygen sensors were affected by temperature. The effect of pressure on both types of sensors can be quantified as exp(VP/R′T), where Vis a coefficient (cm3 mol-1), P is decibars, R′ is the gas constant (831.47 cm3 mol-1 db K-1), and T is kelvins. As water gets colder, V for both sensors increases. For temperatures less than 21°C, V for the membrane sensor is -33.7 ± 0.54cm3 mol-1, and V for the microhole sensor is 0.29 ± 0.31 cm3 mol-1. The V's for calibrations of four oceanic casts had larger ranges than the laboratory experimental data: -27.6 to -34.9 cm3 mol-1 for the membrane sensor, and -0.4 to -2.9 cm3 mol-1 for the microhole sensor. At 10°C, increasing pressure to depths of 5000 m decreases current output of a membrane sensor approximately 50% and increases output of a microhole sensor about 0.6%. For field calibrations, the authors recommend using a constant V obtained by iterations of linear fits. The use of a pressure term with the form exp(VP/R′T) appears to improve field calibrations of membrane oxygen sensors.