An electrochemical, galvanic type, oxygen sensor including scavenging electrodes for continuously reacting any unreacted or partially reacted active gases, such as oxygen, to remove the active gases prior to being dissolved or have been dissolved in the electrolyte resulting in less than accurate output signals. The use of the scavenging electrodes permits the gas sensor to detect active gases in a gas mixture in sub-parts per billion level accurately and without the need for external sparging of the electrolyte with a pure inert gas. The liquid electrolyte may be stored in the a sol-gel medium formed in sensor container and holding the electrolyte in voids or pockets in the medium thereby rendering the oxygen sensor portable with the stored electrolyte and usable within a few minutes of operation.