Maintaining a stable oxygen content in the breathing loop of the diving apparatus with a semi-closed respiratory circuit is essential for the safety of underwater mine clearance operations. This article discusses the influence of modified methods of flushing the breathing apparatus on the quality of the breathing loop ventilation process. The problem of flushing an apparatus is presented in light of the Polish Naval Academy’s change from statistical to deterministic decompression schedules, based on the model of apparatus ventilation, oxygen toxicity hazard and decompression. The examination and determination utilising the required accuracy of the method used for flushing the breathing loop of the device allowed to assess the stable oxygen content during the decompression phase. The assumptions were verified based on a system for measuring oxygen content in the breathing loop.