The O 2 -transmitter Oxytrans TR measures continuously and exactly the content of oxygen in liquids and gases. The sensor is especially designed for breweries and further applications with high requirements, e.g. power plants or bioreactors. The optical principle of measurement is based on the effect of dynamic luminescence quenching by molecular oxygen. The indicator layer on the glass installed in the measuring head is illuminated with a blue-green-light. With this, the indicator molecules are transferred into an excited state and emit a red light and detected by the internal detector. If oxygen is in the medium, this luminescence effect is prevented by energy transfer to the oxygen molecule. After the collision with the indicator molecule the oxygen molecule is transferred from its ground state (triplet state) to its excited singlet state. As a result, the indicator molecule does not emit luminescence and the measurable luminescence signal decreases linear to existing oxygen concentration. This decrease is the basis for the oxygen calculation. The O 2 -concentration can be displayed in different units like ppb, ppm, %oxygen, etc.