Large tanker vessels carrying oil, gas, or chemicals are equipped with an inert gas system to fill up the tank headspace especially during cargo onloading and offloading. In the inert gas systems, oxygen concentration monitoring is required to ensure proper operation of the system. For the oxygen monitoring equipment easy maintenance, effortless calibration and reliability are of primary importance. In cold regions it is also important to avoid tank freezing. A reliable dewpoint transmitter ensures that the inert gas is dry enough.