gas mixture with a low energy content can be fed to the interior of the tank 10 furthermore there is provision for the measurement unit 26 to have a measuring sensor 28 for determining the energy content of the gas mixture 62 and a probe for determining the oxygen content in the crude oil tank 10 the energy content may be determined by means of various methods in this context the term u201c measuring sensor 26 u201d may also embrace a more complex device such as for example a gas chromatograph or a device comprising an infrared sensor for measuring the infrared radiation of a specific wavelength absorbed by the gas mixture the oxygen content may be fed to an indicator unit not illustrated here and serve for monitoring purposes it may however also serve for controlling the feed of inert gas into the interior of the tank 10 by the feed of inert gas the gas mixture 62 can be prevented from being converted into an ignitable gas mixture if for example air has penetrated into the tank interior of the crude oil tank 10 due to leakages there is provision for the device 12 to be mountable on a vehicle the pump unit 25 and the measurement unit 26 and or the compressor 38 being installed on the vehicle by means of the vehicle which is not illustrated here a plurality of crude oil tanks 10 can be serviced by.