as a pt thermometer the pt thermometer 3 is preferred due to its good performance as to long term stability which will be further explained below however it should be appreciated that the absolute temperature sensor may be other thermometers such as but not limited to a metallic resistance thermometer a semi conductor resistance thermometer a thermoelement thermometer a frequency based thermometer and or an emission thermometer in the following the relative temperature sensor 1 will be referred to as an rt sensor the rt sensor 1 comprises a measurement body 7 a reference body 9 and a difference temperature sensing means 11 which is thermally and here in fact physically connected to both the measurement body 7 and the reference body 9 and which senses a temperature difference between the reference body 9 and the measurement body 7 preferably the difference temperature sensing means comprises a plurality of thermoelements thermo piles which are attached to each other forming a block and which are engaged with a respective surface area of each one of the bodies 7 9 further the rt sensor 1 comprises a housing 13 the housing 13 embraces said bodies 7 9 and said sensing means 11 and it is attached with the measurement body 7 and is heat conducting thus the measurement body 7 is exposed to the surrounding environment or substance as regards the heat thereof more specifically it is arranged so that the measurement body and the substance have the same temperature within the measurement precision further the housing 13 is not engaged with the reference body 9 but rather it forms a cavity wherein the reference body 9 is arranged at a distance from the inner wall of the housing 13 and.