thickness of e g about one mm due to the light color of the material of the cap 8 the state of consumption will be readily visible for a possible replacement after corresponding exposure to dust or environmental effects the porous hydrophobic material of the cap 8 especially ptfe or hydrophobized pe ensures that no moisture will penetrate into the gas sensor or the metal body 7 and it will not consequently damage these and compromise their measuring function a calibrating adapter 9 see fig3 which is screwed on the gas sensor and is manufactured for example from a glass fiber reinforced plastic such as polyacryl is located above the metal body 7 with the cap 8 the calibrating adapter 9 is manufactured with perforations or slots openings in order to hinder the diffusion of the gas from the environment through the surface of the cap 8 as little as possible and it has at the upper end a gas admission pipe connection 10 for connecting a feed line for a calibrating gas from a pressurized gas storage unit for example from a pressurized calibrating gas cylinder the calibrating adapter 9 preferably remains on the gas sensor both during the measurement and during the calibration so that if the calibrating gas storage unit is connected to the gas admission pipe connection 10 remote calibration is readily possible if needed by opening the calibrating gas storage unit at any rate without an assembly being needed as before for the changeover this is especially advantageous in case of poorly accessible measuring sites the flow resistance of the porous material of the cap 8 is selected by selecting the layer thickness and or the porosity such that the.