Apparatus for detecting a gas in a headspace of a container

A transmitter-receiver arrangement for an apparatus for measuring a concentration of a first gas and/or for measuring a pressure in a headspace of a container, the transmitter-receiver arrangement defining a measuring zone for accommodating the headspace, and the transmitter-receiver arrangement comprising:

• a transmitter configured to emit electromagnetic radiation covering a wavelength range including an absorption line of the first gas;
• a receiver configured to receive electromagnetic radiation in the wavelength range, the receiver and the transmitter being oriented with respect to each other such as to define a radiation path for the electromagnetic radiation from the transmitter across the measuring zone to the receiver;
• a fixating element for fixing the transmitter-receiver arrangement to the apparatus or to at least one of a filling and a closing machine;
• an electrically controllable actuator arrangement configured to cause at least one of:
• a common translatory displacement and/or rotational displacement of the transmitter and of the receiver relative to the fixating element, and
• a mutual relative translatory displacement and/or rotational displacement of the transmitter relative to the receiver;
• and
• a casing defining an interior space and having an outer surface towards an exterior space, wherein
• the exterior space includes the measuring zone,
• the electrically controllable actuator arrangement is located in the interior space, and
• the outer surface has an arithmetic average roughness of less than or equal to 1.6 micrometers.