Determination of basis weight of a material web using a microwave sensor, whereby the distance between the material web and the surfaces of the microwave sensor is being adjusted to a constant value by air cushions

A method for determining the basis weight of a moving material web, the method comprising the steps of:

• providing at least one microwave sensor including a coupling element configured for coupling microwaves and a reference element, said coupling element and said reference element being parallel to each other and located at a distance from each other such that the moving material web moves therebetween, at least one of said coupling element, said reference element, and said at least one microwave sensor configured to be movable;
• measuring the moving material web;
• adjusting distances of at least one of said coupling element, said reference element, and said at least one microwave sensor to a value for or during said measuring step;
• and
• determining the basis weight through a resonance curve relative to a set vertical distance between said coupling element and said reference element.