Method and apparatus of measuring properties of a moving sheet

An apparatus for measuring properties of a moving cellulose, paper or board sheet, wherein the apparatus comprises

• a resonator sensor configured to measure at least one first parameter of a first resonance caused by the moving sheet in a frequency range 1 GHz to 25 GHz of electromagnetic radiation;
• a transceiver sensor comprising at least a pair of transceiver parts located on opposite sides of the moving sheet configured to measure at least one second parameter of an electromagnetic signal transmitted between the transceiver parts through the moving sheet in a frequency range 25 GHz to 1000 GHz, the at least one first parameter and the at least one second parameter being affected by dry stuff content and a weight of water per unit of area of the moving sheet in a different manner due to dielectric properties of water in the moving sheet, a minimum difference between the frequency ranges related to the first parameter and the second parameter being at least 5 GHz;
• a processing unit configured to determine both the dry stuff content and the weight of water per unit of area on the basis of the at least one first parameter, the at least one second parameter and available information related to a distance travelled by the electromagnetic signal for measuring the at least one second parameter.