and the side walls of the sensor housing the housing is also closed off at the bottom towards the stator of the refiner or segment holder if such is used by a lid 11 the seal 16 is of a particularly suitable somewhat resilient material e g rubber so that it can permit the small movements that the shearing forces give rise to in the measuring surface and still provide a good seal preventing steam and pulp from penetrating into the device the seal preferably also has a dampening effect on inter alia the vibrations occurring during operation in this context it may be mentioned that the load can vary considerably over the refining zone from values in the order of 20n to in the order of 150n for instance in the present case at an estimated mean value of approximately 40n displacements of the measuring surface that can be measured in the order of hundredths of a millimeter are obtained fig5 and 6 illustrate a second embodiment of the present invention in which compensation can take place for the steam pressure that exists in the refiner and which constitutes a part of the normal force pressure on the measuring surface that is measured with the measuring device in accordance with the first embodiment as mentioned earlier the normal force f n which affects the measuring surface comprises both the force from the fiber pressure f fib exerted by the fiber.