Air packed　in an air-bag is put between two contacted matrials,and air pressure generated innside that air-bag is measured.
Air pressure indicated represents the force of contact surface from which the force absorbed by ductility of the material has been deducted.
The correctness of that measured value can be certified by the water
pressure calibration method.
By changing water temperature during calibration, temperature dependence
of the system can also be confirmed. The error has been found
to be extremely small, namely, within 10kPa at45℃.
*The reason why calibration is made by water
The surface of soft materials has a ductility and their surface shape is
always changing when solid materials are in contact, thereby a
resistance is generated due to staggering between two contacting materials.
Such resistance does, on some occasions, not have a reproducibility
in data due to friction, adhering force produced by moisture, or to statical electricity.
Liquid is therefore used for the calibration because it does not produce
a contact pressure and is always subject to an equal force.