The measurements of mechanical stress and pressure are among the most frequently performed measurements. The force per unit area is called the pressure of a fluid if the fluid is in contact with a boundary. The stress in a solid body is usually considered to be caused by a force applied in a certain direction and with a defined sense (e. g., tensile/negative stresses, compressive/positive stresses, torsional stresses). Mechanical stress in a solid body is the relation of force applied to the body in a specified direction to the cross-sectional area of the solid body in this direction. The measurements of mechanical stress or pressure may also be indirect measurements performed in order to determine a force or a mass. The unit of mechanical stress and pressure in the SI system is the pascal: 1 Pa = 1 N/m ². There is another pressure unit used frequently in American or British technical literature: pounds per square inch (psi): 1 psi = 6,895 Pa. The value of atmospheric pressure is also measured in other units such as standard atmosphere [atm], technical atmosphere [at], bar [bar], millimeter of mercury [mmHg], and inch of mercury [inHg]. There are numerical relations between these units: 1 bar = 10 ⁵ Pa, 1 atm = 101,325 Pa, 1 at = 98,066.5 Pa, 1 mmHg = 133.3224 Pa, and 1 inHg = 3,386.389 Pa.