Pressure is a fundamental physical parameter that is expressed in units of force divided by area. The unit of pressure in the avoirdupois system is pounds per square inch, while the unit of pressure in the older metric system is the bar, and the unit of pressure in the modern SI system is pascals (equivalent to newtons per square meter). A chamber pressure measured with a copper crusher gauge would there be expressed as psi (CUP) in the English system, bar (CUP) in the metric system, and MPa (CUP) in the SI system.