Copper units of pressure or CUP, and the related lead units of pressure or LUP, are terms applied to pressure measurements used in the field of internal ballistics for the estimation of chamber pressures in firearms. These terms were adopted by convention to indicate that the pressure values were measured by copper crusher and lead crusher gauges respectively. In recent years, they have been replaced by the adoption of more modern piezoelectric pressure gauges that more accurately measure chamber pressures and generally give significantly higher pressure values. This nomenclature was adopted to avoid confusion and the potentially dangerous interchange of pressure values and standards made by different types of pressure gauges. For example, it makes little sense to describe a maximum pressure as 300 MPa, and in case the pressure has been measured according to the CUP procedure it should be denoted as 300 MPa (CUP).

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.

 

Note:   The above text is excerpted from the Wikipedia article Copper units of pressure, which has been released under the GNU Free Documentation License.