Copper units of pressure or CUP, an 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 the copper crusher and lead crusher gauges respectively. This was necessitated 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. Pressure is a fundamental thermodynamic parameter that is expressed in units of force divided by area. In the avoirdupois system, the units of pressure are pounds per square inch and in the metric system, the units or pressure are newtons per square meter or pascals. A chamber pressure measured with a copper crusher gauge would be expressed as psi (CUP) in the English system or MPa (CUP) in the metric system.