This Pakistan Standard applies to black polythene tube (type 425) for use in cold water services and in flush, overflow and warning and waste pipe applications.
1.2 The type designation (425) is an indication of the recommended maximum Working stress for The material in pounds per square inch at 20°C when in pipe form.
PS 391-1964 history
1964PS 391-1964 Polythene tube for cold water services (Low Density)