Question

How compressability of a substance is measured and which is the instrument used to measure it ans in which unit it is measured?

Answer 1

Compressibility is defined as a measure of the relative volume change of a fluid or solid as a response to a pressure (or mean stress) change. Compressibility is the reciprocal of bulk modulus and SI unit of Bulk modulus is Pascal. So the SI unit of compressibility is inverse Pascal i.e. PA-1.
It is measured by an instrument called Dilatometer.