What is an instrument? 

Essentially, an instrument is a technology used to measure and control a process variable known as an instrument. And it is a device that is used to measure, monitor, display, etc. a process variable. The basic process variables in an instrument are pressure, flow, level and temperature.

Pressure 

Generally pressure is defined as force per unit area and mathematically known as pressure = F/A. And the unit of pressure is PSI, BAR, PASCAL, TORR, Kg/cm 

Flow 

Usually flow is defined as the rate of change of differential pressure. And the unit of flow is liter/minute, gallon/hour, kg/hour. 

Level

Level is basically the difference between two heights. And the level unit is mm, meter, centimeter, MMWC (millimeter water column).