Primary pressure reference standards are used to measure and generate pressures to a high level of accuracy and repeatability for calibration purposes.
These instruments are used to ensure that the measurement uncertainty of all pressure instrumentation which are referred to the standard are kept within performance requirements.
For example water or mercury manometers which are liquid filled U tube columns, use the difference in height between columns to determine the weight (Force) of liquid. Since the diameter of the column is known the cross sectional area (Area) can be calculated and thus the pressure can be determined.
Another example is a Dead Weight Tester which uses a piston and cylinder of a known cross sectional area (Area) and weights of a known mass (Force) which are placed on top of the piston. Thus the pressure applied can be determined very accurately.
Since Primary Pressure Standards are the most accurate pressure measurement instruments available they are typically used as calibration instruments for pressure sensors and gauges.