Introduction:
The dead weight gauge is a type of pressure measuring instrument used to accurately measure high pressures. It is a primary standard device and provides highly precise and accurate pressure measurements. The dead weight gauge operates on the principle of balancing a known force against an unknown force to determine the pressure.

Working Principle:
The working principle of a dead weight gauge is based on the concept of Pascal's law, which states that when pressure is applied to a confined fluid, the pressure is transmitted undiminished in all directions. The dead weight gauge consists of a piston-cylinder assembly, weights, and a base plate. The piston-cylinder assembly is filled with a fluid, typically oil or mercury, which is incompressible. The weights are placed on the piston, creating a known force.

Measurement of Pressure:
To measure pressure using a dead weight gauge, the following steps are followed:

1. Calibrate the Gauge: Before using the dead weight gauge, it needs to be calibrated. This involves determining the relationship between the applied force and the resulting pressure. The dead weight gauge is typically calibrated using a reference pressure source, such as a high precision pressure standard.

2. Set up the Gauge: Place the dead weight gauge on a stable surface and ensure it is level. Attach the appropriate weights to the piston to generate the desired force.

3. Apply Pressure: Connect the dead weight gauge to the system or device whose pressure needs to be measured. The pressure will act on the piston, compressing the fluid inside the piston-cylinder assembly.

4. Balance the Forces: Adjust the weights on the piston until the piston is in equilibrium and the piston and cylinder are floating freely. At this point, the force applied by the weights is equal to the force exerted by the pressure acting on the piston. The pressure can be calculated using the formula: Pressure = Force/Area.

Measurement Range:
The dead weight gauge is capable of measuring high pressures, typically ranging from a few hundred bar to several thousand bar. In the given options, option D states that the dead weight gauge is used for the measurement of pressure about 7000 bar. This means that the dead weight gauge is suitable for measuring pressures up to approximately 7000 bar.

Conclusion:
The dead weight gauge is a reliable and accurate instrument used for measuring high pressures. It operates on the principle of balancing forces and provides precise pressure measurements. In the given options, option D correctly states that the dead weight gauge is used for measuring pressures around 7000 bar.