Piezometers are instruments used for measuring fluid pressure in pipes. However, there are certain conditions under which a piezometer may not be suitable for pressure measurement. One such condition is when the fluid in the pipe is a gas.

Explanation:
- Fluid Pressure Measurement: Piezometers are commonly used in civil engineering and other industries to measure fluid pressure in pipes. They work based on the principle of fluid pressure exerting force on a liquid column, which is then measured using a pressure gauge.
- Pressure Difference: A piezometer can be used for pressure measurement in pipes even when the pressure difference is low. It is designed to accurately measure small pressure differences, making it suitable for a wide range of applications.
- Velocity: The velocity of the fluid in the pipe does not affect the usability of a piezometer for pressure measurement. Piezometers can be used to measure pressure in pipes with high-velocity flows without any issues.
- Highly Viscous Fluids: Piezometers can still be used for pressure measurement in pipes even when the fluid is highly viscous. The viscosity of the fluid does not affect the functioning of the piezometer as long as it can exert pressure on the liquid column inside the device.
- Gas as the Fluid: The main reason why a piezometer cannot be used for pressure measurement in pipes when the fluid is a gas is that gases are compressible. Unlike liquids, gases can be compressed, which means the pressure exerted by a gas can vary significantly with changes in volume. Piezometers are designed to measure pressure based on the force exerted by a liquid column, which is not applicable to gases. Therefore, the measurements obtained from a piezometer would not accurately represent the true pressure of the gas in the pipe.
- Alternative Instruments: For pressure measurement of gases in pipes, other instruments such as pressure transducers or manometers are used. These instruments are specifically designed to measure the pressure of gases, taking into account their compressibility. They provide accurate readings for gas pressure in pipes.
- Conclusion: In summary, a piezometer should not be used for pressure measurement in pipes when the fluid in the pipe is a gas. Gases are compressible, and piezometers are not designed to accurately measure the pressure of compressible fluids.