ANS:D - very low pressure.

Piezometers are primarily used to measure:

• Pressure in water channels, pipes, etc.

Piezometers are simple devices used to measure the pressure at a specific point in a fluid (liquid or gas). They consist of a small tube or pipe inserted into the fluid at the point where pressure measurement is desired. The pressure of the fluid causes it to rise in the tube to a height proportional to the pressure. While piezometers are primarily used to measure pressure in water channels, pipes, and other fluid systems, they can indirectly measure the difference in pressure at two points by comparing the heights of fluid columns in separate piezometers. Piezometers are not typically used to measure atmospheric pressure or very low pressure. For atmospheric pressure measurement, barometers are more suitable devices, while extremely low pressures may require specialized equipment such as vacuum gauges. Therefore, the correct statement is:

• Pressure in water channels, pipes, etc.