To measure very low pressure, we use

ANS:C - manometers

Manometers are devices used to measure the pressure of a fluid by balancing it against a column of another fluid, typically a liquid such as mercury or water. They work on the principle of hydrostatic equilibrium, where the pressure exerted by the fluid being measured is balanced by the weight of the column of the reference fluid. There are several types of manometers, including:

1. U-tube Manometer: This is the simplest form of manometer and consists of a U-shaped tube partially filled with a liquid. One end of the tube is connected to the source of pressure being measured, while the other end is open to the atmosphere. The difference in liquid levels in the two arms of the U-tube provides a measure of the pressure difference between the fluid being measured and the atmosphere.
2. Inclined Manometer: In this type of manometer, the U-tube is inclined at an angle to the vertical. This allows for more precise measurements of small pressure differences because the liquid column moves over a greater distance for a given pressure change.
3. Differential Manometer: A differential manometer compares the pressures of two different fluids or two points in a fluid system. It consists of a U-tube with one end connected to one pressure source and the other end to the second pressure source or point. The difference in liquid levels in the two arms of the U-tube indicates the pressure difference between the two points.
4. Well-Type Manometer: In this type of manometer, the tube is bent into a U-shape with one end submerged in a well containing the fluid whose pressure is being measured. The difference in liquid levels between the well and the other end of the tube provides the pressure measurement.

Manometers are widely used in various industries and applications, including HVAC systems, hydraulic systems, and process industries, for measuring pressure in pipes, vessels, and other equipment. They offer a simple and cost-effective method for measuring pressure accurately.