A Manometer is a device to measure pressures. A common simple manometer consists of a U shaped tube of glass filled with some liquid. Typically the liquid is mercury because of its high density.
Manometers have a long history in the area of fluid mechanical measurements. They essentially consist of a `U’ shaped glass tube, which is filled with some liquid, typically oil, water, or mercury. At its simplest, one end of the U tube will be open to the atmosphere, while the other will be connected to whatever it is that one wishes to measure the pressure of, say a pressurized tank of gas. Manometers can also be used to make pressure measurements of liquid flows. In this case, obviously, it is important that the manometer fluid be different (and heavier!) than the liquid being measured.
If there is a pressure difference between the ends of the manometer, the liquid moves until the pressure difference is balanced by the difference in heights of the ends of the liquid.
The greater the pressure the greater the difference in heights.
Oil is often used rather than water because water evaporates and also oil is less dense which makes the manometer more sensitive.
Pressure difference between two regions is given by
ΔP=d g Δh
This is basically the same equation as P=d g h