Pressure and density variation in a stationary fluid

The pressure in a stationary fluid increases with depth. This variation is rather complicated in the case of air because the density also increases with depth. These variations are very important for aircraft flight and in the next chapter the changes of pressure, density and temperature with height are described in some detail.

In a liquid, matters are much simpler; the density remains almost constant, and the pressure change is directly proportional to the change in depth. In a liquid, the pressure variation is given by the simple expression:

Change in pressure = density X gravity constant X change in depth

Or: Ap = p X g X (change in depth)

This book is about the mechanics of aircraft flight, so you might wonder why we should have any interest in the way that pressure varies in a liquid. The answer is that it is because the easiest way to measure pressure is to use a U – shaped tube containing liquid. This is known as a U-tube manometer, and is described in the next chapter.