Example: Determine the uncertainty in velocity as a function of uncertainty in the head when a pitot static tube is used for air velocity measurement along with an inclined tube manometer. Here, is the displacement of the manometric liquid along the tube. The governing equation is

where is the angle at which the arm of the manometer is inclined with respect to the horizontal and the density of the manometric liquid. We assume and to be precisely known and constant. Hence

Since is small, any uncertainty in its value will lead to large values of . Hence in practice is not used as a measured quantity for determining velocity. Instead the manometer is calibrated inclusive of and is obtained as