A pitot tube is primarily used to measure:

Explanation:

Pitot Tube is a device used for calculating the velocity of flow at any point in a pipe or a channel. 

The pitot tube is used to measure velocity at a point.

In the question velocity at the stagnation point is given which is zero. So here stagnation pressure will be the correct answer. Because this stagnation pressure head is used to calculate the velocity at a point.

V = \(\sqrt{2gh}\)

It is based on the principle that if the velocity of flow at a point becomes zero the pressure there is increased due to the conversion of the kinetic energy into pressure energy. 

Working:

• The liquid flows up the tube and when equilibrium is attained the liquid reaches a height above the free surface of the water stream
• Since the static pressure under this situation is equal to the hydrostatic pressure due to its depth below the free surface the difference in level between the liquid in the glass tube and the free surface becomes the measure of dynamic pressure \(p_0-p=\frac{\rho V^2}{2} = h\rho g\)  where p0 p and V are the stagnation pressure static pressure and velocity respectively at point A
• Such a tube is known as a Pitot tube and provides one of the most accurate means of measuring the fluid velocity
• For an open stream of liquid with a free surface this single tube is sufficient to determine the velocity but for a fluid flowing through a closed duct the Pitot tube measures only the stagnation pressure and so the static pressure must be measured separately.

Mistake PointsIn the option velocity at the stagnation point is mentioned at the stagnation point velocity is already zero there is no need to measure velocity at the stagnation point. the pitot tube is used to measure velocity at any point by measuring the stagnation pressure.