Write a short note on Pitot tube.

-It is a device used for measuring the velocity of flow at any point in pipe or channel.

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

-The pilot tube consist of the glass tube, bent at right angle as shown.

-The liquid rises up in the tube due to the conversion of KE into pressure energy.

-The velocity is determined by measuring the rise of liquid in the tube.

-Consider point 1 and 2 as shown in fig.

Let $P_1=$intensity of pressure at point 1

$P_2=$pressure at point 2

$v_1=$velocity of flow at 1

$v_2=$velocity of flow at 2

H = depth of tube in liquid

h = rise of liquid

Applying Bernoulis equation at point (1) and (2)

$\frac{P_1}{\rho g}+\frac{v_1^2}{2g}+z_1=\frac{P_2}{\rho g}+\frac{v_2^2}{2g}+z_2$

but $z_1=z_2$ and $v_2=0$

$\therefore \frac{P_1}{\rho g}=\text{pressure head at 1=H}$

$\therefore \frac{P_2}{\rho g}=\text{pressure head at 2=(h+H)}$

Substituting we get

$H+\frac{v_1^2}{2g}=h+H$

$\therefore h=\frac{v_1^2}{2g}$ or $v_1=\sqrt{2gh}$

This is theoretical and actual velocity is given by

$v_{1act}=C_v\sqrt{2gh}$

$C_v=\text{coefficient of pitot tube}$

$\therefore$ velocity at any point

$v=C_v\sqrt{2gh}$