Pitot tubes are used to measure flow when a great accuracy is not required and in pipes and ducts of large Dimensions, where to use orifice flanges, flow nozzles or Venturi tubes would result unpractical or too expensive.
Pitot tubes are also used when the static pressure is very low.
Pitot tubes offer the following advantages: simple construction, low cost, easy of mounting in place and none or disregardable line pressure loss.
The tip of Pitot tubes is generally placed in the center of pipes, taking in due account the ratio between mean velocity of fluid in the duct and velocity in the centre, such ratio is normally between 0,83 to 0,85. This ratio is called traverse and will be included in the calculation to have correct dp.
For accurate measurement it is necessary to know the velocity and ratio exactly, this id done by the traverse method, measuring the velocity in different points on two traverse diameters at 90.
Besides, the straight smooth pipe lenght (without restrictions,widenings or bends) upstream of Pitot tube must be at least of 50 pipes diameters.
For Annubar flow elements can be reduced down to 8 times the pipe diameter or less.

Pitot tube factor "Kp"
The pitot tube factor (kp) is the ratio between the average fluid velocity and the maximum velocity at the Point of measurement.
The ratio is:

Kp² =hm/hM
hm = Average dp
Hm = Maximum dp

The pitot tube factor is constant if the flow is in turbulent condition.
The exact value of Kp is obtained from experimental calibration test

Amplification factor Z
Amplification factor Z is a fuction of Kp. Exactly it is the reciprocal of Kp².

Z = 1/Kp²

Z is another constant and depend from the fluid and from Pitot tube characteristics