The Venturi has a rather smooth entry and exit which produces a low ∆P (Delta-P)

It characteristically exhibits less pressure loss than orifice plates of a similar size. However it is normally more costly to build a Venturi than an orifice plate and once it is set in place (installed) it cannot easily be replaced.  If the flow velocity has significantly decreased or increased so that it is “out of range” it may be inconvenient to replace it.

The advantages over an orifice plate design is that there is a large throat compare to a small opening and there is much less change of foreign material building up at the constriction. Normally the foreign material will simply pass through it.

Flow nozzles are different in that they have a smooth entrance/entry and a sharper exit.  It has an advantage over orifice plates in that it can handle abrasive liquids much better. The total pressure loss is also less than that of an orifice.

Flow nozzles are popular in steam services because of their “solidness” and are often used in applications that might be a high velocity and temperature impractical for an orifice plate

One aspect of flow nozzles that appeals to certain engineers is that in compressible flows they are predictable and this is a reason why they might be used to test or check other gas meters.

Pitot Tube Fig 1.2

1. total pressure tap
2. static pressure port

The pitot tube is normally viewed as a spot velocity measurement device. (See pitot tube fig 1.2 a classic example)

As a fluid hits the open port (a) its velocity is reduced and kinetic energy is converted to potential energy (pressure/head) Thus the differential pressure can be measured i.e. static pressure vs impact port pressure.  Using mathematical calculations including the diameter of the pipe /cross sectional area, the flow volume can be calculated. The great advantage of the pitot tube is that it has a negligible effect on pressure drop. However, the impact port must be installed in the pipe where the velocity is equal to the average velocity of the fluid flowing through the  pipe.  The pitot tube can also be designed in a way that the static port is integral to the tube  . In MULTIPORT averaging pitot tubes, the impact pressures from all the ports are averaged to give a more accurate flow rate.