A small tube with variable internal diameter and barbs for demonstrating Bernoulli's principle, as well as for giving a brief description of head loss. 

The barbs are meant to fit standard airline tubing for aquarium air pumps, with the tubing inner diameter being 3/16 of an inch. 

Bernoulli's equation for an incompressible flow at any point along a streamline is as follows:

1/2*v^2 + g*z + p/ρ = constant 

Where:

v=velocity 

g=acceleration from gravity 

z=change in height

p=pressure

ρ=density

By increasing or decreasing the diameter of the flow and thus changing the fluid velocity due to mass conservation, the relative pressure drops. This phenomena is also known as the venturi effect, though a more traditional venturi nozzle would be used to measure the pressure drop over a single constriction. With the opposite being done here over multiple different diameters, the flow is most likely turbulent and so a noticeable loss occurs due to friction and bends in the model at the intake. This can also be a point of discussion. 

Link to relevant wiki articles: 

https://en.wikipedia.org/wiki/Bernoulli%27s_principle

https://en.wikipedia.org/wiki/Venturi_effect

https://en.wikipedia.org/wiki/Hydraulic_head

OPERATION: Choose an intake side, apply tubing to it and all the barbs. Fix model to a surface perpindicular to another surface that a measuring instrument can be applied to. The intake tube plus the water level of the reservoir should be below the level of the tubes being used to measure pressure to ensure there is no overflow. The reservoir should be open to the air. A clip or fold in the tube can serve as a valve. If a constant water level is desired in the reservoir, another closed inverted container filled with water with the mouth at the height of the desired water level can be used to maintain constant input pressure.

The pressure at each point relative to atmosphere can be found using the height of the water in that column (tube) multiplied by the density of the fluid and the force of gravity. (ρ*g*h)

The internal diameters of each section are as follows:

Intake/exit: 3 mm

Smaller mid pipe diameter: 7.5 mm

Larger mid pipe diameter: 15 mm