There are many ingenious approaches to measuring liquid flow rates and today I’d like to focus on one of the liquid flow meters that depends on the Bernoulli principle: the orifice plate flow meter. But first a little background on the Bernoulli principle itself…
It’s something I’ve come across many times in my career, in very different fields. To sum it up very simply, the Bernoulli principle says that when the speed at which a liquid or gas is flowing increases, its pressure decreases.
The principle is used, for example, to raise aircraft off the ground. The profile of a wing is shaped so that air rushes quickly over the top surface but moves more slowly across the under surface. The difference in air pressure gives you the lift. A similarly shaped profile, but upside down, is used for the rear spoiler that pushes a racing car downward and helps it stick to the track.
In a very different area of engineering, the burrow system of a prairie dog includes raised ‘chimneys’ whose shape forces the wind to increase in speed as it flows over the openings. This reduces the air pressure, compared to the non-raised openings of the burrow, causing fresh air to be drawn through the passages for ventilation.
Going back to liquid flow meters, inventors have used various constriction devices to temporarily speed up the flow. By measuring the resulting pressure drop between the slower-flowing liquid and the flow in the constriction, and applying Bernoulli’s equation, it is possible to calculate the flow rate.
The most common types of liquid flow meter using this principle are the orifice plate, nozzle, Venturi nozzle and Venturi tube. The orifice plate liquid flow meter is particularly popular in many applications, largely because it is so simple and inexpensive to construct.
Essentially it consists of a thin plate with a hole in the middle, set within a pipe through which the liquid flows. As it passes through the hole, or orifice, the liquid is forced to increase in speed. The convergence of the liquid that the constriction causes reaches its maximum a short distance after the actual hole. This point is known as the ‘vena contracta’, and the pressure difference between here and the normal section of the pipe is used in the equation.