Which type of flowmeter uses a flat plate with a hole in it which causes a considerable loss of pressure downstream?

The flowmeter that uses a flat plate with a hole in it, leading to a significant loss of pressure downstream, is the orifice meter. The orifice plate is a simple device installed in a pipeline, and it features a hole that creates a restriction in the flow area. When fluid passes through the orifice, its velocity increases as it encounters the smaller diameter of the hole, resulting in a drop in pressure downstream, per Bernoulli’s principle. This principle of fluid dynamics explains how the pressure differential created by the orifice can be used to determine the flow rate of the fluid.

The orifice plate is widely utilized in various applications mainly because it is easy to manufacture and install and can offer a reliable means of measuring flow in both low and high flow conditions. However, it's important to note that while orifice meters are effective, they do introduce a considerable pressure loss, making them less efficient than other types of flowmeters in applications where maintaining pressure is critical.