Describe the function of differential pressure flow meters.

Differential pressure flow meters are designed to measure the flow rate of a fluid by assessing the pressure drop that occurs across a flow obstruction, such as an orifice plate, venturi tube, or nozzle. When fluid flows through these obstructions, the velocity of the fluid increases, causing a decrease in pressure according to Bernoulli's principle. The differential pressure created between two points – before and after the obstruction – is directly related to the fluid's flow rate.

This relationship allows the flow meter to calculate how much fluid is passing through the system based on the pressure difference detected. By utilizing equations derived from fluid dynamics, particularly the principle of conservation of mass and energy, engineers can determine the flow rate accurately.

The other options do not accurately describe the primary function of differential pressure flow meters. While measuring fluid velocity or density may be related to flow, those tasks are not the specific or direct function of these particular devices. Assessing temperature changes is also not relevant to the operation of differential pressure flow meters, which focus on pressure differential rather than thermal properties.