Which of the following instruments is considered a positive displacement instrument?

A gear meter is considered a positive displacement instrument because it measures fluid flow by trapping a fixed volume of fluid and then displacing it through the meter. As the gears rotate, the space between the gears and the casing captures a specific volume of the fluid. This trapped volume is then moved downstream with each revolution of the gears. The number of gear rotations directly correlates to the volume of fluid that has passed through the meter, allowing for accurate flow measurement regardless of changes in fluid properties or flow conditions.

Positive displacement meters like gear meters are particularly useful in applications where precise flow measurement is crucial, such as in the oil and chemical industries. This characteristic of capturing and measuring a specific volume provides a high degree of accuracy across various flow rates and fluid viscosities.

Other types of flow measurement instruments, such as rotameters and magnetic flow meters, operate based on different principles. A rotameter measures flow based on the variable area principle, while a magnetic flow meter uses electromagnetic fields to measure the flow of conductive liquids. These methods do not rely on displacing a fixed volume of fluid, which distinguishes them from positive displacement instruments like the gear meter.