Which method is NOT used to measure flow rate in a system?

A thermocouple measures temperature, not flow rate. It operates based on the principle of thermoelectricity, where a voltage is generated in a circuit made of two different metals exposed to a temperature differential. While temperature readings can be important in many systems, they do not provide a direct measurement of how much fluid is moving through a system or its flow rate.

In contrast, the other methods listed are specifically designed for measuring flow rate. A Venturi meter uses the principle of fluid dynamics where the velocity of a fluid increases as it passes through a constricted section of pipe, resulting in a pressure drop that can be measured to calculate flow rate. A magnetic flow meter works by applying a magnetic field to the fluid and measuring the voltage generated as it flows, which is directly proportional to the flow rate. An orifice plate creates a pressure differential as fluid passes through an opening, allowing flow rate calculations based on the pressure drop before and after the orifice. Each of these methods effectively provides a means to quantify how much fluid is moving in a system, making them suitable for flow measurement.