Pressure-based flow meters constrict the flow of a gas or liquid into a narrow channel where its specific pressure can be measured. Using this pressure, the concentration of the liquid or gas and Bernoulli's principle, the meter can then derive the rate of flow. Some pressure-based flow meters dedicated specifically to gasses utilize multiple channels in order to measure flow in multiple dimensions.
Bucket-and-stopwatch flow meters are some of simplest means of flow measurement. These meters interrupt the normal flow of a liquid or gas in order to measure how long it takes to fill a container. Other mechanically based flow meters operate using turbines. The current in the pipe turns the turbine, and measuring the rate at which the turbine turns gives the rate of the flow.
If the substance being measured is capable of conduction (like water), its flow can be measured using a magnetic meter. By using the strength of current induced by the flow, one can determine how fast the charged liquid is moving. Alternatively, some flow meters use ultrasonic pulses similar to a speed detector: by measuring the flow velocity at two different points in time, one can determine a more accurate flow speed.
Optical flow meters measure flow using light, particularly for measuring gas flows. The gas being measured passes between two lasers, which determine the time a single particle takes to move between them. Measuring a larger number of particles, the meter is then able to determine the average flow speed of the gas.
Open channel flow meters operate using a non-constricted pipe and calculate the flow using a combination of depth and velocity. Some similar flow meters introduce dye or some additive to the flow and then measure its concentration after a specific period of time.