Transit Time Meter
For measuring the flow of large pipes, transit time meters were created in the petroleum industry. They were successfully used in open channel flow for water measurement applications. Transmit time meters use ultrasonic waves are similar to Doppler meters or an https://coltraco.com/ultrasonic-flow-meter/, but instead of bouncing sound waves off water particles like a Doppler Meter, transmit time meters send ultrasonic waves between two sensors along the length the pipe. They then use the transmit times of the sound waves for water velocity computations. The speed of sound in water can be determined from the shift in ultrasonic transit times that is caused by the water’s velocity. Because of the complexity and number of sensors required, transmit time meters can be more expensive than Doppler flowmeters. The ability to cut the flow into horizontal sections and measure velocity across each section can make them more precise.
Common Flow Metering Methods for Full Pipe Applications
Venturi meters measure flow in pressure or full pipes. The Venturi effect uses a converging section to constrict flow. The continuity equation shows that the converging section has less cross sectional area and therefore, a higher velocity within the throat. The Bernoulli Principle and conservation of energy result in a higher throat velocity, which results in a lower pressure in the throat. The Bernoulli equation can be used to calculate flow by measuring the pressure drop at the converging section. Because of the possibility that pressure measurement ports in wastewater applications can become blocked, venturi meters are more commonly used in water metering.
Turbine Flow Meters
Turbine flow meters, a type of mechanical flow meter, use a rotating turbine to measure the flow in the stream. The turbine’s rotational speed is proportional to the flow rate. Turbine meters can only be used in water applications because of the possibility of wastewater solids clogging and blocking the turbine.
Magnetic flow meters are used to measure the flow through a pipe by applying a magnetic field. The magnetic field creates an electronic potential difference, thanks to Faraday’s Law and electromagnetic induction. This can be measured using electrode sensors. The electronic potential difference’s magnitude is proportional to water velocity. The continuity equation can then be used to calculate flow. Magnetic meters have one advantage: the metering section of the magnetic meter is the same size as the pipe adjacent, so there is no head loss. Magnetic meters are most commonly used for full pressure pipe applications. However, open-channel magnetic meters can be purchased.
There are many methods to measure flow. To help you choose the right flow metering type, it is essential to know the differences between flow measurement techniques.