Flow measurement

Permanent magnets in the rotating gears can transmit a signal to an electric reed switch or current transducer for flow measurement.

The fluid, most commonly water, enters in one side of the meter and strikes the nutating disk, which is eccentrically mounted.

Strainers are generally required to be installed in front of the meter to protect the measuring element from gravel or other debris that could enter the water distribution system.

They are often approved by Underwriters Laboratories (UL) or Factory Mutual (FM) or similar authorities for use in fire protection.

As the magnets in the blades spin past the sensor, the paddle wheel meter generates a frequency and voltage signal which is proportional to the flow rate.

The original Pelton wheels were used for the generation of power and consisted of a radial flow turbine with "reaction cups" which not only move with the force of the water on the face but return the flow in opposite direction using this change of fluid direction to further increase the efficiency of the turbine.

This method is widely used to measure flow rate in the transmission of gas through pipelines, and has been used since Roman Empire times.

[7][8] The Dall tube is a shortened version of a Venturi meter, with a lower pressure drop than an orifice plate.

As with these flowmeters the flow rate in a Dall tube is determined by measuring the pressure drop caused by restriction in the conduit.

A typical averaging pitot tube consists of three or more holes (depending on the type of probe) on the measuring tip arranged in a specific pattern.

These flowmeters are particularly sensitive to temperature changes affecting the fluid viscosity and the diameter of the flow element, as can be seen in the governing Hagen–Poiseuille equation.

Small particles which accompany natural and industrial gases pass through two laser beams focused a short distance apart in the flow path in a pipe by illuminating optics.

Optical flowmeters are very stable with no moving parts and deliver a highly repeatable measurement over the life of the product.

Because distance between the two laser sheets does not change, optical flowmeters do not require periodic calibration after their initial commissioning.

The level of the water is measured at a designated point behind weir or in flume using various secondary devices (bubblers, ultrasonic, float, and differential pressure are common methods).

Acoustic Doppler velocimetry (ADV) is designed to record instantaneous velocity components at a single point with a relatively high frequency.

If the density and specific heat characteristics of the fluid are constant, the meter can provide a direct mass flow readout, and does not need any additional pressure temperature compensation over their specified range.

For more aggressive gases, the meter may be made out of special alloys (e.g. Hastelloy), and pre-drying the gas also helps to minimize corrosion.

In either case, the vehicle's electronic control unit interprets the sensor signals as a real-time indication of an engine's fuel requirement.

Another method of flow measurement involves placing a bluff body (called a shedder bar) in the path of the fluid.

This sensor is often a piezoelectric crystal, which produces a small, but measurable, voltage pulse every time a vortex is created.

Since the frequency of such a voltage pulse is also proportional to the fluid velocity, a volumetric flow rate is calculated using the cross-sectional area of the flowmeter.

Sonar flowmeters have been widely adopted in mining, metals processing, and upstream oil and gas industries where traditional technologies have certain limitations due to their tolerance to various flow regimes and turn down ratios.

The applied magnetic field is pulsed, which allows the flowmeter to cancel out the effect of stray voltage in the piping system.

An LFF measures the integrated or bulk Lorentz force resulting from the interaction between a liquid metal in motion and an applied magnetic field.

With wide-beam illumination transit time ultrasound can also be used to measure volume flow independent of the cross-sectional area of the vessel or tube.

For the Doppler principle to work, there must be a high enough density of sonically reflective materials such as solid particles or air bubbles suspended in the fluid.

This is in direct contrast to an ultrasonic transit time flowmeter, where bubbles and solid particles reduce the accuracy of the measurement.

Analysis of this shifted wavelength can be used to directly, and with great precision, determine the speed of the particle and thus a close approximation of the fluid velocity.

In one general LDV class, the two beams are made to intersect at their focal points where they interfere and generate a set of straight fringes.