The present invention relates to the measurement of gas or liquid flow, and more particularly, but not exclusively, to a flow meter apparatus, for measurement of substance (say gas or liquid) flow through a conduit.
The measurement of liquid or gas flow under pressure in enclosed pipes has historically been performed through the use of mechanical flow meters.
Typically, in the mechanical flow meters, a first mechanism has moving parts which move upon interaction with a gas or a fluid flowing through a pipe, and the movement of the parts is mechanically transmitted to moving parts of a second mechanism used to register the amount of water or gas flowing through the pipe.
For example, piston meters, also known as rotary piston or semi-positive displacement meters, operate on the principle of a piston rotating within a chamber of known volume. For each rotation, an amount of water passes through the piston chamber. As the piston rotates, a needle dial and an odometer type display are advanced.
A turbine flow meter translates the mechanical action of the turbine rotating in a liquid flow around an axis into a user-readable rate of flow.
The turbine's wheel is set in the path of a fluid stream. The flowing fluid impinges the wheel's blades, imparting a force to the blades surfaces and setting the wheel in motion. When a steady rotation speed is reached, the rotation speed is proportional to fluid velocity.
Turbine flow meters are used for the measurement of both gas and liquid flow.
With turbine meters, the flow direction is generally straight through the meter, allowing for higher flow rates and less pressure loss than displacement-type meters.
Turbine meters have become the meters of choice for large commercial users, fire protection, and as master meters for water distribution systems.
A woltmann meter comprises a rotor with helical blades inserted axially in the flow, much like a ducted fan. Woltmann meters may be considered a type of turbine flow meter. Woltmann meters are commonly referred to as helix meters, and are popular at larger sizes.
A nutating disk meter is probably the most commonly used meter for measuring water supply.
With a nutating disk meter, the substance, most commonly water, enters in one side of the meter and strikes a nutating disk, which is eccentrically mounted. The disk must then nutate about the vertical axis, since the bottom and the top of the disk remain in contact with a mounting chamber. A partition separates the inlet and outlet chambers. As the disk nutates, it gives direct indication of the volume of the liquid that has passed through the meter as volumetric flow is indicated by a gearing and register arrangement, which is connected to the disk.
Some mechanical flow meters are rather pressure-based.
Pressure-based flow meters typically rely on Bernoulli's principle, either by measuring the differential pressure within a constriction, or by measuring static and stagnation pressures to derive the dynamic pressure.
For example, a Venturi meter constricts the flow in some fashion, and pressure sensors measure the differential pressure before and within the constriction. This method is widely used to measure flow rate in the transmission of gas through pipelines, and has been used since Roman Empire times.
Optical flow meters use light to determine flow rate.
In one example, small particles which accompany natural and industrial gases pass through two laser beams focused in a pipe by illuminating optics. Laser light is scattered when a particle crosses the first beam. The detecting optics collects scattered light on a photo detector, which then generates a pulse signal. If the same particle crosses the second beam, the detecting optics collect scattered light on a second photo detector, which converts the incoming light into a second electrical pulse. By measuring the time interval between the two pulses, the gas velocity may be calculated.
Another currently used flow meter is a magnetic flow meter in which a magnetic field is applied to a metering tube, which results in a potential difference proportional to the flow velocity perpendicular to the flux lines. The physical principle at work is Faraday's law of electromagnetic induction. The magnetic flow meter requires a conducting fluid, e.g. water, and an electrical insulating pipe surface, e.g. a rubber lined nonmagnetic steel tube.
Ultrasonic flow meters measure the difference of transit time of ultrasonic pulses propagating in and against flow direction. This time difference is a measure for the average velocity of the fluid along the path of the ultrasonic beam. By using the absolute transit times both the averaged fluid velocity and the speed of sound can be calculated, as known in the art.