Flow measurement

Flow measurement is the quantification of bulk fluid or gas movement. It can be measured in a variety of ways.

Dependent on the quantity measured different symbols are used. The volumetric flow rate is usually given the symbol $Q$ and the mass flow rate the symbol $\dot m$ .

Units of measurement

Volumetric flow rate is sometimes measured in "standard cubic centimeters per minute" (abbreviation sccm), a unit acceptable for use with SI except that the additional information attached to the unit symbol. The SI standard would be m³/s (with any appropriate prefix, with temperature and pressure specified). The term "standard" indicates that the given flow rate assumes a standard temperature and pressure. Many other similar abbreviations are also in use, such as standard cubic feet per minute or per second. Other units used include gallons (U.S. liquid or imperial) per minute, liters per second, bushels per minute, and acre-feet per day.

Mechanical flow meters

There are three main types of mechanical meter:-

>Piston Meter >Woltmann Meter >Jet Meter.

Piston meters, or Semi-Positive displacement meters are the most common in the UK and are used for almost all meter sizes up to and including 40mm (1 1/2"). The piston meter operates on the principle of a piston rotating within a chamber of known volume. For each rotation, an amount of water passed through the piston chamber.

Woltman meters, commonly referred to as Helix meters are popular at larger sizes. Jet meters (single or Multi-Jet) are increasing in popularity in the UK at larger sizes and are commonplace in the EU, largely due to the inferior quality of drinking water compared to the UK.

Another method of measurement, known as a venturi meter, is to constrict the flow in some fashion, and measure the differential pressure that results across 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.

Another simple method of measurement uses an orifice plate, which is basically a plate with a hole through it. It is placed in the flow and constricts the flow. It uses the same principle as the venturi meter in that the differential pressure relates to the velocity of the fluid flow (Bernoulli's equation).

A variation on this method is to measure the pressure changes that are created by the vortices that are caused by the flow passing an object. It is usually measured with a quartz crystal. This kind of flow-meter is called a vortex-meter.

Measurement of the pressure within a pitot tube in the flowing fluid, or the cooling of a heated element by the passing fluid are two other methods that are used. These types of sensors are advantageous in that they are rugged, so not easily damaged in an extreme environment.

A pitot tube is an L shaped tube which is also able to measure fluid flow. An advantage is that it does not disturb the flow as much as a venturi meter or an orifice plate would. It works by measuring the difference between the static pressure and the dynamic pressure.

Magnetic, ultrasound and coriolis flow meters

Modern innovations in the measurement of flow rate incorporate electronic devices that can correct for varying pressure and temperature (i.e. density) conditions, non-linearities, and for the characteristics of the fluid.

Magnetic flow meters

The most common flowmeter a part from the mechanical flow meters, is the magnetic flow meter. A magnetic field is applied to the metering tube, which results in a potential difference proportional to the flow velocity perpendicular to the flux lines. The physical principle at work is electromagnetic induction. The magnetic flow meter requires a conducting fluid, e.g. water, and an electrical insulating pipe surface, e.g. a rubber lined steel tube.

Ultrasonic flow meters

Ultrasonic flowmeters measure the difference of the propagation time (transit time) of ultrasonic pulses propagating in (normally an inclination angle around 30 to 45° is used) flow direction and against the flow direction. This time difference is a measure for the averaged 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. Using the two transit times

t_{up}

and

t_{down}

and the distance between receiving and transmitting transducers

L

and the inclination angle

\alpha

one can write the equations

v = \frac{L}\;\frac

and

c = \frac{L}{2}\;\frac

, where

v

is the average velocity of the fluid along the sound path and

c

is the speed of sound.

Measurement of the doppler shift resulting in reflecting an ultrasonic beam off the flowing fluid is another recent, accurate innovation made possible by electronics.

Coriolis flow meters

Using the Coriolis effect that causes a laterally vibrating tube to distort, a direct measurement of mass flow can be obtained in a Coriolis flow meter. Futhermore a direct measure of the density of the fluid is obtained. Coriolis measurement can be very accurate and is very insensitive to variations in the medium that is measured, the same measurement tube can be used for measuring anything between Hydrogen gas and for instance Peanut butter without recalibration.

External links

Fluid dynamics | Measurement

Durchflusssensor | Caudalímetro | Fluimetro

Gourt Home::Gourt :: Flow measurement