The working of Ultrasonic flow measurement system by measuring phase difference is shown in the figure below. The two peizo-crystals p1 and p2 working both as transmitter and receiver of signals alternatively are mounted conveniently, so that the ultrasonic signals are transmitted between them as well as through the liquid. Switch ‘sw’ is utilized to supply p1 and p2 alternately from an oscillator simultaneously connecting the detector to p2 and p1 respectively. The detector is designed to measure the transit time from upstream to downstream and vice versa via phase shift measurement. If C is the velocity of the ultrasonic wave and v is the fluid velocity, then for a distance b between the crystals pa and p2, the phase shifts between the two cases are

Pd1 = wb/(C + v) and

Pd2 = wb/(C – v)

Phase shift difference is proportional to the liquid flow rate.

Pd2 – Pd1 = 2wvb/ {(C-v)(C+v)} = 2wvb/C²

Ultrasonic Flow Measurement using Phase Difference

The measurement through this method using the phase shift method provides the instrument good linearity. The dynamic response is limited by the switching frequency. Material of the flow channel should be chosen not to allow acoustic transmission. Plastic is often chosen for the purpose.

The phase measurement flow system has also been used with interdigital transducers. Acoustic wave is transmitted and received across the flow line by two output transducers, one upstream and anther downstream equidistant from the transmitter. The transmitter has uniformly spaced metallic fingers for radiating the sound wave into the liquid from a solid-liquid boundary without focusing on either direction. Some transducers can act as receivers as well.

The working of Ultrasonic flow measurement system by measuring frequency difference is shown in the figure below. t/2 is the time over which the signal is received and is also the time over which the signal is not received by the receiver when the trigger works. Two sets of piezo-crystals, with the set consisting of a transmitter and a receiver, are mounted as shown in the figure and wave trains are sent at an angle a with the liquid flow direction The systems are in part, closed loop so that the receiver picks up the wave train, detects and sends it to the computing part of the amplifier. The amplified signal is used to retrigger the generator to send the second train of pulses. The repetition frequency will depend on the transmit time of the wave train across the length l. Thus, the frequencies can be written by the equations,

f1 = (C + vCosa)/2l

f2 = (C – vCosa)/2l

The frequency difference, f1 – f2 = vCosa/l 

Ultrasonic Flow Measurement using Frequency-Difference

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The parameter used for the measurement of flow is the speed of flow, and is measured in terms of the potential difference induced when the moves in a tubing/pipe with a transverse magnetic field impressed. The working of an electromagnetic flowmeter can be understood from the figure below.

Electromagnetic Flowmeter

Suppose the flow of liquid whose conductivity is atleast about 10^-5 S/cm and is flowing through an insulating tube of diameter‘d’ and is placed in a magnetic field of flux density ‘B’, then to a pair of electrodes mounted perpendicular both to the magnetic field and fluid, as shown in the figure above, a voltage ‘e’ is induced which is dependent on the flux density and liquid flowing velocity v, according to Faraday’s law.

Thus,

E = -B.d.v * 10^-8 V

The equation written above can also be obtained in a generalized way through Lorentz force which deflects the charge carriers in the fluid as the conductor in the magnetic field. The force F is given as

F = c. (v*B)                                                                                          c – Charge

 An insulating pipe is usually chosen to avoid short-circuiting and a non-magnetic pipe is selected for allowing the magnetic field to penetrate through the liquid. A plastic tube or a metal tube lined with neoprene is well suited. Some other materials that are used for lining are Teflon (PTFE), polyurethane, ceramic (99.5% Al₂O₃) base, and polysulphate. These are chosen also on the basis of process fluids, their corrosivity, temperature, and so on. The electrodes are usually of stainless steel mounted flush with the inside face of the tube. Titanium, tantalum, hastelloy have also been used.

The plane of the measuring electrodes is generally chosen to be vertical such that the formation of air bubbles or deposition of non-conducting solids does not break the electrical circuit.

As the measured quantity is in volts and since its magnitude is small, parasitic effects are to be carefully considered. Suitable amplifiers are necessary to amplify the signal. The input impedance of the amplifiers are to be properly selected since the liquid resistance between the electrodes for small conductivity liquids are larger than usual.

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Generally flow is measured in two ways , volumetric basis and on the basis of weight .The flow of solids are usually measured in terms of mass per unit time or weight per unit time. Liquid flow is measured volumetrically or in the basis of weight. Gaseous flow is normally measured volumetrically.

When we are dealing with flow meters there are two terms called ‘turn down’ and ‘rangeability’. Turn down is defined as the ratio of full-scale flow to the minimum flow, which can be measured within a stated accuracy. If the turn down is in the ratio of 20:1, it means that the flow meter can measure from 20 per cent to 100 percent of the scale. This gives the accuracy of the meter. Rangeablity is the ratio of maximum to minimum range to which the meter can be calibrated (Refer: Calibration of flow meters ) 

There are many techniques used for flow measurement (Refer: How to Select a Flow meter).  Let us now look at some of the flow meters used in the industry.

1) Mechanical type flow meters

• Piston Meters

Piston meters  or the rotary piston is semi positive displacement meter  consists of a rotating piston in a chamber whose volume is known. They are used for domestic water measurement

• Variable area meter

The variable area meter or the rotameter  is available for a wide range of liquids but are commonly used for measurement of air and water

• Turbine flow meter

The turbine flow meter converts the rotating of the turbine into a human readable scale and to the display

• Single jet meter

It consists of an impeller with radial vanes which is impinged with a single jet

• Woltmann meter

A rotor and helical blades are inserted axially into the flow   in a Woltmann meter . it is considered as a turbine flow meter

• Paddle wheel meter

Similar to single jet meter except the fact that the impeller will be small .

• Current meter

It is used to determine the flow though a large structure like a penstock in a hydro electric plant . the measurement is done by averaging the flow velocity over a large area

• Nutating disc meter

Here a nutating disc which is ergonomically mounted is used to determine the fluid flow. Nutating disc meter is usually used in the measurement of water supply

• Pelton wheel

A pelton wheel turbine which is also known as the radial turbine converts the mechanical action of the pelton wheel rotating in a liquid to user readable form

• Multiple jet meter

Multiple jet meter is a velocity type meter which works similar to a single jet meter except the fact that here the flow is directed equally to the impeller by the ports

• Oval gear meter

it is a positive displacement meter which is used to measure the flow

• Inferential meter

Inferential meter reduces the volumetric flow by measuring some properties of the  liquid

2) Pressure based flow meters

• Orifice plate
• Venturi meter
• Dall tube
• Pitot tube
• Multi hole pressure probe
• Differential pressure transmitters

3) Optical flow meters

4) Open channel flow measurement

• Level to flow measurement
• Area/velocity measurement
• Dye testing
• Acoustic  Doppler velocimetry

5) Thermal flow meters

6) Vortex flow meters
7) Electromagnetic flow meters
8) Ultrasonic flow meters
9) Mass flow meters

• Angular momentum mass flow meter –
• Corioles mass flow meters
• Thermal mass flow

10) Laser Doppler flow measurement

Calibration of flow meters

Flow meters are considered to be pretty accurate and in ideal case they are not affected by its environmental conditions.  But in practical case we have to consider various environmental factors too. We can see that due to improper installation and other factors industrial flow measurements are often prone to errors for avoiding these errors we have to calibrate the flow meters. Usually in situ methods are employed for calibrating flow meters.

How to Select a Flow meter

You know a flow meter is an instrument used by measure the linear or non linear mass flow rate in gases or liquids. Flow meters are classified into different basis according to the method used to measure the rate of flow.  All these types of flow meters have their own merits and demerits. For selecting the  best flow meter we must  consider many aspects like the process conditions, turndown requirements, accuracy, installation requirement and so on.  We are giving some guide lines which will help you to select the best flow meter for your requirement

1. First enquire about the type of flowmeter  which has been  used in similar application before. This is the simple and most popular method. There are many manufactures around the planet making flow meters are they will be having websites. In the product specifications they will be mentioning its applications.
2.  Another option is using the most familiar type of flow meter such as the differential pressure flow meter.  We will not recommend this as it’s not a much accurate option.  You should do further check before finalizing the flow meter.
3.  Some other factors according to which flow meters are chosen are based on service, rangeability, pressure loss, accuracy, installation requirements, cost.
4. Special attention should be taken for flowmeter intended to be used on sour service applications. NACE requirements, testing requirements etc should be  carefully considered.
5. When choosing flow meters employed in hazardous area requirements, winterization requirements etc all requirements should be full filled .
6. Special service applications such as custody transfer metering , multiphase flow metering etc. needs specific types of flowmeters  in order to  suit their requirements and hence the same shall be carefully considered.
7. The installation requirements of the corresponding flow meter should be satisfied, as the piping layouts can be affected by straight run requirements of flowmeters.

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