Hot Wire Anemometer (Thermal Method)

Basic Principle of Hot wire Anemometer

When an electrically heated wire is placed in a flowing gas stream, heat is transferred from the wire to the gas and hence the temperature of the wire reduces, and due to this, the resistance of the wire also changes. This change in resistance of the wire becomes a measure of flow rate.

Description of Hot wire Anemometer

The main parts of the arrangement are as follows:

• Conducting wires placed in a ceramic body.
• Leads are taken from the conducting wires and they are connected to one of the limbs of the wheat stone bridge to enable the measurement of change in resistance of the wire.

Operation of Hot wire Anemometer

There are two methods of measuring flow rate using a anemometer bridge combination namely:

• Constant current method
• Constant temperature method

Constant current method

• The bridge arrangement along with the anemometer has been shown in diagram. The anemometer is kept in the flowing gas stream to measure flow rate.
• A constant current is passed through the sensing wire. That is, the voltage across the bridge circuit is kept constant, that is, not varied.
• Due to the gas flow, heat transfer takes place from the sensing wire to the flowing gas and hence the temperature of the sensing wire reduces causing a change in the resistance of the sensing wire. (this change in resistance becomes a measure of flow rate).
• Due to this, the galvanometer which was initially at zero position deflects and this deflection of the galvanometer becomes a measure of flow rate of the gas when calibrated.

Constant temperature method

• The bridge arrangement along with the anemometer has been shown in diagram. The anemometer is kept in the flowing gas stream to measure flow rate.
• A current is initially passed through the wire.
• Due to the gas flow, heat transfer takes place from the sensing wire to the flowing gas and this tends to change the temperature and hence the resistance of the wire.
• The principle in this method is to maintain the temperature and resistance of the sensing wire at a constant level. Therefore, the current through the sensing wire is increased to bring the sensing wire to have its initial resistance and temperature.
• The electrical current required in bringing back the resistance and hence the temperature eof the wire to its initial condition becomes a measure of flow rate of the gas when calibrated.

Application of Hot wire Anemometer

In research applications, they are extensively used to study varying flow conditions.

Pitot Tube Analysis Video Post

Analysis of Pitot tube,

I hope you will enjoy this video

Turbine Type Anemometer:

When a rotor attached to a permanent magnet polarized at 90’ to the axis of rotation, and if the rotor is made to rotate due to the fluid velocity V, the rotating magnetic fluid will be cut by the pickup coil generating voltage pulses. The frequency of voltage pulses is proportional to flow rate. Hence the measure of frequency of voltage pulses becomes a measure of flow rate.

Description of Turbine Meter:

The main parts of the turbine meter are as follows:

A turbine wheel (rotor)
A permanent magnet polarized at 90’ to the axis of rotation which is attached to one of the blades of the rotor.
A pickup coil placed external to the meter housing.
A frequency meter or counter attached to the pickup coil.

Operation of Turbine Meter:

The turbine meter is fixed to the pipe carrying the fluid whose volume flow rate is to be measured.
The fluid strikes the blades of the rotor and makes it to rotate. When the rotor rotates, the permanent magnet attached to the rotor blade also rotates, which in turn produces a rotating magnetic field.
Each time the magnet passes the pickup coil, the magnetic flied is being cut generating a voltage pulse. The frequency of voltage pulses is indicated by a frequency meter.
This frequency of voltages pulses becomes a measure of flow rate when calibrated.
The volume flow rate “Q” is given by Q = F/C

Where F = total number of pulses
C = flow Coefficient

Applications:

They are extensively used in weather stations to measure wind velocity.
They are used to measure water flow in rivers and streams.
Compact models are used to measure flow in tubes and pipes.

Advantages of turbine Meter:

Recording and controlling can be done from a distance (telemetry).
High accuracy (error is -/+ 0.5%).
Has good dynamic response.
The pressure drop in the fluid is low.
It is easy to install and maintain.

Limitations of turbine meter:

Error increases if the flow rate is low.
Bearing friction and wear may alter the linear output of this instrument.
For a distance, a straight run of pipe ahead of instrument is required.

What is a Pitot Tube or Total Pressure Probe

A probe is a device used for point pressure measurement in a flowing fluid. This point measurement of pressure is done to determine fluid flow rate. The most popular probe is the “PITOT TUBE” which is one of the total pressure probes. The Pitot tube measures the combined pressure (static pressure + impact pressure). The pitot tube has one impact opening and eight static openings as shown in the diagram. The impact opening is provided to sense impact pressure and the static opening are provide to sense static pressure.

Operation of Pitot Tube:

The pitot tube is introduced in the fluid flow area where point pressure details is required (which is an indirect measure of flow rate).

The pressure in the outer tube is the static pressure in the line. The total pressure in the inner tube is greater than static pressure. That is, total pressure is the static pressure plus the impact pressure. The differential pressure (P1-P2) is measured using a differential pressure sensor. This differential pressure becomes a measure of flow rate at that point where the pitot tube is present in the flowing fluid.

Applications of pitot tube

Pitot tubes are extensively used in laboratories to measure velocity, pressure and flow rates of fluids.

Advantages of pitot tube:

It causes no pressure loss in the flowing fluid.
Pitot tube is cheap and very easy to install.

Limitations of Pitot tube

It is difficult to obtain proper alignment of the pitot tube with flowing direction.
It cannot be used in fluids with suspended solids and impurities.
The fluid velocity should be high in order to get a measurable pressure difference.

What is Rotameter or Variable-Area Meter?

Rotameter or Variable Area meter is an instrument used to measure the flow rate of a fluid by using a simple float (let us say a small triangle shaped block) to float in the moving fluid. The operation of the Rotameter is explained in detail below.

Parts of the Rotameter:

The main parts of a Rotameter are as follows:

A tapered transparent glass tube graduated to read flow rate directly.
A float is used whose density is greater than the flowing fluid. The float’s diameter is such that it completely blocks the inlet of the tapered transparent glass tube.

Operation of Rotameter:

As the fluid whose flow rate is being measured comes and touches the bottom portion of the float blocking the inlet of the tapered transparent glass tube, the float starts to rise when the following happens:

Pressure of flowing fluid + flowing buoyancy is greater than downward pressure due to weight of the float.

When the float rises, an annular space is created between the periphery of the float and the inner wall of the tapered transparent glass tube. This annular space which is concentric opening through which the fluid passes to the other side of the instrument keeps on increasing until the following happens:

(pressure of the flowing fluid) + (fluid buoyancy) = (Downward pressure due to weight of the float)

When this happens, the float stops rising further and stops at a particular position, that is, the float comes to equilibrium.

Thus, increase in flow rate will make the float to rise higher and vice versa. That is, the position of the float becomes a direct indication of flow rate. Hence the tapered transparent glass tube can be graduated suitably by proper calibration to get a direct indication of flow rate by noting the position of the float with respect to the graduations on the tapered tube.

The instrument has to be designed in such a manner so that the effects of changing viscosity and density are minimized leaving only pressure of the flowing fluid as a variable.

Application of the Rotameter:

1. Rotameter can be used to measure flow rates of corrosive fluids.
2. It is particularly useful to measure low flow rates

Advantages of Rotameter:

1. Flow conditions are visible.
2. Flow rate is a linear function (uniform flow scale).
3. Can be used to measure flow rates of liquids, gases and vapours.
4. By changing the float, tapered tube or both, the capacity of the Rotameter can be changed.

Limitations of Rotameter:

They should be installed vertically.
They cannot be used in measurements of moving objects.
The float will not be visible, when colored fluids are used, that is, when opaque fluids are used.
For high pressure and temperature fluid flow measurements, they are expensive.
They cannot be used for fluids containing high percentage of solids in suspension.

How to use venturi meter for measuring flow rate

Basic principle:

When a venture meter is placed in apipe carrying the fluid whose flow rate is to be measured, a pressure drop occurs between the entrance and throat of the venturimeter. This pressure drop is measured using a differential pressure sensor and when calibrated this pressure drop becomes a measure of flow rate.

Construction of Venturi meter

The following are the main parts and areas of venture meter:

1. The entry of the venture is cylindrical in shape to match the size of the pipe through which fluid flows. This enables the venture to be fitted to the pipe.
2. After the entry, there is a converging conical section with an included angle of 19’ to 23’.
3. Following the converging section, there is a cylindrical section with minimum area called as the throat.
4. After the throat, there is a diverging conical section with an included angle of 5’ to 15’.
5. Openings are provided at the entry and throat (at sections 1 and 2 in the diagram) of the venture meter for attaching a differential pressure sensor (u-tube manometer, differential pressure gauge, etc) as shown in diagram.

Operation of venturi meter:

1. The fluid whose flow rate is to be measured enters the entry section of the venturi meter with a pressure P1.
2. As the fluid from the entry section of venturi meter flows into the converging section, its pressure keeps on reducing and attains a minimum value P2 when it enters the throat. That is, in the throat, the fluid pressure P2 will be minimum.
3. The differential pressure sensor attached between the entry and throat section of the venturi meter records the pressure difference(P1-P2) which becomes an indication of the flow rate of the fluid through the pipe when calibrated.
4. The diverging section has been provided to enable the fluid to regain its pressure and hence its kinetic energy. Lesser the angle of the diverging section, greater is the recovery.

Application:

1. It is used where high pressure recovery is required.
2. Can be used for measuring flow rates of water,gases,suspended solids, slurries and dirty liquids.
3. Can be used to measure high flow rates in pipes having diameters in a few meters.

Advantages of venturi meters

1. Less changes of getting clogged with sediments
2. Coefficient of discharge is high.
3. Its behaviour can be predicted perfectly.
4. Can be installed vertically, horizontally or inclinded.

Limitations

• They are large in size and hence where space is limited, they cannot be used.
• Expensive initial cost, installation and maintenance.
• Require long laying length. That is, the veturimeter has ti be proceeded by a straight pipe which is free from fittings and misalignments to avoid turbulence in flow, for satisfactory operation. Therefore, straightening vanes are a must.
• Cannot be used in pipes below 7.5cm diameter.

Flow Measurement Using Flow Nozzle:

Basic Principle of Flow Nozzle:

When a flow nozzle is placed in a pipe carrying whose rate of flow is to be measured, the flow nozzle causes a pressure drop which varies with the flow rate. This pressure drop is measured using a differential pressure sensor and when calibrated this pressure becomes a measure of flow rate.

Description of Flow Nozzle:

The main parts of flow nozzle arrangement used to measure flow rate are as follows:

1. A flow nozzle which is held between flanges of pipe carrying the fluid whose flow rate is being measured. The flow nozzle’s area is minimum at its throat.
2. Openings are provided at two places 1 and 2 for attaching a differential pressure sensor (u-tube manometer, differential pressure gauge etc.,) as show in the diagram.

Operation of flow Nozzle:

1. The fluid whose flow rate is to be measured enters the nozzle smoothly to the section called throat where the area is minimum.
2. Before entering the nozzle, the fluid pressure in the pipe is p1. As the fluid enters the nozzle,the fluid converges and due to this its pressure keeps on reducing until it reaches the minimum cross section area called throat. This minimum pressure p2 at the throat of the nozzle is maintained in the fluid for a small length after being discharged in the down stream also.
3. The differential pressure sensor attached between points 1 and 2 records the pressure difference (p1-p2) between these two points which becomes an indication of the flow rate of the fluid through the pipe when calibrated.

Applications of Flow Nozzle

1. It is used to measure flow rates of the liquid discharged into the atmosphere.
2. It is usually used in situation where suspended solids have the property of settling.
3. Is widely used for high pressure and temperature steam flows.

Advantages of flow Nozzle

1. Installation is easy and is cheaper when compared to venturi meter
2. It is very compact
3. Has high coefficient of discharge.

Limitations

1. Pressure recovery is low
2. Maintenance is high
3. Installation is difficult when compared to orifice flow meter.

How to Measure Flow Using Orifice Meter

In this post, i am interested in covering the details about how to measure flow using Orifice meter. And i am starting it from the scratch.

Basic Principle of Orifice Meter

When an orifice plate is placed in a pipe carrying the fluid whose rate of flow is to be measured, the orifice plate causes a pressure drop which varies with the flow rate. This pressure drop is measured using a differential pressure sensor and when calibrated this pressure drop becomes a measure flow rate. The flow rate is given by.

Where, Qa = flow rate
Cd = Discharge coefficient
A1 = Cross sectional area of pipe
A2 = Cross sectional area of orifice
P1, P2 = Static Pressures

Description of Orifice Meter

The main parts of an orifice flow meter are as follows:

• A stainless steel orifice plate which is held between flanges of a pipe carrying the fluid whose flow rate is being measured.
• It should be noted that for a certain distance before and after the orifice plate fitted between the flanges, the pipe carrying the fliud should be straight in order to maintain laminar flow conditions.
• Openings are provided at two places 1 and 2 for attaching a differential pressure sensor (U-tube manometer, differential pressure gauge etc) as shown in the diagram.

Operation of Orifice Meter

• The detail of the fluid movement inside the pipe and orifice plate has to be understood.
• The fluid having uniform cross section of flow converges into the orifice plate’s opening in its upstream. When the fluid comes out of the orifice plate’s opening, its cross section is minimum and uniform for a particular distance and then the cross section of the fluid starts diverging in the down stream.
• At the upstream of the orifice, before the converging of the fluid takes place, the pressure of he fluid (P1) is maximum. As the fluid starts converging, to enter the orifice opening its pressure drops. When the fluid comes out of the orifice opening, its pressure is minimum (p2) and this minimum pressure remains constant in the minimum cross section area of fluid flow at the downstream.
• This minimum cross sectional area of the fluid obtained at downstream from the orifice edge is called VENA-CONTRACTA.
• The differential pressure sensor attached between points 1 and 2 records the pressure difference (P1 – P2) between these two points which becomes an indication of the flow rate of the fluid through the pipe when calibrated.

Applications of Orifice Meter

1. The concentric orifice plate is used to measure flow rates of pure fluids and has a wide applicability as it has been standardized.
2. The eccentric and segmental orifice plates are used to measure flow rates of fluids containing suspended materials such as solids, oil mixed with water and wet steam.

Advantages of Orifice Meter

1. It is very cheap and easy method to measure flow rate.
2. It has predictable characteristics and occupies less space.
3. Can be use to measure flow rates in large pipes.

Limitations of Orifice Meter

1. The vena-contracta length depends on the roughness of the inner wall of the pipe and sharpness of the orifice plate. In certain cases it becomes difficult to tap the minimum pressure (P2) due to the above factor.
2. Pressure recovery at downstream is poor, that is, overall loss varies from 40% to 90% of the differential pressure.
3. In the upstream straightening vanes are a must to obtain laminar flow conditions.
4. Gets clogged when the suspended fluids flow.
5. The orifice plate gets corroded and due to this after sometime, inaccuracy occurs. Moreover the orifice plate has low physical strength.
6. The coefficient of discharge is low.

Note: the materials used for maintaining orifice plate are stainless steel, phosper bronze, nickel and monel.

Electromagnetic Flow meter

A video about Working of Electromagnetic Flow meter.

Updated Video on Electromagnetic flow measuring principle

Thank for Watching, any comments???

Head type flowmeters – Based on Differential Pressure Measurement

In these head type flowmeters, some devices is inserted into a pipe carrying fluid. It obstructs the flow of fluid and creates a pressure difference on either side of the device. The most commonly used devices are as follows:

1. Orifice plate.
2. Venture plate.
3. Flow nozzle.
4. Doll flow tube.
5. Pilot tube.

The basic principle of all such devices is that due to obstruction, the velocity of the fluid increases and the pressure decreases. Then the volume flow rate is proportional to the square root of pressure difference across the obstruction. To measure pressure difference, diaphragm based differential pressure transducer is used.

Orifice plate:

The orifice plate is a metal disk with a concentric hole as shown in the figure below. It is the simplest device used in almost all industrial application because of cheapness and availability in wide range of sizes.

There are certain limitations of the orifice plate. For very high flow rates, the permanent pressure losses are very high. Over a period of time, the sharp edge of the hole wear out and the particles in the flowing fluid build up behind the hole reducing diameter. Hence discharge coefficient gradually changes. This problem can be eliminated by using eccentric hole near the bottom of the pipe which sweeps out built-up particles behind the plate. Sometimes bubbles of vapour or gas tend to built up behind plate and obstruct the flow. This can be avoided by mounting the orifice plate in vertical run of the pipe.

Flow nozzle:

As compared to the orifice plate, flow nozzle is better option as no possibility of solid particles or bubbles of gas sticking in the flow restriction. The flow across nozzle is as shown in the figure below.

The measurement accuracy of the flow nozzle is very high as no harm of getting warned out. But the cost is comparatively higher as fabrication of the flow nozzle is difficult. Also permanent pressure losses are also high similar to the orifice plate. The flow nozzles are typically used for the steam flow measurement.

In the next post i will discuss about venturi plate, doll flow tube, pilot tube.

Flow Measurement Devices

The important instruments used to measure flow have been listed below:


Secondary or rate meters
Head Type Flow Meters
Obstruction meters

• Orifice meter
• Flow nozzle
• Venturi meter
• Rota meter (variable area meter)

Pitot tube (Total Pressure Probe)


Special Methods

• Magnetic flow meter
• Ultrasonic flow meter (travel time difference method and the oscillating loop system)
• Turbine type anemometer or turbine meter
• Hot Wire anemometer (constant current and constant temperature methods)

I will be discussing all of these devices listed above, if you need to know about any device, please use comment box or chat box.

How to Measure Flow using Magnetic Flow Meter

Basic principle:

When a flowing conducting fluid is subjected to a transverse magnetic field, the flowing conducting fluid cuts the magnetic field and causes a voltage to be induced. This induced voltage is proportional to the fluid velocity, that is, flow rate.

Construction of Magnetic flow meter:

The main parts of this instrument are as follows:

A conducting fluid is flowing through a non-magnetic and non-conducting pipe, whose flow rate is to be measured.

Two electrodes are attached in opposite sides of pipe carrying the conducting fluid. These electrodes are in contact with the flowing conducting fluid.

The pipe is surrounded by an electromagnet which produces which produces a magnetic field.

Operation of a Magnetic Flow Meter:

This magnetic flow meter is based on farady’s law of induced voltage which is given as follows,

E = BLV
Where,
E = induced voltage(volts)
B = flux density (gauss)
L = Length of conductor which is the diameter of the pipe (cm)
V = Average velocity of conductor (fluid) in cm/sec

When the conducting fliud flows through the pipe which is subjected to a magnetic field, the conducting fluid cuts the magnetic field and due to this a voltage is induced. As the magnetic field is constant, voltage obtained across the electrodes will be directly proportional t average fluid velocity and diameter (length) and hence becomes a measure of volume flow rate.

Applications of a Magnetic Flow Meter:

Used to measure flow rates of conducting fluids.
Used to measure flow rates of slurries, corrosive and abrasive fluids.
Magnetic flow meter is used to measure bidirectional flows by reversing connections which can be done automatically.

Advantages of magnetic flow meter

These meters do not cause obstruction to flow and hence cause no pressure drop.
For measurements, it does not matter whether the flow is laminar or turbulent.
It gives accurate results.
Its reliability is high, that is, it gives a standard performance for an elongated period of time.
It can handle greasy materials and fluids containing suspended solids.
The measurement is independent of viscosity, density, temperature and pressure.

Limitations of Magnetic Flow Meter:

The fluid whose flow rate is to be measured should satisfy certain conduction conditions.
The fluid should be full in the pipe to get accurate results.
Air and gas bubbles in the fluid will cause errors.
When certain fluids, the electrodes might get coated with scales and this will affect the output signal. However, this can be taken care off by cleaning the electrodes.
In many cases, the output voltage is low and hence requires amplification.