Orifice Meter: Construction, Working, Formula & Applications

An Orifice Meter is a differential-pressure flow-measuring device used to determine the flow rate of liquids, gases, and steam through a pipe. It works by placing a thin orifice plate with a precisely machined opening inside the pipeline. As fluid passes through the opening, its velocity increases and its pressure decreases. The resulting pressure difference is measured and used to calculate the flow rate.

Orifice meters are widely used in process industries, power plants, oil and gas facilities, chemical plants, water systems, and other industrial applications because they are simple, compact, and relatively inexpensive.

Table of Contents

What Is an Orifice Meter?

An Orifice Meter is a type of differential-pressure flow meter used to measure the flow rate of a fluid in a closed pipeline.

The basic arrangement consists of an orifice plate installed between pipe flanges and pressure taps located upstream and downstream of the plate. When fluid flows through the smaller opening in the plate, its velocity increases while its static pressure decreases.

The pressure difference across the orifice is related to the flow rate. A differential-pressure transmitter can measure this pressure difference, allowing the flow rate to be calculated.

The basic relationship is:

Q \propto \sqrt{\Delta P}

where:

$Q$ = volumetric flow rate
$\Delta P$ = differential pressure across the orifice

Thus, a larger pressure difference generally indicates a higher flow rate.

Orifice Meter Construction or Parts

An Orifice Meter consists of several components that work together to create a controlled pressure drop and measure the flow rate of fluid through a pipeline. The main parts include the inlet section, orifice plate, flow conditioner, outlet section, pressure taps, differential-pressure transmitter, impulse lines, flanges, and gaskets.

The major parts of an Orifice Meter are explained below.

1. Inlet Section

The inlet section is the portion of the pipeline through which the fluid enters the Orifice Meter. It provides a passage for the fluid before it reaches the orifice plate.

A sufficiently long and properly aligned inlet section helps establish a stable flow profile before the fluid reaches the restriction. Elbows, valves, pumps, and other disturbances close to the orifice can affect the flow profile and introduce measurement errors.

2. Orifice Plate

The orifice plate is the main flow-measuring element of an Orifice Meter. It is a thin metal plate with a precisely machined opening installed across the pipeline.

When fluid passes through the smaller opening, its velocity increases and its static pressure decreases. This creates the differential pressure required to determine the flow rate.

For a typical concentric orifice plate, the bore is located at the center of the plate.

The diameter ratio is defined as:

\beta = \frac{d}{D}

where:

$d$ = orifice bore diameter
$D$ = internal pipe diameter

The diameter ratio is an important parameter in Orifice Meter sizing and flow calculation.

3. Flow Conditioner

A flow conditioner may be installed upstream of the orifice plate to improve the flow profile before the fluid reaches the measurement section.

Upstream pipe fittings such as bends, valves, reducers, and tees can produce swirling or asymmetric flow. A properly selected flow conditioner helps reduce these disturbances and produces a more uniform velocity profile.

However, a flow conditioner is not necessarily a separate component in every Orifice Meter installation. Its use depends on the piping arrangement, applicable measurement standard, and required measurement performance.

4. Outlet Section

The outlet section is the part of the pipeline located downstream of the orifice plate. After passing through the restriction, the fluid continues through this section while the pressure gradually recovers from the minimum-pressure region.

The downstream pressure is measured at the appropriate pressure tap and is used together with the upstream pressure to determine the differential pressure.

5. Pressure Taps

Pressure taps are provided at specified locations upstream and downstream of the orifice plate.

The upstream tap senses the pressure $P_1$ , while the downstream tap senses $P_2$ . The differential pressure is:

\Delta P = P_1 – P_2

This pressure difference is the primary measurement used to calculate the flow rate.

The location and type of pressure taps are important because different tap arrangements can produce different measurement characteristics.

6. Differential-Pressure Transmitter

A differential-pressure (DP) transmitter measures the pressure difference between the upstream and downstream pressure taps.

The transmitter receives the two pressure signals and produces an output proportional to the differential pressure. In an industrial measurement system, this signal can be sent to a flow indicator, PLC, DCS, or other control system.

Because flow is approximately proportional to the square root of differential pressure, square-root extraction is commonly applied:

Q \propto \sqrt{\Delta P}

7. Impulse Lines

Impulse lines connect the pressure taps on the pipeline to the differential-pressure transmitter.

They carry the sensed process pressure from the upstream and downstream sides of the orifice plate to the transmitter. Proper routing, installation, and maintenance of impulse lines are essential because blockages, leakage, condensation, or incorrect elevation arrangements can affect the pressure measurement.

8. Pipe Flanges

Flanges are used to connect the orifice plate assembly to the pipeline and hold the plate securely in position.

They also provide a convenient arrangement for inspection, removal, and replacement of the orifice plate during maintenance.

9. Gaskets

Gaskets are installed between mating flange surfaces to prevent fluid leakage.

The gasket should be compatible with the process fluid, pressure, and temperature. Incorrect gasket installation or protrusion into the flow path can disturb the flow and affect measurement accuracy.

10. Flow Pipe or Metering Section

The flow pipe provides the passage through which the measured fluid travels. The internal diameter of the pipe is an important parameter because it is used with the orifice bore diameter to determine the diameter ratio.

The pipe should be properly aligned and maintained in accordance with the applicable flow-measurement requirements.

Orifice Meter Parts at a Glance

Part	Function
Inlet section	Allows fluid to enter the metering section
Orifice plate	Creates the controlled restriction and pressure difference
Flow conditioner	Helps improve the upstream flow profile when required
Outlet section	Carries fluid downstream of the orifice plate
Pressure taps	Sense upstream and downstream pressure
DP transmitter	Measures differential pressure
Impulse lines	Connect pressure taps to the transmitter
Pipe flanges	Hold and connect the orifice plate assembly
Gaskets	Prevent leakage at flange connections
Flow pipe	Provides the passage for the fluid

The orifice plate, pressure taps, and differential-pressure measurement system form the core of an Orifice Meter. The inlet and outlet piping, along with proper flow conditioning and installation, help ensure that the pressure difference represents the actual flow conditions accurately.

Working Principle of Orifice Meter

The Orifice Meter works on the principles of Bernoulli’s equation and conservation of mass. It measures fluid flow by creating a restriction in the pipeline and measuring the resulting pressure difference.

When fluid flows through the orifice plate, the available flow area decreases. According to the continuity principle, the fluid velocity increases as it passes through the smaller opening. At the same time, Bernoulli’s principle indicates that an increase in fluid velocity is accompanied by a decrease in static pressure.

The pressure upstream of the orifice plate is relatively high, while the pressure downstream is lower. These pressures are sensed through pressure taps installed at suitable locations around the orifice plate.

The differential pressure is:

\Delta P = P_1 – P_2

where:

P1 = upstream pressure
P2 = downstream pressure
$\Delta P$ = differential pressure across the orifice

The differential-pressure transmitter measures this pressure difference and sends the flow signal to the control or monitoring system. Since the flow rate is approximately proportional to the square root of the differential pressure, the relationship can be expressed as: $Q\propto\sqrt{\Delta P}$

Working Steps of an Orifice Meter

The operation of an orifice meter can be summarized as follows:

Fluid enters the metering section through the upstream pipe.
The fluid approaches the orifice plate with a relatively high static pressure.
The orifice plate restricts the available flow area.
Fluid velocity increases as it passes through the orifice opening.
Static pressure decreases because of the increase in velocity and the associated flow losses.
Upstream and downstream pressure taps sense the pressure at the selected locations.
The differential-pressure transmitter measures the pressure difference.
The flow rate is calculated from the measured differential pressure using the appropriate orifice-meter equation.

Thus, an orifice meter converts fluid flow into a measurable differential pressure, which can then be used to determine the flow rate.

Orifice Meter Diagram

A simplified arrangement of an orifice flow meter is shown conceptually below:

The orifice plate produces a restriction in the pipe. The upstream pressure $P_1$ is higher than the downstream pressure $P_2$ , and the difference between these pressures is used to determine the flow rate.

Types of Orifice Plates

Orifice plates can be classified according to the shape and position of the opening.

1. Concentric Orifice Plate

A concentric orifice plate has a circular hole located at the center of the plate.

It is the most common type and is suitable for many liquid, gas, and steam-flow applications.

2. Eccentric Orifice Plate

In an eccentric orifice plate, the opening is offset from the center.

It can be useful when the fluid contains suspended solids or when it is desirable to reduce the accumulation of liquid or solids near the restriction.

3. Segmental Orifice Plate

A segmental orifice has a segment-shaped opening.

It is particularly useful for fluids containing suspended solids, gases carrying liquids, or applications where clogging could be a concern.

Hydraulic Coefficients of an Orifice Meter

The performance of an orifice meter is described using several hydraulic coefficients. These coefficients help relate the actual flow behavior to the ideal conditions assumed in theoretical calculations.

The four important coefficients associated with an orifice meter are:

Coefficient of Contraction
Coefficient of Velocity
Coefficient of Resistance
Coefficient of Discharge

Coefficient of Contraction

The coefficient of contraction represents the reduction in jet area as the fluid passes through the orifice. It is defined as the ratio of the cross-sectional area of the jet at the vena contracta to the area of the orifice opening.

C_c = \frac{A_c}{A_o}

where:

C_c = coefficient of contraction
A_c = area of the jet at the vena contracta
A_o = area of the orifice opening

Coefficient of Velocity

The coefficient of velocity accounts for the difference between the actual velocity of the jet and its theoretical velocity at the vena contracta.

It is defined as the ratio of actual jet velocity to theoretical jet velocity.

C_v = \frac{V_{\text{actual}}}{V_{\text{theoretical}}}

where:

C_v = coefficient of velocity
V_actual = actual velocity of the jet
V_theoretical = theoretical velocity of the jet

Coefficient of Resistance

The coefficient of resistance indicates the energy loss caused by the restriction and associated flow effects as the fluid passes through the orifice.

It is related to the head loss produced by the orifice and can be expressed as the ratio of the head loss to the available velocity head.

K = \frac{h_L }{ V^2/2g}

where:

K = coefficient of resistance
h_L = head loss due to the orifice
V = reference fluid velocity
g = acceleration due to gravity

Coefficient of Discharge

The coefficient of discharge accounts for the difference between the actual discharge and the theoretical discharge predicted by the ideal-flow equation.

It is defined as:

C_d = \frac{Q_{\text{actual}}}{Q_{\text{theoretical}}}

where:

C_d = coefficient of discharge
Q_actual = actual discharge through the orifice
Q_theoretical = theoretical discharge

For an orifice meter, the coefficient of discharge is particularly important because it incorporates the effects of contraction, velocity variation, and other real-flow effects that are not fully represented by the ideal theoretical equation.

Orifice Meter Discharge Formula

For an ideal incompressible flow, the theoretical discharge through an orifice can be expressed as:

Q_t = A_o \sqrt{\frac{2\Delta P}{\rho(1-\beta^4)}}

where:

$Q_t$ = theoretical flow rate
$A_o$ = area of orifice opening
$\Delta P$ = pressure difference
$\rho$ = fluid density
$\beta=d/D$

The area of the orifice opening is:

A_o = \frac{\pi d^2}{4}

Therefore:

Q_t = \frac{\pi d^2}{4}\sqrt{\frac{2\Delta P}{\rho(1-\beta^4)}}

The actual discharge is lower than the theoretical discharge because of friction, contraction, and other flow effects.

Therefore:

Q = C_d A_o \sqrt{\frac{2\Delta P}{\rho(1-\beta^4)}}

This is one of the most commonly used forms of the orifice meter discharge formula.

Orifice Flow Meter Formula

The general orifice flow meter formula for an incompressible fluid is:

Q = C_d \frac{\pi d^2}{4} \sqrt{\frac{2\Delta P}{\rho(1-\beta^4)}}

Since:

\beta = \frac{d}{D}

the equation can also be written as:

Q = C_d \frac{\pi d^2}{4} \sqrt{\frac{2\Delta P}{\rho\left[1-\left(\frac{d}{D}\right)^4\right]}}

where:

$Q$ = actual volumetric flow rate in $\text{m}^3/\text{s}$
$C_d$ = coefficient of discharge
$d$ = orifice diameter in m
$D$ = pipe diameter in m
$\Delta P$ = differential pressure in Pa
$\rho$ = fluid density in $\text{kg/m}^3$

For a given installation, the exact flow calculation may require additional correction factors depending on the fluid, Reynolds number, pressure-tap arrangement, pipe geometry, and applicable measurement standard.

Derivation of Orifice Meter Formula

Consider a pipe of diameter $D$ containing an orifice plate with an opening of diameter $d$ .

Let:

$A_1$ = pipe cross-sectional area
$A_2$ = orifice area
$V_1$ = velocity upstream
$V_2$ = velocity through the restriction
$P_1$ = upstream pressure
$P_2$ = downstream pressure

From the continuity equation:

Q = A_1 V_1 = A_2 V_2

Therefore:

V_1 = \frac{Q}{A_1}

and

V_2 = \frac{Q}{A_2}

For ideal flow, Bernoulli’s equation gives:

\frac{P_1}{\rho} + \frac{V_1^2}{2} = \frac{P_2}{\rho} + \frac{V_2^2}{2}

Rearranging:

\frac{P_1 – P_2}{\rho} = \frac{V_2^2 – V_1^2}{2}

Therefore:

\Delta P = \frac{\rho}{2}\left(V_2^2 – V_1^2\right)

Using the continuity relationship and the area ratio, the theoretical flow rate becomes:

Q_t = A_2 \sqrt{\frac{2\Delta P}{\rho(1-\beta^4)}}

Considering actual flow effects through the coefficient of discharge:

Q = C_d A_2 \sqrt{\frac{2\Delta P}{\rho(1-\beta^4)}}

Since:

A_2 = \frac{\pi d_2^2}{4}

we obtain:

Q = C_d \frac{\pi d_2^2}{4} \sqrt{\frac{2\Delta P}{\rho(1-\beta^4)}}

Orifice Meter Calculation

An orifice meter calculation requires several parameters.

Typically, you need:

Pipe diameter $D$
Orifice diameter $d$
Differential pressure $\Delta P$
Fluid density $\rho$
Coefficient of discharge $C_d$

1. Determine the Diameter Ratio

\beta = \frac{d}{D}

2. Find the Orifice Area

A_o = \frac{\pi d^2}{4}

3. Calculate Flow Rate

Q = C_d A_o \sqrt{\frac{2\Delta P}{\rho(1-\beta^4)}}

The resulting value gives the volumetric flow rate when consistent SI units are used.

Sample Problem With Solution

Consider an orifice meter installed in a water pipeline with the following data:

Pipe diameter $D=100$ mm
Orifice diameter $d=50$ mm
Differential pressure $=20$ kPa
Water density $\rho=1000$ kg/m³
Coefficient of discharge $C_d=0.61$

1. Convert Diameters

D=0.1\,\mathrm{m}

d=0.05\,\mathrm{m}

2. Calculate Diameter Ratio

\beta=\frac{0.05}{0.1}=0.5

3. Calculate Orifice Area

A_o=\frac{\pi(0.05)^2}{4}

A_o=0.0019635\,\mathrm{m^2}

4. Calculate Flow Rate

The differential pressure is:

\Delta P=20,000\,\mathrm{Pa}

Using the orifice flow meter formula:

Q=0.61(0.0019635)\sqrt{\frac{2(20,000)}{1000(1-0.5^4)}}

Therefore:

Q\approx0.000117\,\mathrm{m^3/s}

or approximately:

Q\approx0.117\,\mathrm{L/s}

This example demonstrates the basic orifice meter calculation. In an actual industrial installation, the calculation should use the appropriate standardized equation and correction factors for the particular service.

Pressure Loss in an Orifice Meter

One important limitation of an Orifice Meter is its permanent pressure loss.

The pressure decreases as fluid passes through the restriction. Some of this pressure is recovered downstream, but a portion is permanently lost because of turbulence and friction.

This permanent pressure loss means that an orifice meter generally consumes more pumping or compression energy than a Venturi meter for the same service.

Therefore, pressure loss should be considered when selecting an orifice meter for an industrial process.

Orifice Meter Specifications

The specifications of an Orifice Meter depend on the pipe size, fluid being measured, operating conditions, orifice plate material, and applicable design standards. Important specifications generally include the orifice bore, pipe diameter, pressure rating, temperature range, and diameter ratio.

Typical considerations include:

Parameter	Typical Specification or Consideration
Orifice bore diameter	Selected according to the required flow range and differential pressure
Pipe diameter	Depends on the size of the process pipeline
Diameter ratio (β)	Commonly selected within the applicable design-standard range
Operating temperature	Depends on the plate material, gaskets, fittings, and process conditions
Operating pressure	Determined by the pressure rating of the flanges, plate assembly, and piping system
Orifice plate material	Selected according to fluid properties, temperature, pressure, and corrosion requirements
Pressure tapping	Selected according to the applicable orifice-meter design standard
Differential pressure	Sized according to the required flow range and allowable permanent pressure loss

For many industrial applications, the orifice plate is designed and sized according to recognized flow-measurement standards. Therefore, values such as maximum operating pressure and temperature should not be treated as fixed limits for every orifice flow meter. The final specification should always be established from the process conditions and the selected design standard.

Advantages of Orifice Meter

An Orifice Meter offers several advantages:

Simple construction
Low initial cost
No moving parts
Compact design
Easy installation
Suitable for liquids, gases, and steam
Easy replacement of the orifice plate
Suitable for high-pressure applications
Can provide good repeatability when properly designed and installed
Widely accepted for industrial flow measurement

Disadvantages of Orifice Meter

Despite its simplicity, an Orifice Meter has some limitations:

Causes permanent pressure loss
Requires relatively straight pipe lengths
Accuracy can be affected by upstream disturbances
Orifice plates can wear over time
The opening can become damaged or contaminated
Flow rangeability is limited compared with some modern flow meters
Differential-pressure measurement requires additional instrumentation
Small pressure differences can become difficult to measure accurately at very low flow rates
Improper installation can produce significant measurement errors

Applications of Orifice Meter

Orifice meters are used for flow measurement in a wide range of industries and process systems, including:

Power Plants: Measuring water, steam, compressed air, fuel gas, and other process flows.
Oil and Gas Industry: Measuring gas and liquid flows in process, transmission, and utility systems.
Chemical Plants: Measuring process fluids, gases, and utility streams.
Water Treatment Plants: Measuring water flow through pipelines and treatment systems.
Compressed-Air Systems: Monitoring compressed-air consumption and distribution.
Steam Systems: Measuring steam flow using an orifice plate and differential-pressure measurement system.
Process Industries: Measuring general-purpose liquid and gas flows where differential-pressure flow measurement is suitable.

Orifice Meter Applications in Industrial Instrumentation

In industrial instrumentation, the orifice meter forms part of a flow measurement and control system. The orifice plate creates a differential pressure, while a differential-pressure transmitter measures this pressure difference and converts it into an electrical signal.

The signal is then sent to a PLC, DCS, flow indicator, or other control system for monitoring, display, recording, or control.

A typical instrumentation arrangement is:

Fluid Flow
↓
Orifice Plate
↓
Differential Pressure
↓
DP Transmitter
↓
Flow Signal
↓
PLC / DCS / Flow Indicator

The control system can use square-root extraction to convert the measured differential pressure into a flow signal because flow is approximately proportional to the square root of differential pressure.

For example, if the differential pressure increases four times, the corresponding flow increases approximately two times, provided the fluid properties and other operating conditions remain constant.

Orifice Meter vs Venturi Meter

Both Orifice and Venturi meters use differential pressure to measure flow, but their construction and performance differ.

Parameter	Orifice Meter	Venturi Meter
Construction	Thin plate with opening	Converging and diverging tube
Cost	Lower	Higher
Size	Compact	Larger
Installation	Relatively simple	Requires more space
Pressure loss	Higher	Lower
Maintenance	Simple	More involved
Moving parts	None	None
Accuracy	Good when properly installed	Generally higher
Replacement	Plate can be replaced easily	Whole meter section is more substantial
Application	General industrial flow measurement	Applications where low pressure loss is important

The choice depends on factors such as accuracy, available space, pressure-loss requirements, operating conditions, maintenance requirements, and cost.

Precautions for Orifice Meter Installation

Proper installation helps the Orifice Meter provide accurate and reliable flow measurements. Follow these precautions:

Maintain proper pipe alignment: Align the pipe and orifice plate correctly to avoid disturbing the flow profile and introducing measurement errors.
Use the correct orifice plate: Select the plate bore, thickness, material, and geometry according to the fluid, operating conditions, and measurement requirements.
Provide adequate straight pipe length: Maintain the required straight pipe length upstream and downstream of the orifice plate according to the applicable flow-measurement standard.
Keep the orifice opening clean: Remove deposits, corrosion, and other obstructions that could alter the effective bore and affect flow measurement.
Check pressure tappings: Install the pressure taps correctly and keep the taps and impulse lines free from blockage or leakage.
Check for leakage: Properly tighten and seal flange joints, pressure connections, and impulse-line connections to prevent fluid leakage.
Consider fluid properties: Account for density, viscosity, temperature, pressure, and compressibility when sizing the orifice meter and calculating the flow rate.
Inspect the plate periodically: Check the orifice plate for erosion, corrosion, deformation, and deposits, particularly when measuring corrosive, abrasive, or contaminated fluids.

Orifice Meter Flow Measurement Accuracy

The accuracy of an Orifice Meter depends not only on the equation but also on the quality of the installation.

Important factors include:

Correct plate dimensions
Correct bore diameter
Proper edge condition
Correct pressure-tap location
Adequate straight pipe length
Stable flow profile
Correct differential-pressure measurement
Proper density and process-property data
Correct transmitter configuration
Appropriate calibration

For critical custody-transfer or high-accuracy applications, standardized calculation procedures should be followed rather than relying on a simplified equation alone.

Conclusion

An Orifice Meter is one of the simplest and most widely used differential-pressure flow-measuring devices. It uses an orifice plate to create a controlled restriction in a pipeline and measures the resulting pressure difference to determine fluid flow.

The orifice meter discharge formula relates flow rate to differential pressure, fluid density, orifice diameter, pipe diameter, and coefficient of discharge. Proper plate design, pressure-tap arrangement, installation, and maintenance are essential for obtaining reliable measurements.

Because of its simple construction, low cost, compact size, and wide industrial applicability, the Orifice Meter remains an important flow-measurement method in power plants, process industries, water systems, chemical plants, and oil and gas facilities.

Frequently Asked Questions

Q1. What is an Orifice Meter?

An Orifice Meter is a differential-pressure flow meter that measures fluid flow by creating a pressure difference across an orifice plate.

Q2. What is the difference between an orifice meter and an orifice plate?

The orifice plate creates the restriction in the pipeline, while the orifice meter refers to the complete flow-measuring system, including the orifice plate, pressure taps, and differential-pressure measurement equipment.

Q3. What is the principle of an Orifice Meter?

An Orifice Meter works mainly on the principles of Bernoulli’s equation and continuity. The restriction increases fluid velocity and produces a measurable pressure drop.

Q4. What fluids can an Orifice Meter measure?

Orifice meters measure the flow of liquids, gases, and steam when engineers properly design and install the meter for the specific service.

Q5. Why does an Orifice Meter cause pressure loss?

The restriction creates turbulence and energy dissipation in the flowing fluid. Consequently, only part of the pressure drop is recovered downstream.

Read Next:

What Is an Orifice Meter?

Orifice Meter Construction or Parts

1. Inlet Section

2. Orifice Plate

3. Flow Conditioner

4. Outlet Section

5. Pressure Taps

6. Differential-Pressure Transmitter

7. Impulse Lines

8. Pipe Flanges

9. Gaskets

10. Flow Pipe or Metering Section

Orifice Meter Parts at a Glance

Working Principle of Orifice Meter

Working Steps of an Orifice Meter

Orifice Meter Diagram

Types of Orifice Plates

1. Concentric Orifice Plate

2. Eccentric Orifice Plate

3. Segmental Orifice Plate

Hydraulic Coefficients of an Orifice Meter

Coefficient of Contraction

Coefficient of Velocity

Coefficient of Resistance

Coefficient of Discharge

Orifice Meter Discharge Formula

Orifice Flow Meter Formula

Derivation of Orifice Meter Formula

Orifice Meter Calculation

1. Determine the Diameter Ratio

2. Find the Orifice Area

3. Calculate Flow Rate

Sample Problem With Solution

1. Convert Diameters

2. Calculate Diameter Ratio

3. Calculate Orifice Area

4. Calculate Flow Rate

Pressure Loss in an Orifice Meter

Orifice Meter Specifications

Advantages of Orifice Meter

Disadvantages of Orifice Meter

Applications of Orifice Meter

Orifice Meter Applications in Industrial Instrumentation

Orifice Meter vs Venturi Meter

Precautions for Orifice Meter Installation

Orifice Meter Flow Measurement Accuracy

Conclusion

Frequently Asked Questions

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