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คำตอบที่ 7
The Venturi Meter
In the venturi meter the fluid is accelerated through a converging cone of angle 15-20o and the pressure difference between the upstream side of the cone and the throat is measured and provides the signal for the rate of flow.
The fluid slows down in a cone with smaller angle (5-7o) where most of the kinetic energy is converted back to pressure energy. Because of the cone and the gradual reduction in the area there is no "vena contracta". The flow area is at minimum at the throat.
High pressure and energy recovery makes the venturi meter suitable where only small pressure heads are available.
A discharge coefficient - cd - of 0.975 may be taken as standard, but the value varies noticeably at low values of the Reynolds number.
The pressure recovery is much better for the venturi meter than for the orifice plate.
The venturi tube is suitable for clean, dirty and viscous liquid and some slurry services.
The rangeability is 4 to 1.
Pressure loss is low.
Typical accuracy is 1% of full range.
Required upstream pipe length 5 to 20 diameters.
Viscosity effect is high
Relative cost is medium
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