STUDY OF FLOWMETERS

1. VENTURIMETERA flow meter is a device that measures the rate of flow or quantity of a moving fluid in an open or closed conduit. The most widely used flow metering principle involves placing a fixed area flow restriction of some type in the pipe or duct carrying the fluid. This flow restriction causes a pressure drop that varies with the flow rate. Thus, measurement of the pressure drop by means of a suitable differential-pressure pick up allows flow rate measurement. The important flow meters are
A venturi meter is a device which is used to measure the rate of flow or discharge in a pipe. It works on principle of measurement of pressure drop in the direction of flow that facilitates in the determination of the discharge. The pressure drop is created by the reduction of flow area in the flow direction. It consists of three parts: convergent cone, throat and divergent cone. The convergent cone is of shorter length so as to accelerate the fluid with lesser energy loss and the divergent cone is made longer which avoids the flow separation and reduces losses. By measuring the pressure drop with the help of manometer the flow rate can be calculated by applying Bernoulli’s equation.
2. ORIFICE METER
The orifice meter consists of a flat circular plate provided with a circular hole called orifice fitted concentric with the axis of the pipe. The jet coming out of the orifice plate gets contracted to a minimum cross section which is called vena contracta and then expands to occupy the full pipe. At vena contracta the stream lines will be parallel to each other and perpendicular to the cross sectional area of flow. The pressure drop created between the upstream section of the orifice plate and vena contracta is used to compute the discharge through the pipe.
3. NOTCHES
A notch may be defined as an opening provided in the side of a tank or vessel such that the liquid surface in the tank is below the top edge of the opening. It is used for measuring the rate of flow of a liquid though a small channel or a tank.
4. ROTAMETER
The rotameter consists of a vertical conical transparent tube provided with calibrated graduations. A rotor or a float having sharp circular edge is provided inside the tube. As the liquid flows from the tube from bottom the float rises to a level corresponding to the rate of flow and sharp edge indicates the discharge on scale.
5. PITOT TUBE
This is a simple tube used to measure the velocity at a point. The pitot tube works with the principle of reducing the velocity at a point to zero is called stagnation point and the pressure rise is used to determine the velocity. The pressure rise is known as stagnation pressure. The Pitot tube in its simple form is a simple glass tube bent at the right angles with its tip facing the flow direction.
Fig 1.5: Pitot tube