Flow measurements began to be known since 1732 when Henry Pitot regulated the amount of fluid flowing. In fluid measurements it is necessary to determine the amount and vector of the flow velocity at a point in the fluid and how the fluid changes from point to point. Measurement or censorship of fluid flow can be classified as follows:
1. Quantity measurement
These measurements provide clues that are proportional to the total quantity that has been flowing in a given time. The fluid flows through the primary elements successively in a quantity more or less isolated by alternately filling and emptying the measuring vessel of known capacity. Quantity measurements are classified according to:
a. Gravimetry gauge or weight measurement
b. Volumetry gauge for liquids
c. Volumetric meter for gas
2. Flow rate measurement
The flow rate of Q is a function of the area of pipes A and velocity of fluid flowing through the pipe, namely:
Q = AV
but in practice, the speed is uneven, larger at the center. Thus the average measured velocity of a liquid or gas may differ from the actual average speed. These symptoms can be corrected as follows:
Q = KAV
where K is a constant for a given pipe and illustrates the relationship between actual average velocity and measured speed. Constant values can be obtained through experiments. Flow rate measurements are used to measure the velocity of a liquid or gas flowing through a pipe. These measurements are grouped again by jemis of measured materials, liquids or gases, and according to the properties of the primary elements as follows:
a. Measurement of flow rates for liquids
1) the type of deflection vane
2) the type of rotary propeller
3) helical vane type
4) type of turbine
5) the combination meter
6) magnetic flow meter
7) ultrasonic flow meter
8) range flow meters ( vortex )
9) swirl measure ( swirl )
b. Measurement of gas flow rate
1) the type of deflection vane
2) the type of rotary propeller
3) thermal type
3. Measurement of pressure differential method
The most widely used type of flow meter is the differential pressure measurement. In principle, the difference in cross sectional area of the stream is reduced by the resulting speed increase, thus increasing the energy of movement or kinetic energy. Since energy can not be created or eliminated (Law of energy transfer), then this kinetic energy increase is obtained from the pressure energy that changes.
Moreover, if the fluid moves through a uniform (pipe) carrier at a low velocity, then the motion of each particle is generally parallel along the pipe wall line. If the flow rate increases, the peak point is reached when the particle movement becomes more random and complex.
The approximate speed at which this change occurs is called the critical velocity and flow at a higher rate of speed is called turbulent and at a lower rate of speed is called a laminer. The critical speed is also called the Reynolds number, written without dimension:
Where :
D = dimension of cross section of fluid flow, usually diameter
ρ = fluid density
V = fluid velocity
μ = the absolute speed of the fluid
The critical speed limits for pipes are usually between 2000 and 2300. The flow measurement of this method can be done in many ways eg using venturi pipe, pitot pipe, orifice plate (narrow opening), turbine flow meter, rotameter, thermal way, using active radio material, electromagnetics, sonic ulters and gyro flowmeter.