Flow Sensor Principle
a. Flow Sensors Based on Pressure Differences
This method is based on Bernoulli’s Law which states the relationship:
Where:
P = fluid pressure
ρ = period of fluid type
v = speed of fulida
g = Earth’s gravity
h = high fluid (elevation)
Figure 3.59 Constitution Law
If h 1 and h 2 are made equal in height then
Caution: The above formula applies only to Laminer flows , ie streams that meet the principle of continuity.
Pitot pipes, orifice plate, venturi pipe and flow Nozzle using Bernoulli’s law above. The basic principle is to form a slight change of velocity from the fluid flow to obtain observable pressure changes. The fluid flow velocity change can be made by changing the diameter of the pipe, this relationship is obtained from the fluid flow control law.
Consider the following formula: A 1 . D 1 = A 2 . D2 , where: A = the cross-sectional area of the pipe, B = the fluid discharge
Since fluid flow is directly related to fluid velocity, it is clear that the fluid velocity can be changed by altering the diameter of the pipe.
b. Orifice Plate
The measuring instrument consists of a pipe where the inside is given a hollow plate smaller than the diameter of the pipe. Pressure sensor placed side of the inlet plate (P 1 ) and one on the side of the outlet plate plate (P 2 ). If there is flow from the inlet to the outlet, then the pressure P 1 will be greater than the outlet pressure P 2 .
The main advantages of this Orfice plate are from:
1. Simple construction
2. The size of the pipe can be made exactly the same as the size of the connection pipe.
3. The price of making the tool is quite cheap
4. Output is large enough
The disadvantages of using this method are:
1. If there is a solid part of the fluid flow, then the solid part will be collected on the plate side of the inlet.
2. The reach of measurement is very low
3. Turbulent flow may result in a measurement error so large because it does not follow the Laminer flow principle .
4. Not possible when used to measure low pressure fluid flow.
Figure 3.60 Orifice Plate
