Problem 4: A venturi meter is measuring the flow of water; it has a main diameter of 3.5 cm tapering down to throat diameter of 1.0 cm. If the pressure difference is measured to be 18 mm Hg, what is the speed of the water entering the venturi throat?
Solution
Given
Main diameter = 3.5 cm = 0.035 m = d1(say)
Cross-sectional area of main dimeter
Throat diameter = 1.0 cm = 0.01 m = d2(say)
Area of the throat diameter = 
Pressure difference = 18 mm Hg = 18 × 133 = 2394 pascals
Density of water = ρ = 1000 kg/m3
Pre-requisite
Before putting hand on the problem, we should know that,
(1) 1 mm of Hg = 133 pascal
(2) If ‘d’ is the diameter of the cross-sectional area, then the area ‘A’ of the cross-section is given by,
A = πr2
where ‘r’ is the radius of the cross-sectional area = d/2.
Required: Velocity of water, v
Formula
Put values in the formula;
