Experiment 6
Measurement of the speed of an airflow using Pitot-static tube and
Venturi meter
Data Collection, Processing and Presentation
The aim of the experiment was to determine the speed of an airflow using two
different techniques, which are both often used in everyday life. The first technique was with
the Pitot tube and the second one uses the Venturi tube.
1. Pitot tube
The speed of airflow can be determined, if the change in pressure between two holes
in the Pitot tube is known. The formula for the speed of airflow is derived using Bernoulli’s
equation as shown bellow:
p1 
v2
2
 p2  0
p  p2  p1 
v
; ( z1  z2 )
 v2
2
2p

where v is the velocity of airflow,  is the density of air and p is the change in pressure,
which can be calculated p   water gh .
The density of air is  = 1.2 kg/m3 and the density of water is water = 1000 kg/m3.
Measured results and airspeeds at various distances are presented in table 1.
1
2
3
4
s [cm]
20  1
30  1
40  1
50  1
h [cm]
2.0  0.1
0.3  0.1
0.2  0.1
0.0  0.1
v [m/s]
18  1
71
61
01
Table 1: measured difference in levels of water (h) and calculated airspeeds (v) at various distances from the
nozzle of the vacuum cleaner (s)
2. Venturi meter
Another way of determining the speed of airflow is with Venturi meter. The speed can
be calculated using the both diameters (d1 and d2) of the tube and the change in pressure at the
beginning of the tube and the pressure in the narrow part of the tube:
(1) p1 
 v12
2
 p2 
 v2 2
2
d 
(2) V  v1S1  v2 S2  v2  v1  1 
 d2 
p  p1  p2 
v1 
 v12
2
( ) 
d1 4
d2
2
 v12
2
2 p
 ( dd12 ) 4  1
The diameter at the beginning of the tube was d1 = (5.0  0.1) cm and at the narrow
part d2 = (3.0  0.1) cm. The change in pressure can be determined with the change of water
levels in U-tube connected to the Venturi meter p   water gh . Measured differences levels of
water (h) and calculated airspeeds are shown in table 2.
1
2
3
4
5
s [cm]
10  1
20  1
30  1
40  1
50  1
h [cm]
3.9  0.1
3.0  0.1
1.3  0.1
0.6  0.1
0.4  0.1
v [m/s]
10  1
91
61
41
31
Table 2: measured difference in levels of water (h) and calculated airspeeds (v) at various distances from the
nozzle of the vacuum cleaner (s)
3. Comparison of two techniques
Speed of airflow at different distances from the nozzle of the vacuum cleaner was
calculated using two different techniques. The values are compared in table 3.
1
2
3
4
5
s [cm]
10  1
20  1
30  1
40  1
50  1
vpitot tube [m/s]
18  1
71
61
01
vventuri meter [m/s]
10  1
91
61
41
31
v [m/s]
10  1
13  1
61
51
21
Table 3: comparison of the values of airflow gathered with Pitot tube and with Venturi meter
A graph of the average speed of the airflow versus the distance of the nozzle of the
vacuum cleaner is shown in figure 1. The value at the point s = 10 cm was omitted, because
the speed, due to technical problems, could not be measured with Pitot tube.
20
18
16
14
v [m/s]
12
10
8
6
4
2
0
10
15
20
25
30
35
40
45
50
s [cm]
Figure 1: a graph of the speed of the airflow versus the distance from the nozzle
Conclusion and Evaluation
Results of the experiment support the assumption that the speed of airflow is
decreasing as the distance from the nozzle of the vacuum cleaner increases. This is because
the moving air is spreading (due to diffusion with outside air) as it moves. Therefore the speed
has to decrease as shown with the formula of conservation of the volume flow rate:
V  S1v1  S2v2  v2  v1 SS
1
2
S2  S1  v2  v1
On the other side, more complex conclusion cannot be deduced from the results,
because the equipment used, did not produce reliable results.
If the speed of airflow measured with Pitot tube and Venturi meter is compared, then
can be seen that the speeds are very different. That means the results are not reliable. Why is
there such a big difference between the two techniques? The tubes used could have some
small pores, where air would leak out. Secondly and more probable reason is because the
change in pressure could not be measured very accurately. The water level was oscillating all
the time, so the difference of the levels could not be determined exactly.
For more accurate results, the different technique for measuring the difference in
pressures should be used. Using digital manometer would certainly produce much better
results. Another suggestion is that the U-tube should be enlarged so that more liquid would be
in the tube. In that way the amplitude of the oscillations would be much smaller. Furthermore,
instead of using water in the U-tube, other liquids, which are more viscose, could be used (oil,
mercury…)