Inlet Calibration
Object : To calibrate the inlet by the use f the traversing pitot-static tube.
Procedure :
A range of volume flow rates are set up using the resistance screens single or in combinations, first
with one fan only running and for higher flows with fans No1 and 2 in parallel.
At each setting the inlet pressure drop is measured to atmosphere using an inclined manometer. Fort
his, connect the negative side of the manometer tube to the inlet pressure tapings and leave the
positive side open to the atmosphere. At the same time, the mean velocity pressure is determined by
traversing the pitot-statics tube. This is also connected to an inclined manometer, the pitot-probe
being connected to the positive side and the static tube to the negative side, so that velocity
pressure is indicated directly. Measurements are made at positions according to the log-linear rule
follows:
0.032D, 0.135D, 0.321D, 0.679D, 0.865D, 0.968D
Where D=diameter of the duct at the traverse cross-section.
Atmospheric temperature and pressure should also be recorded to determine air density for the
calculation of mean velocity.
Results and Calculations:
Diameter of the duct, D=290 mm =0ç29m
Positions in the duct from the center (to the nearest mm.) at which the center of the Pitot - static
tube should be set:
0.032D
136
i)
ii)
0.135D
106
0.321D
52
0.679D
-52
0.865D
-106
0.968D
-136
Plot a graph of inlet pressure drop (pi) against mean velocity pressure (Pv), obtained by
averaging the 6 values of Pv.
From the graph obtain a relationship expressing Pv in terms of Pi, i.e. Pv=K. Pi where K is
constant of proportionality.
Pv =1/2p v2 (N/m2 or Pa)
Rearringing gives V= sqrt(2*Pv)/p) (m/s)
Where V= mean air velocity in the duct
P= air density (kg/m3)
=0.4627B/(273+t)
Where B=atmospheric pressure in mm. Hg.
And t=atmospheric temperature in C0
Q= air volumetric flow rate (m3/s)
A= cross-sectional area of flow = 0.066m2
Then Q=A*V = 0.066*sqrt(2*Pv/p) in here sqrt means square root
= 0.0934 sqrt(K*Pi/p) m3/s
Anemometer Calibration
Object: To determine the true air vaelcity from an anemometer by calibrating it against a pitotstatic tube.
Procedure: A range of velocities are set up in the duct as in previous experiment. The
anemometer is adjust to be axial with the flow and th epitot-static tube postioned so the that it
is axial and in line with the centre of the anemometer vane whell.
Connect the Pitot-static tube to an inclined manometer to measure velocity pressure (Pv). At
each velocity setting, take three readings of Pv and velocity with the Pitot-static tube and
anemometer respectively. These are then averaged and the true air velocity as read on the
anemometer (Va).
Atmospheric temperature and pressure should be recoreded to determine air density.
Yaw of a Pitot-static Tube
Object: To determine the effect of mis-aligment of Pitot-static tube to an airstream.
Definition : “Yaw” or “Angle of Yaw” is the angle between the axis of the instrument and
direction of the mean velocity
Procedure:
Axially align the Pitot-static tube i,rr the centre of the duct checking that the protractor reads
zero degrees (if not then adjust it to be so.) Connect the Pitot-static tube to an inclined manometer
to measure velocity pressure (Pv).
Set up a suitable velocity pressure' say about 80 Pa; this may be achieved by using 2 fans in
parallel with one 8 mesh/inch. resistance screen in the inlet. Connect the inlet pressure tappings to
an inclined manometer to check that mean air velocity does not vary during the experiment.
Measure the velocity pressure ir the axial position (Pvo) and at 50 increments between +- 450
measured on the protractor.
Note the inlet pressure drop and atmospheric temperature and pressure.
Record results.
Part B : Static Pressure Variation with Angle of Yaw
With thhe Pitot static tube positioned as in Part A, disconnected the total pressure tapping. Set
up the static presuure to about 200 Pa; this may be achieved using 2 fans in parallelwith one 8
mesh/inch resistance screen in the inlet. Again check to ensure there are no variations in velocity.
Measure the static pressure in axial position (Pso) and at 50 increments between +-450
Note the inlet pressure drop and atmospheric temperature and pressure.
Yaw of an Anemometer:
Object: To determine the effect of misalignment of an anemometer to an airstream.
Definition: “yaw” and angle of yaw” is the angle between the axis of instrument and direction of the
mean velocity.
Procedure: Axially align the anemometer and check that the protractor reads zero degrees
Set up a suitable air velocity, say about 10m/sec; this may be achieved using No:3 fan with two 8
mesh/inch an done 20 mesh/inch resistance screens in the inlet.
Connect the inlet pressure tapings to an inclined manometer to check that mean air velocity does not
vary during the experiment.
Measure the anemometer count as described in previous section. Repeating this 3 times to achieve
satisfactory mean value of air velocity Vao. Read the anemometer at 50 of increments of yaw
between ±400 measured on the protractor.
Note the inlet pressure drop and atmospheric temperature and pressure.
Inlet Calibration Typical Results
Atmospheric Temperature: 15 0C
Atmospheric Pressure: 762.5 mm. Hg.
Inlet
Pressure
Pi (Pa)
13
21
30
37
50
73
127
Pitot-Static Reading, Pv (Pa) at
136
(mm.)
8
12
20
24
43
46
105
106
(mm.)
14
21
32
36
50
72
132
52
(mm.)
16
24
34
40
51
79
132
-52
(mm.)
15
24
33
40
52
80
130
Pv=(64.2/100)*Pi =0.942 *Pi (pa)
Q= 0.0934 sqrt(0.942Pi/p) = 0.0907 sqrt(pi/p)
M=p.Q= p*0.0907 sqrt(pi/p) = 0.0907 sqrt(pi*p)
-106
(mm.)
13
21
32
39
50
75
128
Mean
Pv
(pa)
-136
(mm.)
9
14
22
28
35
56
103
12.5
19.3
28.8
34.5
46.8
68
121.6
MEAN VELOCITY PRESSURE PV (PA)
INLET PRESSURE AGAINST VELOCITY
PRESSURE
140
120
100
80
60
40
20
0
0
20
40
60
80
100
120
140
INLET PRESSURE (PI)
Anemometer Calibration: Typical Results
P=(0.4627*768.5)/ (273+16) = 1.23 kg/m3
Thus true velocity
V= sqrt(2*Pv/1.23) = 1.27 sqrt(Pv)
The Graph of Va against V shows a linear relationship over the range tested. The equation for the
straight-line is:
V= 0.98 * Va m/s
If the count obtained on the anemometer is 600m in 1 minute, then :
Va=600/60 = 10 m/s
Giving V= 0.980 * 10 =9.8 m/s
Velocity Pressure and Anemometer Readings
Atmospheric temperature: 16 0C
Atmospheric pressure = 768.5 mm. Hg.
Velocity
Pressure,
Pv (Pa)
11
11
11
13
13
13
15
15
16
Anemometer
Counts (m)
Start
200
700
100
700
300
700
400
800
200
Stop
500
1000
400
1000
600
1000
700
1100
500
Lapse
Time (s)
Second
69.0
69.4
69.4
64.7
64.8
64.7
59.4
59.3
59.2
Mean Air
Velocity
Va (m/s)
4.33
Mean Pv
(pa)
11
True Air
Velocity
V (m/s)
4.21
4.63
13
4.58
5.06
15.3
4.97
20
20
20
24
24
24
31
32
31
40
41
40
46
45
46
56
54
56
300
800
300
600
100
600
300
800
300
400
200
900
800
600
300
300
000
700
700
1200
700
1000
500
100
700
1200
700
1000
800
1500
1400
1200
900
900
600
1300
69.5
69.5
69.5
63.2
63.2
63.1
55.6
55.3
55.7
73.2
72.6
73.1
68.8
69.0
68.3
62.6
62.3
61.2
5.76
20
5.68
6.33
24
6.22
7.2
31.3
7.15
8.19
40.3
8.05
8.73
45.6
8.58
9.68
55.3
9.45
Yaw of a Pitot-static Tube : Typical Results
From the graph of log. (% Pressure Variations) Vs. log. (Angle of Yaw) 5% error iin velocity pressure
occurs around ±20 0 of jaw of the Pitot-static tube, and 5% error in static head occurs around +- 250
yaw, showing the velocity pressure measurement is a little more sensitive to misalignment.
Velocity Pressure variation with Angle of Yaw
Atmospheric temperature = 15 0C
Atmospheric pressure = 762.5 mm. Hg.
Inlet Pressure drop = 65 Pa
Angle of Yaw, α
(degrees)
0
5
10
15
20
25
30
35
40
45
0
-5
-10
-15
-20
-25
-30
Velocity Pressure
Pvα
81
81
80
80
78
75
69
63
52
46
81
81
80
79
76
72
66
Pvα / Pv0
100 (1-Pvα-Pvo)
1.000
1.000
0.988
0.988
0.963
0.926
0.852
0.778
0.642
0.568
1.000
1.000
0.988
0.975
0.938
0.889
0.815
0
0
1.2
1.2
3.7
7.4
14.8
22.2
35.8
43.2
0
0
1.2
2.5
6.2
11.1
18.5
-35
-40
-45
0
60
50
42
81
0.741
0.617
0.519
1.000
25.9
38.3
48.1
0
Psα / Ps0
100 (1-Psα-Pso)
1.000
1.005
1.015
1.024
1.034
1.049
1.059
1.068
1.088
1.112
1.000
1.010
1.015
1.024
1.029
1.044
1.059
1.073
1.098
1.117
1.000
0
0.5
1.5
2.4
3.4
4.9
5.9
6.8
8.8
11.2
0
1.0
1.5
2.4
2.9
4.4
5.9
7.3
9.8
11.7
0
Static Pressure variation with Angle of Yaw
Atmospheric temperature = 15 0C
Atmospheric pressure = 762.5 mm. Hg.
Inlet Pressure drop = 65 Pa
Angle of Yaw, α
(degrees)
0
5
10
15
20
25
30
35
40
45
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
0
Static Pressure
Pvα
205
206
208
210
212
215
217
219
223
228
205
207
208
210
211
214
217
220
225
229
205
Yaw of pitot static tube
Static Pressure ratio
Velocity Pressure ratio
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
-60
-40
-20
0
20
40
Angle of yaw
Yaw of anemometer
Angle of
Yaw α
Anemometer
Counts m
0
5
10
15
20
25
30
35
40
0
-5
-10
-15
-20
-25
-30
-35
-40
0
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
Seconds
60.8
60.6
60.3
59.9
59.3
60.3
63.8
69.4
74.8
60.8
60.1
59.5
59.3
59.2
60.8
64.8
68.0
75.9
60.8
Mean Air
Velocity
Va (m/s)
8.22
8.25
8.29
8.35
8.43
8.29
7.84
7.2
6.68
8.22
8.32
8.4
8.43
8.45
8.22
7.72
7.35
6.61
8.22
Vaα / Vao
1.000
1.003
1.008
1.015
1.025
1.008
0.953
0.876
0.813
1.000
1.012
1.022
1.025
1.027
1.000
0.938
0.894
0.804
1.000
60
Yaw of anemometer
1.05
1
Velocity Ratio
0.95
0.9
0.85
0.8
0.75
0.7
-50
-40
-30
-20
-10
0
Angle of Yaw
10
20
30
40
50