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1. Lafarge Process Kit
Methods
Version 2
Direction Industrielle
Direction Assistance des
Procédés
MEASUREMENT OF AIR FLOW
RATE WITH PITOT TUBE
Page : 1/13
Date : Dec 01, 2008
© Copyright 2006, Lafarge Plâtres
All rights reserved. No part of this document may be reproduced, stored in a retrieval system, translated, or transmitted in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without prior written permission of Lafarge Plâtres. CONFIDENTIAL The information contained herein is confidential and proprietary to
Lafarge Plâtres. It may not be disclosed or transferred, directly or indirectly, to any third party without the explicit written permission of Lafarge Plâtres
SUMMARY
1. PURPOSE............................................................................................................................. 1
2. FIELD OF APPLICATION...................................................................................................... 1
3. PRINCIPLE ........................................................................................................................... 1
4. SAFETY................................................................................................................................. 2
5. EQUIPMENTS....................................................................................................................... 2
6. WORKING INSTRUCTIONS ................................................................................................. 3
6.1. Location of the measurement hole.................................................................................. 3
6.2. Evaluation of the number of measurement points........................................................... 5
6.3. Measurement of air flow with Pitot tube .........................................................................10
6.4. Calculation of the air flow rate........................................................................................12
1. PURPOSE
Measure the air flow rate inside an air duct using the Pitot tube.
2. FIELD OF APPLICATION
All LAFARGE Gypsum Division production sites.
3. PRINCIPLE
Determine the average gas velocity through a pipe section using the Pitot tube. To measure
relevant figures, the section of the pipe or air duct should have uniform velocity repartition and
should not present any vortex zone or return flow. Furthermore the measurement is usually
considered difficult when the gas velocity is lower than 5 m/s.
Dynamic Pressure = Total Pressure – Static Pressure
Total
Pressure
Static
Pressure

2. Lafarge Process Kit
Methods
Version 2
Direction Industrielle
Direction Assistance des
Procédés
MEASUREMENT OF AIR FLOW
RATE WITH PITOT TUBE
Page : 2/13
Date : Dec 01, 2008
© Copyright 2006, Lafarge Plâtres
All rights reserved. No part of this document may be reproduced, stored in a retrieval system, translated, or transmitted in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without prior written permission of Lafarge Plâtres. CONFIDENTIAL The information contained herein is confidential and proprietary to
Lafarge Plâtres. It may not be disclosed or transferred, directly or indirectly, to any third party without the explicit written permission of Lafarge Plâtres
Reminder:
- Do not use the Pitot tube in a corrosive and dusty environment.
- If the relative humidity is high, some water could jam the Pitot Tube. In this case,
remove the tube and shake it.
4. SAFETY
For the measurement of air flow, the technician in charge of the test must wear safety gloves.
5. EQUIPMENT
Equipment recommendations
Equipment Supplier Price estimation
Pitot tube (ref: NPL), Length:
1000mm, Diam.: 8mm
KIMO (France) 400 Euros
Manometer MP 200
(datalogger)
KIMO (France) 600 Euros

3. Lafarge Process Kit
Methods
Version 2
Direction Industrielle
Direction Assistance des
Procédés
MEASUREMENT OF AIR FLOW
RATE WITH PITOT TUBE
Page : 3/13
Date : Dec 01, 2008
© Copyright 2006, Lafarge Plâtres
All rights reserved. No part of this document may be reproduced, stored in a retrieval system, translated, or transmitted in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without prior written permission of Lafarge Plâtres. CONFIDENTIAL The information contained herein is confidential and proprietary to
Lafarge Plâtres. It may not be disclosed or transferred, directly or indirectly, to any third party without the explicit written permission of Lafarge Plâtres
6. WORKING INSTRUCTIONS
6.1. Location of the measurement hole
The pipe section must be located on the straight part of the pipe in order to have uniform shape
and a constant diameter.
The location of the measurement hole has to comply with the following conditions :
1. The upstream distance should be at least equal to 5 times the hydraulic diameter (Dh).
2. The downstream distance should be at least equal to 5 times the hydraulic diameter (Dh) when
the pipe outlet is not connected (stack) or when the downstream perturbations can have an
important influence on the measurement. About the angled pipe, this distance is twice the
hydraulic diameter from the angled pipe.
5 x Dh
minimum 2 x Dh
distance minimum
measurement distance
section
5 x Dh 5 x Dh
minimum minimum
distance gas gas distance
When the downstream perturbation When the downstream perturbation
has an important influence has a slight influence

4. Lafarge Process Kit
Methods
Version 2
Direction Industrielle
Direction Assistance des
Procédés
MEASUREMENT OF AIR FLOW
RATE WITH PITOT TUBE
Page : 4/13
Date : Dec 01, 2008
© Copyright 2006, Lafarge Plâtres
All rights reserved. No part of this document may be reproduced, stored in a retrieval system, translated, or transmitted in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without prior written permission of Lafarge Plâtres. CONFIDENTIAL The information contained herein is confidential and proprietary to
Lafarge Plâtres. It may not be disclosed or transferred, directly or indirectly, to any third party without the explicit written permission of Lafarge Plâtres
Hydraulic diameter: Dh
Dh : hydraulic diameter
S : pipe section
P : pipe perimeter
P
S
4
h
D ×
=
• In the case of a circular pipe which diameter is noted “D”; it is easy to verify that :
D
circular
h
D =
• In the case of a rectangular pipe which length is noted “L” and width is noted “l”; it is easy to
verify that :
)
l
L
(
l
.
L
2
gular
tan
rec
h
D
+
×
=
• In the case of a square pipe which side is noted “L” ”; it is easy to verify that :
L
square
h
D =

5. Lafarge Process Kit
Methods
Version 2
Direction Industrielle
Direction Assistance des
Procédés
MEASUREMENT OF AIR FLOW
RATE WITH PITOT TUBE
Page : 5/13
Date : Dec 01, 2008
© Copyright 2006, Lafarge Plâtres
All rights reserved. No part of this document may be reproduced, stored in a retrieval system, translated, or transmitted in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without prior written permission of Lafarge Plâtres. CONFIDENTIAL The information contained herein is confidential and proprietary to
Lafarge Plâtres. It may not be disclosed or transferred, directly or indirectly, to any third party without the explicit written permission of Lafarge Plâtres
6.2. Evaluation of the number of measurement points
Circular pipe
The method to follow is the “Log-Tchebycheff method” :
• “n” is the number of measurement points per radius (total number of points = 2n)
• “i” appoints one measurement point (1 ≤ i ≤ 2n)
diameter 'D
1st
radius
2nd
radius
i
1 Y1
2n
Yi
Y2n
There is n measurement points per radius and thus 2n measurement points per diameter. Those
points are symmetric to each other from the center of the pipe. There’s no measurement on the
center of the pipe.
“Yi” is calculated as per the equation : Yi = Ci x D using the table presented here under.
Due to the symmetry from the center of the pipe : Ci+ C2n+1-i = 1
n 1
st
radius 2
nd
radius
3
C1 = 0.0320
C2 = 0.1375
C3 = 0.3125
C6 = 0.9680
C5 = 0.8625
C4 = 0.6875
4
C1 = 0.0240
C2 = 0.1000
C3 = 0.1940
C4 = 0.3345
C1 = 0.9760
C2 = 0.9000
C3 = 0.8060
C4 = 0.6655
5
C1 = 0.0240
C2 = 0.1000
C3 = 0.1940
C4 = 0.3345
C4 = 0.3565
C1 = 0.0240
C2 = 0.1000
C3 = 0.1940
C4 = 0.3345
C4 = 0.6435

6. Lafarge Process Kit
Methods
Version 2
Direction Industrielle
Direction Assistance des
Procédés
MEASUREMENT OF AIR FLOW
RATE WITH PITOT TUBE
Page : 6/13
Date : Dec 01, 2008
© Copyright 2006, Lafarge Plâtres
All rights reserved. No part of this document may be reproduced, stored in a retrieval system, translated, or transmitted in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without prior written permission of Lafarge Plâtres. CONFIDENTIAL The information contained herein is confidential and proprietary to
Lafarge Plâtres. It may not be disclosed or transferred, directly or indirectly, to any third party without the explicit written permission of Lafarge Plâtres
Example : n= 3 with a 300 mm pipe.
In that case the total measurement points is 6 (Y1 to Y6). The hereunder table
presents the results of calculation :
1
st
radius 2
nd
radius
i Ci Yi (mm) i Ci Yi (mm)
1 C1 = 0.0320 10 6 C6 = 0.9680 290
2 C2 = 0.1375 41 5 C5 = 0.8625 259
3 C3 = 0.3125 94 4 C4 = 0.6875 206
D = 300 mm
Y1 = 10
Y2 = 41
Y3 = 94
Y4 = 206
Y5 = 259
Y6=290
1 2 3 4 5 6

7. Lafarge Process Kit
Methods
Version 2
Direction Industrielle
Direction Assistance des
Procédés
MEASUREMENT OF AIR FLOW
RATE WITH PITOT TUBE
Page : 7/13
Date : Dec 01, 2008
© Copyright 2006, Lafarge Plâtres
All rights reserved. No part of this document may be reproduced, stored in a retrieval system, translated, or transmitted in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without prior written permission of Lafarge Plâtres. CONFIDENTIAL The information contained herein is confidential and proprietary to
Lafarge Plâtres. It may not be disclosed or transferred, directly or indirectly, to any third party without the explicit written permission of Lafarge Plâtres
Rectangular pipe
The equal areas are represented on the drawing here under :
L/2n
l/2n
l
n points
N
p
o
i
n
t
s
L
• “N” is the number of measurement points on the length “L”.
• “n” is the number of measurement points on the width “l”.
The number of equal areas is : N x n
The smaller distance from one measuring point to the wall is
n
2
L
for the length.
The smaller distance from one measuring point to the wall is
n
2
l
for the width.
The following tables are presenting some examples for the values of : N, n,
n
2
L
,
n
2
l
.
For different square pipes : 1
l
L
= and for different rectangular pipes : 1
l
L

8. Lafarge Process Kit
Methods
Version 2
Direction Industrielle
Direction Assistance des
Procédés
MEASUREMENT OF AIR FLOW
RATE WITH PITOT TUBE
Page : 8/13
Date : Dec 01, 2008
© Copyright 2006, Lafarge Plâtres
All rights reserved. No part of this document may be reproduced, stored in a retrieval system, translated, or transmitted in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without prior written permission of Lafarge Plâtres. CONFIDENTIAL The information contained herein is confidential and proprietary to
Lafarge Plâtres. It may not be disclosed or transferred, directly or indirectly, to any third party without the explicit written permission of Lafarge Plâtres
For a square area 1
l
L
=
L (m) N L/2n (cm) l (m) n l/2n (cm)
0.5 2 12.5 0.5 2 12.5
1.0 4 12.5 1.0 4 12.5
1.5 5 15.0 1.5 5 15.0
2.0 6 16.7 2.0 6 16.7
2.5 7 17.9 2.5 7 17.9
3.0 8 18.8 3.0 8 18.8
For a rectangular area 5
.
1
l
L
=
L (m) N L/2n (cm) l (m) n l/2n (cm)
0.75 3 12.5 0.5 2 12.5
1.50 5 15.0 1.0 4 12.5
2.25 7 16.1 1.5 5 15.0
3.00 8 18.8 2.0 6 16.7
3.75 8 23.4 2.5 7 17.9
The curve here under is a resume of those results :
Number of measuring points
as a function of L and l
0
1
2
3
4
5
6
7
8
9
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
L or l (m)
N
or
n

9. Lafarge Process Kit
Methods
Version 2
Direction Industrielle
Direction Assistance des
Procédés
MEASUREMENT OF AIR FLOW
RATE WITH PITOT TUBE
Page : 9/13
Date : Dec 01, 2008
© Copyright 2006, Lafarge Plâtres
All rights reserved. No part of this document may be reproduced, stored in a retrieval system, translated, or transmitted in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without prior written permission of Lafarge Plâtres. CONFIDENTIAL The information contained herein is confidential and proprietary to
Lafarge Plâtres. It may not be disclosed or transferred, directly or indirectly, to any third party without the explicit written permission of Lafarge Plâtres
Example : Rectangular pipe
length L = 50 cm
width l = 40 cm
Considering the curve here above :
For the length : N = 2
For the width : n = 2
The distances between each point are presented in the table here under :
L (m) 0.5
L (m) 0.4
N 2
n 2
L/2N (mm) 125
l/2n (mm) 100
The drawing for the measurement points location is the following one :
500
400
100
125
The total measurement points is : N x n = 4 x 4 = 16

10. Lafarge Process Kit
Methods
Version 2
Direction Industrielle
Direction Assistance des
Procédés
MEASUREMENT OF AIR FLOW
RATE WITH PITOT TUBE
Page : 10/13
Date : Dec 01, 2008
© Copyright 2006, Lafarge Plâtres
All rights reserved. No part of this document may be reproduced, stored in a retrieval system, translated, or transmitted in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without prior written permission of Lafarge Plâtres. CONFIDENTIAL The information contained herein is confidential and proprietary to
Lafarge Plâtres. It may not be disclosed or transferred, directly or indirectly, to any third party without the explicit written permission of Lafarge Plâtres
6.3. Measurement of air flow with Pitot tube
• Connect the plastic pipe to the Pitot tube to
measure the Static Pressure as shown on
the attached picture.
• Connect the plastic pipe to the Pitot tube to
measure the Total Pressure as shown on
the attached picture.

11. Lafarge Process Kit
Methods
Version 2
Direction Industrielle
Direction Assistance des
Procédés
MEASUREMENT OF AIR FLOW
RATE WITH PITOT TUBE
Page : 11/13
Date : Dec 01, 2008
© Copyright 2006, Lafarge Plâtres
All rights reserved. No part of this document may be reproduced, stored in a retrieval system, translated, or transmitted in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without prior written permission of Lafarge Plâtres. CONFIDENTIAL The information contained herein is confidential and proprietary to
Lafarge Plâtres. It may not be disclosed or transferred, directly or indirectly, to any third party without the explicit written permission of Lafarge Plâtres
• Measure the Static Pressure in one
location. Protect the measurement hole on
the air duct with a card board.
• Record the value from the display
• Measure the Total Pressure. Protect the
measurement hole on the air duct with card
board.
• Record the value from the display
(averaged value (minimum integration time
: 30s*, 3 values per location, the highest
one is kept).
• Move the Pitot tube on the section pipe
according to the defined location of
measurement points.**

12. Lafarge Process Kit
Methods
Version 2
Direction Industrielle
Direction Assistance des
Procédés
MEASUREMENT OF AIR FLOW
RATE WITH PITOT TUBE
Page : 12/13
Date : Dec 01, 2008
© Copyright 2006, Lafarge Plâtres
All rights reserved. No part of this document may be reproduced, stored in a retrieval system, translated, or transmitted in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without prior written permission of Lafarge Plâtres. CONFIDENTIAL The information contained herein is confidential and proprietary to
Lafarge Plâtres. It may not be disclosed or transferred, directly or indirectly, to any third party without the explicit written permission of Lafarge Plâtres
* :For the Pitot tube measurement, the dynamic pressure is measured at 6 different locations per
diameter and with 30seconds at least as integration time. In the following example, more than 30
seconds is not necessary as the cumulative mean is quite stable (see following chart, to be
checked for each casewith the datalogger, the signal can be less stable than that sometimes).
D ata / C u m u lative m ean and stand ard d eviation
30
35
40
45
50
55
60
65
70
0 20 40 60 80 100 120 140 160 180 200
Data
/
Cumulative
mean
0
1
2
3
4
5
6
7
8
9
10
Cumulative
standard
deviation
D ynam ic pressure C um ulative m ean C um ulative standard deviation
** : Some specific connections (with plastic electrical fittings…) can be done in order to avoid
movements of the Pitot tube and to make safely the measurement :

13. Lafarge Process Kit
Methods
Version 2
Direction Industrielle
Direction Assistance des
Procédés
MEASUREMENT OF AIR FLOW
RATE WITH PITOT TUBE
Page : 13/13
Date : Dec 01, 2008
© Copyright 2006, Lafarge Plâtres
All rights reserved. No part of this document may be reproduced, stored in a retrieval system, translated, or transmitted in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without prior written permission of Lafarge Plâtres. CONFIDENTIAL The information contained herein is confidential and proprietary to
Lafarge Plâtres. It may not be disclosed or transferred, directly or indirectly, to any third party without the explicit written permission of Lafarge Plâtres
6.4. Calculation of the air flow rate
To calculate the value of the air flow, use Thermicalc software as shown below:
Key in the “yellow cells” the following figures:
- Dynamic pressure (calculated with Total Pressure and Static Pressure figures
measured with the Pitot tube following the above test method)
- Static pressure (measured with the Pitot tube following the above test method)
- Dry temperature (following the method from the Lafarge Process Kit)
- Wet Bulb temperature (following the method from the Lafarge Process Kit)
- Diameter of the section
- O2 and CO2 content (if you cannot measure those figures, do an assessment of
the figures according to the location of the air flow measurement)
- Atmospheric pressure (according to the location of the plant)