PCERT LEV COURSES
Page 1
MATHS REVISION For P601/P602/P604
Velocities and Velocity Pressure
Although for most of you your instrument will automatically calculate the duct velocity
at a traverse point—for P601/P602 and P603 you must be able to do this calculation
yourselves from first principles using the equations below:-
V
=
1.29√Vp
V = Velocity in m/s
Vp = Velocity Press (Pa)
√ = the square root sign on your calculator ! :-)
The above equation therefore calculates a Velocity from a Velocity Pressure
Sometimes though—you need to do the opposite.
To calculate a Velocity Pressure from a Velocity you need to use the following
version of the equation
Vp
=
(V/1.29)2
(hint—always do your calcs inside the brackets
before you square the answer!)
It is the same equation as the first one – it is just jumbled about to get the Vp on the
left.
In a duct you will collect a number of Velocity Pressures in each of two travers holes
at a single Test Point. It is accepted by the examiners that you simply add all the
Velocity Pressure up (usually 12 readings) and take an average (Nerd warning—actually it is not
the most mathematically correct method because of the square root sign “√” in there—but that is for another day :-))
Before we look at some examples—remember that the units we use are Pa (Pascals)
for pressure and m/s for velocity
And …… the above equations only work for air at standard temperature (200C) and
pressure (1013mB). If your temp is significantly above 200C then you can’t use these
versions of the equations !! You have been warned :-)!!
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PCERT LEV COURSES
Page 2
Velocity from Velocity Pressures
We are going to use the following equation. You will need a simple calculator with a
square root (√) button.
V
=
1.29√Vp
Let us assume that you have collected 12 Velocity Pressure readings (6 at each
traverse point = 12 in total) from a Test Point in a duct. You have added all 12 up and
divided by twelve and therefore have the Average Velocity Pressure.
Average Velocity Pressure = 145Pa
Now put that figure into the equation:
V
=
1.29√Vp
=
1.29√145
The first thing you do is put the 145 into your calculator and hit the √ button
So you should get the answer √145 = 12.04
Now to complete the answer we need to multiply the √145 (ie 12.4) by 1.29
V
=
1.29√Vp
=
1.29√145
=
1.29 x 12.04 =
15.5m/s
So in this example the Velocity would be 15.5m/s
Examples:
Velocity Pressure (Vp)
Velocity
126
=
1.29√126
=
1.29 x 11.2
=
14.5m/s
185
=
1.29√185
=
1.29 x 13.6
=
17.5m/s
200
=
1.29√200
=
1.29 x 14.1
=
18.2m/s
100
=
1.29√100
=
1.29 x 10
=
12.9m/s
© Oxyl8 2017
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v1.1
PCERT LEV COURSES
Page 3
Velocity Pressure (Pa) from Velocity (m/s)
We are going to use the following equation from Page 1:-
Vp
=
(V/1.29)2
(hint—remember—always do your calcs inside the brackets
before you square the answer!)
In this case—you will be given the Velocity in m/s and you will be asked to calculate
the Velocity Pressure (Vp) in Pa (Pascals).
So in the exam they could ask (in the Open Book part!) the following:“If the measured Velocity was 24.3m/s—what Velocity Pressure would that equate to in
the duct”?
Vp
=
(V/1.29)2
=
(24.3/1,29)2
=
18.4 x 18.4
=
338.6Pa
=
(18.84)2
Now—have a go at doing the following examples using the same method:-
Velocity
Velocity Pressure
12 m/s
16.4 m/s
18.1 m/s
20.8 m/s
35.2 m/s
Answers: 86.5 Pa, 161.6 Pa, 196.9 Pa, 260 Pa, 744.6 Pa
© Oxyl8 2017
www.oxyl8.com
v1.1