Practical guide
Air flow measurements
in ducts according to
DIN EN 12599.
Air flow measurements in ducts according to DIN EN 12599
Introduction.
Nowadays, we spend most of the day
Ensuring adequate air exchange and
in closed rooms. This is why heat-
thus determining the volume flow is an
ing, ventilation and air conditioning
important quality factor when it comes
(HVAC) systems are installed which are
to commissioning and operating HVAC
intended to ensure pleasant ambient
systems. The reliable determination of
conditions indoors. Ventilation is of
air velocity in ducts is one of the most
particular importance here. Firstly, it
challenging measurements which a
is not only used to provide fresh air,
ventilation and air conditioning techni-
but also for extracting pollutants, for
cian has to carry out.
instance removing excess humidity
from rooms.
2
Contents:
1. Importance of air velocity.......................................................................... 04
2. M
easurement of the correct air velocity...................................................... 05
3. The right measuring location..................................................................... 06
3.1 Flow profiles in the duct....................................................................... 07
3.2 Distance from disruptions.................................................................... 08
4. The measurement method......................................................................... 10
4.1 Trivial method...................................................................................... 11
4.2 Centroidal axis method........................................................................ 12
4.3 Calculation of the volume flow............................................................. 13
5. Evaluation of the readings
6. The measurement report
........................................................................ 14
.......................................................................... 20
3
Air flow measurements in ducts according to DIN EN 12599
1. Importance of
air velocity
In line with the motto: "The more, the
On the other hand, too low a volume
merrier", HVAC systems are often
flow can also be problematic. The
operated with air volumes that are too
people in the room have too little
high. This excessive requirement leads
fresh air to breathe in. The indoor air
to increased operating costs. Energy
is "stale", because the CO2 content
expenditure for the fan rises, because
in the room is too high. Low volume
a larger volume of air has to be moved
flows can also have negative impacts
through the system. However, costs
on the system's hygiene: there is the
are also incurred for conditioning
risk of germ formation in the system
the air (cooling, heating, humidifying
when movement of the humidified air
or dehumidifying) and these can be
in the ducts is too slow. A correctly set
reduced when the system is set cor-
HVAC system therefore not only helps
rectly. In addition, a high air exchange
make the indoor climate comfortable,
often leads to draughts occurring in
but also helps save costs.
the room, making people feel uncomfortable.
4
2. M
easurement of the
correct air velocity
The key parameter for evaluating the
In order to meet quality requirements
functional capability of the HVAC
when it comes to determining volume
system is the air volume flow. This is
flow, there are different standards all
the product of flow velocity and duct
over the world dealing with the correct
area. Since, in practice, flow velocity in
measurement of flow velocities. In
the duct cross-section is not the same,
addition to EN 12599, which is the
an individual point measurement
leading standard in Germany and
does not suffice when it comes to
large parts of Europe, there are also
determining the average air velocity.
EN 16211 and ASHRAE 111. What all
Disruptors, such as dampers, elbows
methods have in common is that the
and the like, have an influence on
measuring points are distributed over
the velocity profile in the duct, which
the duct cross-section according to
means a so-called grid measurement
the size of the duct in line with defined
has to be carried out at several
specifications, that a distinction
locations in the duct.
is made between rectangular and
round ducts and that the readings are
averaged.
We will now go into the correct
measurement of volume flow
according to EN 12599.
5
Air flow measurements in ducts according to DIN EN 12599
3. The right measuring location
The decisive factor when it comes to
• Minimum distances from disruptions
meaningful measurements is selecting
must be adhered to: at least 6-times
a suitable measuring point. This is
the hydraulic diameter downstream,
already established by the system
and 2-times the hydraulic diameter
planner in the execution plan (project
upstream
plan). The following criteria must be
• The measuring points must be
taken into account here:
easily accessible and there must be
• Air flow measuring points must be
enough space available for handling
allowed for on all main ducts and
on supply lines to rooms with high
requirements.
the measuring instrument.
• The flow must be free of any return
flow or swirling
Round
Rectangular
Square
Dh = diameter
Dh = edge length
Calculation of the hydraulic diameter Dh for different duct shapes.
6
Dh =
2×duct length×duct width
duct length+duct width
3.1. Flow profiles in the duct
Air which flows through a duct does
Turbulent flow.
not have a uniform velocity. As a
In this case, the flow velocities are to
rule, the air in the middle flows
a large extent identical right across
faster than at the duct wall. There
the duct diameter, but the velocity
are greater resistances at the duct
does drastically fall at the duct wall.
wall due to friction and these have to
However, the flow lines are non-
be overcome. A distinction is made
directional, in other words the air
between two basic flow profiles:
moves chaotically and with a high
Laminar flow.
Laminar flow involves a uniform air
flow with the flow lines running parallel
to one another. There is no turbulence
and a distinct maximum velocity in the
middle of the duct. The average flow
velocity is approximately at a third of
the duct diameter. As soon as the air
velocity rises, laminar flow increasingly
converts into turbulent flow.
degree of friction.
All mixed forms between these two
ideal forms are possible, with every
disruptor (such as dampers, elbows,
valves, volume flow regulators, etc.)
changing the flow profile.
In practice, a so-called grid
measurement over the whole duct
cross-section is indispensable for
reproducible results.
7
Air flow measurements in ducts according to DIN EN 12599
Laminar
Turbulent
Max
A(m2)
1/3D mean
Min
Laminar and turbulent air flow velocity. Different flow profiles are generated, depending on the flow velocity.
3.2. Distance from disruptions
Ideal flow profiles are almost
can be considerably reduced.
exclusively found in very long ducts
In practice, dampers, valves,
which run in a straight line and where
elbows and other bends prevent the
there are no disruptions. For this
development of a consistent flow.
reason, minimum distances from
In unfavourable circumstances, this
disruptions have to be adhered to.
results in the maximum of the flow
profile not being in the middle of
Where the distance from disruptions
the duct, but being shifted towards
is sufficient, the number of measuring
the edge, in more problematic
points which have to be measured
circumstances there may also be
spread across the duct cross-section
return flows or areas with no flow.
8
As a rule, return flows are diminished
distorted that a large number of
after a distance of 2-times the
measuring points are required to keep
hydraulic diameter from the disruption,
the measurement uncertainty low.
Distance
7 x Dh
Balanced flow
profile
5 m/s
10 m/s
5 m/s
10 m/s
5 m/s
10 m/s
however the flow profile is so strongly
Distance Distance
2 x Dh
1 x Dh
Distorted flow
profile
D=250
Flow profile with
return flow
AIR
Irregularities in the flow profile are diminished as the distance from the disruption increases. The greater the distance from the disruption, the more uniform the flow profile and the more precise the measurement or the smaller
the number of measuring points required.
9
Air flow measurements in ducts according to DIN EN 12599
4. The measurement method
The representative average flow
round ducts. DIN EN 12599 envisages
value in the duct cross-section
the following two measurement
has to be established to determine
methods:
the air volume flow. To do this, the
measurement area is split into partial
areas and the velocity is determined
at the central point of the partial
areas. This method is called grid
measurement. The method for dividing
the duct cross-section into partial
• the trivial method for measurements
in air ducts with a rectangular or
square cross-section
• the centroidal axis method for
measurements in ducts with a
circular cross-section
areas is different for rectangular and
yi
A
Grid
xi
Duct
Measurement plane
B
Division of the duct cross-section according to the trivial method. The measuring points are at the centre points of
the areas.
10
4.1. Trivial method
The trivial method does not assume
the number of measuring points has
any special velocity distribution in the
to be increased accordingly. It is large
duct. The duct cross-section is simply
enough when the reading fluctuations
divided into several measurement
within a partial area are so small that
areas with an identical size. The
the values measured at the centre
measuring point is in the middle of the
points can be considered to be mean
partial area.
values in the context of the specified
measuring accuracy.
Where there is a uniform velocity
profile, this enables a meaningful
The reading for air volume flow for the
measurement result to be achieved
whole duct is then calculated as an
even with a small number of
arithmetic mean from the partial area
measuring points. Where there are
readings.
larger differences in the flow velocities,
11
Air flow measurements in ducts according to DIN EN 12599
4.2. Centroidal axis method
The procedure for the centroidal axis
axis is the radius (y) which bisects the
method which should be used in round
partial area.
ducts is similar. It involves the circular
duct cross-section being divided into
Since it cannot always be assumed
rings of an equal area and a circle in
that the flow will move forwards in a
the middle. The measuring location in
rotationally symmetrical way in the
the ring area and in the inner circle is
duct, two measurement planes should
on the centroidal axis of each partial
be selected for round ducts which are
area. In this respect, the centroidal
at 90° angles to one another.
Measuring
points
Rings
Centroidal
axes
Determination of the measuring locations of a circular cross-section with the centroidal axis method.
12
4.3. Calculation of the volume flow
The velocity readings determined via
the trivial method or centroidal axis
method should be used to calculate
the average flow velocity, from which
the air volume flow should then be
calculated. The calculation is carried
out using the following formula:
V = A v 3600
V = volume flow in m³/h
v = mean flow velocity in m/s
A = flow cross-section in m²
Example:
A cross-section A of 0.5 m² and a
measured mean velocity of 4 m/s gives a
volume flow of 7200 m³/h
Determination of the volume flow based on the mean
flow velocity and the duct cross-section
13
Air flow measurements in ducts according to DIN EN 12599
5. Evaluation of the readings
There is a requirement in DIN EN
therefore always be seen in connection
12599 for the accuracy of the air
with the distance from disruptions,
volume flow to be determined with a
because these are decisive when it
measurement uncertainty of ±10%.
comes to irregularity in the profile.
Here, the question that now has to
be asked is how accurate was the
Step 1: Determining the
measurement which has just been
irregularity of the flow profile
carried out. DIN EN 12599 also
The required number of measuring
provides answers to this.
points in a specified duct crosssection depends on the irregularity
In addition to the uncertainty of the
(distortion) of the flow profile. The
measuring instrument and the probe
diagram below shows an empirical
used, the irregularity of the flow profile
relationship between the relative
is a crucial factor for determining
distance a/Dh (distance from the
the total error. Where there is a
disruption expressed as quantity
large profile irregularity, the required
of hydraulic diameters) and the
measurement uncertainty of ±10%
irregularity U of the flow profile (as a
can only be achieved with a number
percentage). It can be seen that the
of measuring points that is just as
profile irregularity diminishes as the
large, but this is very time-consuming.
distance increases.
The number of measuring points must
14
a/Dh
10
Determination of the irregu-
8
larity U of the flow profile ac-
7
cording to the distance from
6
the disruption.
5
Example:
4
For a measurement at a
distance of two-times the
3
hydraulic diameter, U is 40%
(reading direction see yellow
arrows).
2
On the other hand, for a = 4
Dh, U is under 20% (green
arrows).
1
4
5
6
7
8
9
10
20
30
40
50 %
15
Air flow measurements in ducts according to DIN EN 12599
Step 2: Determining the number of
read off the number of necessary
required measuring points
measuring points required to adhere
With the value for U which has been
to a particular specified measuring
determined using the diagram, from
accuracy.
the following table you can now
Uncertainty of the measurement τu as a %
Number of
Irregularity of the profile U as a %
measuring
points
2
10
20
30
40
50
4
6
12
20
28
36
42
5
5
11
17
24
31
36
6
5
10
15
21
27
32
8
4
8
13
18
23
27
10
3
7
12
16
20
24
20
2
5
8
11
14
16
30
2
4
7
9
11
14
50
1
3
5
7
8
10
100
1
2
3
5
6
7
200
1
1
2
3
4
5
Uncertainty of the measurement depending on the number of measuring points.
Example: For U = 40% and a specified measurement uncertainty of τU = ±15%, 20 measuring points are needed
(yellow marks, reading direction from top to bottom and then to the left). For U = 20%, 8 measuring points suffice
(green marks).
16
Tip: As you increase the measuring distance from the disruption, you can
reduce the number of required measuring points and thus the effort involved in
the measurement, without impairing the measuring accuracy.
Step 3: Calculating the irregularity
duct cross-section into four quadrants
of the flow profile
with an equal area and determine the
Using your readings, you can now
arithmetic mean of the readings for
check the irregularity of the flow profile
each of the quadrants.
arithmetically. To do this, split the
17
Air flow measurements in ducts according to DIN EN 12599
x
x
x
V2
V1
x
x
x
x
x
Division of the duct
cross-section into four
quadrants. A mean value
is generated in each of
x
x
x
V4
V3
x
x
x
x
give the irregularity of the flow profile
vmax – vmin
2v
U (*100) = irregularity of the flow profile
as a %
Vmax (m/s) = m
aximum of the arithmetic
mean of all four quadrants
Vmin (m/s) = m
inimum of the arithmetic
mean of all four quadrants
V (m/s) = arithmetic mean of the velocity
in the whole cross-section
18
measured flow velocities.
x
The highest and lowest mean value will
U=
the quadrants from the
according to the following formula:
Step 4: Total error calculation
Uncertainty due to influences at
according to DIN EN 12599
the measuring point and measuring
In addition to the measurement
instrument errors (the accuracy of
uncertainty due to (flow) influences at
the measuring instrument and/or the
the measuring point, there are other
probes) have the greatest influence in
possible error sources which may
this respect. With modern measuring
need to be taken into account:
instruments, like the testo 400,
• Measurement uncertainty when
reading off
• Measurement uncertainty of the
mean value (with fluctuating
measurement parameter)
these are automatically taken into
account when the total uncertainty is
calculated, thus supporting standardcompliant performance of the
measurement and documentation of
the results.
• Error of the measuring instrument
display (measuring instrument
error)
• Measurement uncertainties of the
material values, e.g. air density
• Uncertainties with conversion
19
Air flow measurements in ducts according to DIN EN 12599
6. Evaluation of the readings
A full measurement report must
be issued to the client, at the
latest at the time of handover. This
measurement report covers details
of the building and project name,
the specific measuring location, the
agreed target value, the measuring
instrument used, the recorded
readings and the uncertainty of the
Subject to change, including technical modifications.
measurement results, along with the
date and place of the measurement.
The testo 400 enables you to
complete this measurement report
directly in the measuring instrument
and to send it by e-mail. It can’t get
any more effective than that.
www.testo.com