CEN/TR 13582:2021

SLOVENSKI STANDARD
01-april-2021
Nadomešča:
SIST CR 13582:2001
Vgradnja merilnikov toplote - Smernice za izbiro, vgradnjo in delovanje merilnikov
toplote
Installation of thermal energy meters - Guidelines for the selection, installation and
operation of thermal energy meters
Installation von thermischen Energiemessgeräten - Richtlinien für Auswahl, Installation
und Betrieb von thermischen Energiemessgeräten
Compteur d’énergie thermique installation - Lignes directrices pour la sélection,
l’installation et le fonctionnement des compteurs d’énergie thermique
Ta slovenski standard je istoveten z: CEN/TR 13582:2021
ICS:
17.200.10 Toplota. Kalorimetrija Heat. Calorimetry
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

CEN/TR 13582
TECHNICAL REPORT
RAPPORT TECHNIQUE
January 2021
TECHNISCHER BERICHT
ICS 17.200.10
English Version
Installation of thermal energy meters - Guidelines for the
selection, installation and operation of thermal energy
meters
Compteur d'énergie thermique installation - Lignes Installation von thermischen Energiemessgeräten -
directrices pour la sélection, l'installation et le Richtlinien für Auswahl, Installation und Betrieb von
fonctionnement des compteurs d'énergie thermique thermischen Energiemessgeräten

This Technical Report was approved by CEN on 4 January 2021. It has been drawn up by the Technical Committee CEN/TC 176.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2021 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 13582:2021 E
worldwide for CEN national Members.

Contents Page
European foreword . 4
Introduction . 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Selecting a metering device for thermal energy . 7
4.1 General. 7
4.2 Metrological characteristics . 8
4.3 Environmental classifications . 8
5 Dimensioning . 9
5.1 General. 9
5.2 Determining the thermal energy power . 9
5.3 Thermal energy load . 9
5.4 Thermal energy power for water heating . 10
5.5 Thermal energy power for ventilation and air conditioning systems . 11
5.6 Thermal energy power for cooling systems . 11
5.7 Thermal energy power for engineering purposes . 11
6 Determining the flow rate . 12
6.1 Principles of thermodynamics . 12
7 Selecting a flow sensor for a thermal energy meter . 13
8 Checking the flow sensor design after commissioning . 14
8.1 General. 14
8.2 Operating conditions . 14
8.3 Flow sensors . 15
8.4 Temperature sensors . 20
8.5 Calculators . 23
9 Arranging of meters for thermal energy . 24
9.1 General. 24
9.2 Environment . 24
9.3 Flow sensors . 25
9.4 Temperature sensors . 28
9.5 Calculators . 32
10 Installing thermal energy meters . 33
10.1 General. 33
10.2 Mechanics . 33
10.3 Connecting to pipes . 33
10.4 Electrical connections . 33
10.5 Commissioning . 34
11 Monitoring operation . 34
11.1 General. 34
11.2 Measuring cooling supply using water or anti-freeze mixtures as medium . 34
11.3 Requirements for the system arrangement of cooling measurements . 39
12 Other liquids than water . 42
12.1 Introduction . 42
12.2 Physical impact . 42
12.3 Flow measurement . 44
12.4 Temperature difference measurement . 49
12.5 Calculator . 49
Bibliography . 50

European foreword
This document (CEN/TR 13582:2021) has been prepared by Technical Committee CEN/TC 176 “Thermal
energy meters”, the secretariat of which is held by SIS.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.

Introduction
Metering devices for thermal energy (heat and cooling meters) are only working correctly and
consistently if the system design considers the minimum and maximum ratings for temperature,
temperature difference and flow rate according to the approved ranges. The metering device should be
selected for the approved legal range and the application area. The thermal energy meter should be
installed according to the valid requirements. During commissioning the thermal energy meter is checked
for both correct installation and full functionality and afterwards sealed against unauthorized opening.
According to the European harmonized standard EN 1434-6 a commissioning is obligatory to ensure that
the metering device accurately measures the planned or predicted consumption.
Installing the metering devices or their sub-assemblies incorrectly (e.g. an incorrect combination of
temperature sensors with non-approved pockets) does not guarantee the measuring accuracy. Hence,
the measurement deviations may exceed the permissible error limits. National calibration laws state that
the metering point operator should ensure that the metering device is set up, connected, handled and
maintained correctly to guarantee the measuring accuracy. Incorrect measurements result in bills that
cannot be used in business transactions.
The metering point operator is in district heating networks responsible for a proper installation and
commissioning of the metering devices. The metering point operator can also delegate this task to a
service company. The building owner or the building owner’s representative (e.g. a metering service
company) is in sub metering applications responsible for a proper installation and commissioning of the
metering devices.
The EN 1434 standards provide technical principles and practical advice in selecting, installing and
commissioning of thermal energy meters. However, because a standard cannot cover all areas
completely, this report shall assist users of thermal energy meters.
1 Scope
The EN 1434 standards provide technical principles and practical advice in selecting, installing and
commissioning of thermal energy meters. However, because a standard cannot cover all areas
completely, this document assists users of thermal energy meters.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
EN 1434-1, Thermal energy meters - Part 1: General requirements
EN 1434-2, Thermal energy meters - Part 2: Constructional requirements
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 1434-1 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1
DH (network)
district heating system, DC: district cooling system
3.2
meter: thermal energy meter
heat meter or cooling meter
3.3
water
domestic water
3.4
hot water
domestic hot water
3.5
fluid additive
fluid used to supplement a shortage of the heat transfer medium due to leaks
3.6
fluid
heat transfer medium in a DH/DC system
3.7
MID
Measurement Instrument Directive 2014/32/EU
4 Selecting a metering device for thermal energy
4.1 General
A thermal energy meter consists of the following three parts: a flow sensor, a temperature sensor pair
and a calculator (see Figure 1).
These sub-assemblies can be defined as complete instruments, combined instruments or hybrid
instruments (see EN 1434-1).
The calculator unit calculates the energy consumption using the signals from the temperature sensor pair
and the flow sensor.
The minimum temperature difference of the calculator shall not fall below the smallest permissible value
(according to MID the minimum temperature difference is 3 K).
The temperature sensors are usually platinum resistance thermometers of type Pt 100, Pt 500 or Pt 1000.
The sensor pair determines the temperature difference between the inlet (flow) and outlet (return) of
the thermal conveying medium.
The flow sensor is granted an error limit of 2 % to 5 %. Due to faulty design, incorrect installation or wear
the wider error limits of this part/sub-assembly of a meter is exceeded occasionally. This case can be
avoided by selecting the correct flow sensor. An overview of the different types of flow sensors is given
in 8.3.7.
Key
1 inlet
2 outlet
3 calculator
4 outlet temperature sensor
5 inlet temperature sensor
6 flow sensor
7 thermal load
Figure 1 — Thermal energy meter
When operating the heat exchanger circuit system, one may discover that the chosen thermal energy
meter design is not applicable due to the actual requirements.
Flow sensors that are designed for higher flow rates may not have the required accuracy at low flow rates.
If the actual flow rate is below the minimum permissible flow rate, measurements may be skipped until
the measurement fails completely.
Fast changes in energy consumption that place high demands on the dynamics of the meter may cause
significant deviations in the measurement accuracy of the accumulated energy. Fast-response meters
provide measurement characteristics that reduce this deviation (see 8.5.4)
The effects of dirt deposits and flow disturbances over the entire service life of the flow sensors shall be
considered when selecting a meter.
4.2 Metrological characteristics
The accuracy classes and the maximum permissible relative errors of thermal energy meters are
described in EN 1434-1. Be aware that some national regulations do not allow the use of class 3 meters
at all and that other national regulations do not allow the use of class 3 meters for e.g. for q 6 m /h and
p
higher.
Class 2 accuracy is the most frequently used accuracy class for flow sensors.
Due to the very high requirements on both flow sensors and test equipment, the availability of class 1
flow sensors is very limited.
4.3 Environmental classifications
The environmental classes are described in EN 1434-1. Thermal energy meters have an environmental
classification A, B and C regarding Domestic/Industrial EMC requirements and Indoor/Outdoor ambient
conditions.
Table 1 — Relationship between EN 1434-1 and MID re. EMC levels
EN 1434-1 MID (2014/32/EU)
Domestic EMC level Class A and B E1
Industrial EMC level Class C E2
Meters with Class C (E2) marking can be used also in domestic installations, but meters with Class A and
B (E1) shall not be used in industrial installations (see Table 1).
Classes A and C are defined for indoor installations with +5 °C to 55 °C ambient temperature.
Class B is defined for outdoor installation. Since the availability of thermal energy meters for outdoor
installation is limited, special care shall be taken to select a suitable meter or to select a suitable protective
cabinet.
Most thermal energy meters are installed in locations without any vibration. For such installations,
meters with the mechanical class M1 are suitable. In case some vibrations may occur at the installation
site a meter with class M2 shall be selected. In case of more intense vibrations a meter with class M3 shall
be selected (see Table 4 for more details).
5 Dimensioning
5.1 General
When selecting a thermal energy meter, it is important to determine the upper and lower flow limits for
the flow sensor as required by the operating conditions. Based on the range for nominal flow q and
p
minimum flow q one needs to select a suitable flow sensor from the various devices offered by different
i
manufacturers. This selection results in the nominal diameter of the measuring line where the flow
sensor shall be installed.
Simply selecting a flow sensor according to the nominal diameter of an existing pipe is not necessarily
correct. Otherwise the coverage of the lower flow range may be insufficient.
It is often good practice that flow sensor sizes of one nominal diameter smaller than the pipe are chosen
when the expected average flow rates are low.
The thermal energy output commissioned with the customer and the maximum inlet and outlet
temperature for the planned application build the base for calculating the thermal energy supply.
In transfer stations for district heating and cooling, the fluid flow rate shall be limited to the
commissioned value by using a flow rate limiter and/or a differential pressure controller. The controller
protects the consumer circuit and the flow sensor from overloading. Arrange the controller in series after
the flow sensor in the outlet to avoid additional disturbances in the flow profile before the flow sensor.
The expected yearly average flow rate, when known, should preferably be around 2/3 of the nominal
flow q of the flow sensor. As for each flow sensor size the nominal flow q corresponds with about 2 m/s
p p
average flow velocity. This is the basis for the relationship between DN and q , and it minimizes the risk
p
of cavitation as well as loss of accuracy due to wrong meter size.
5.2 Determining the thermal energy power
The metering point operator should perform calculations to determine the thermal energy power only as
a check. Contracted values shall be specified by the customer exclusively.
5.3 Thermal energy load
5.3.1 Standard thermal energy load in new builds
The standard heat load in new buildings and major redevelopments should be determined by a qualified
project engineer, e.g. according to EN 12831-1:2017, Clause 6.
5.3.2 Thermal energy load of buildings with no standard load calculation
If existing buildings are being connected to a thermal energy supply with no standard load calculation,
one could use an approximation or estimation method to determine the thermal energy load for
dimensioning the flow sensor.
If a building connected to a district heating or cooling supply already contains a central heating system,
an approximate thermal energy load can be calculated from an average of the last three years’ annual
consumption, an outside-temperature (see Figure 2) and the expected full usage hours.
Maximum values stored in the thermal energy meter can also be used to determine the output.

Distribution of outdoor temperatures for five European locations, 1881–2000. An indoor temperature has
been added as example.
Key
X hours per year
Y temperature, °C
1 Palermo, Italy
2 Florence, Italy
3 Strasbourg, France
4 Helsinki, Finland
5 Kiruna, Sweden
6 effective indoor temperature 17 °C
1)
Figure 2 — Outdoor temperature duration in Europe
5.4 Thermal energy power for water heating
The thermal energy power for water heating usually needs to be determined by a qualified project
engineer according to accepted engineering standards (e.g. EN 12831-1).
Using a priority control for the water heating and taking advantage of the building’s heat storage capacity
it may be possible to provide the required thermal energy output for short-term peaks of water heating
without having a significant drop in room temperature.
If a priority control is used the qualified project engineer can select the higher value of the required
thermal energy power between the thermal energy output for central heating or cooling and the thermal
energy power for water heating. The higher value is the deciding factor in the selection of the flow sensor.
Parallel operations shall be considered separately.

1) Source reference: Svend Frederiksen, Svend Werner. 2013. District Heating and Cooling. Studenterlitteratur
AB, Lund. Source reference: Figure 4.2 from “District Heating and Cooling” Svend Frederiksen, Svend Werner
ISBN 978-91-44-08530-2.
See Figure 3 for outdoor temperature duration in Europe.

Key
X number of normal apartments
Y required power (kW)
2)
Figure 3 — Outdoor temperature duration in Europe
5.5 Thermal energy power for ventilation and air conditioning systems
The thermal energy power required for ventilation and air conditioning systems should be calculated by
a qualified project engineer.
Depending on climatic requirements, the flow sensor may encounter flow rate peaks during the low load
season if there are ambient inlet temperatures in the district thermal energy network. These peaks shall
be investigated and considered for dimensioning the flow sensor.
5.6 Thermal energy power for cooling systems
In bifunctional systems the flow sensor shall be selected by the maximum flow required for either heat
or cooling. The power should be calculated by a qualified project engineer.
5.7 Thermal energy power for engineering purposes
When supplying heating or cooling for industrial and commercial engineering, it is recommended that
the requirements of the customer and the customer’s qualified project engineer regarding the flow sensor
design are checked. A modulating operating curve in the district thermal energy network can cause
increased flow rate values, especially when there are power peaks in the low load season. This shall be
considered when dimensioning the flow sensor.

2) Source reference: Svend Frederiksen, Svend Werner. 2013. District Heating and Cooling. Studenterlitteratur
AB, Lund. Source reference: Figure 4.2 from “District Heating and Cooling” Svend Frederiksen, Svend Werner
ISBN 978-91-44-08530-2
6 Determining the flow rate
6.1 Principles of thermodynamics
6.1.1 General
The nominal heat or cooling load specifies the thermal energy power required at the measuring point in
the (projected) application.
The maximum thermal energy released or absorbed by a thermal conveying medium in a heating or
cooling circuit is calculated as follows:

Q= kV⋅⋅θθ− (1)
( )
io
where

is the thermal energy power, e.g. kW;

Q

is the flow rate of the thermal conveying medium, in m /h;
V
k is the thermal coefficient, in kWh/m K;
θ is the design temperature in inlet, in °C;
i
θ is the design temperature in outlet, in °C.
o
Convert the formula to calculate the flow rate for the design case.


Q
V= (2)
k⋅θθ−
( )
io
6.1.2 Total maximum power for heating or cooling
Add the outputs specified in Clause 5 to calculate the total thermal energy power.
n

QQ=
(3)
tot
i
∑
i=1
where
 is the total thermal energy power, in kW;
Q
tot
 is the standard thermal load, in kW;
Q
 is the thermal power for hot water heating, in kW;
Q
 
Q Q
NOTE     If using priority control, use only the larger value, either or .
1 2
 is the thermal energy power for ventilation and air conditioning systems, in kW;
Q
 is the thermal energy power for engineering purposes, in kW.
Q
For cooling systems, the thermal load has to be added the same way (identical).
6.1.3 Inlet and outlet temperature
The difference between inlet and outlet temperatures is the temperature difference.
 
∆=Θθ −θ K (4)
inlet outlet 
In general, the inlet temperature of the heating or cooling medium is regulated by the outside
temperature and the outlet temperature is based on the design and operating mode of the heating system.
6.1.4 Thermal coefficient
The thermal coefficient k shall be determined according to EN 1434-1. For example, with θ = 100 °C,
inlet
θ = 50 °C the approximate value of 1,15 [kWh/(m K)] can be expected for outlet meters and
outlet
1,12 [kWh/(m K)] for inlet meters.
7 Selecting a flow sensor for a thermal energy meter
Because the design case described above occurs only for a few days of the year, a flow sensor shall be
selected so that it ensures that the smallest possible deviation occurs over the whole range of the year.
The range of the most frequent flow values (main operation range) at the measuring point is the deciding
factor for selecting the flow sensor.
The operating range of the flow sensor shall be within the approved range which is spread between the
smallest flow q and the nominal flow q .
i p
A flow sensor shall be selected to fulfil all the following criteria:
— the nominal flow q of the flow sensor is as close as possible to the calculated flow rate;
p
— the minimum flow q of the flow sensor is smaller than/equal to the minimum flow of the thermal
i
energy circuit;
— the maximum flow q of the flow sensor is reserved for short term overload (1 hour per day; 200
s
hours per year) in the thermal energy circuit.
If the minimum flow rate of the thermal energy circuit is not covered by the minimum flow q of the flow
i
sensor, it shall be checked whether a smaller flow sensor will cover the design case better.
To achieve the minimum flow rate of the thermal energy circuit, a flow sensor with a higher dynamic
range, q /q , shall be selected.
p i
The nominal flow q shall not be exceeded when selecting the flow sensor.
p
Selecting a flow sensor may be easier if technical measures are taken to reduce the fluctuation range of
the flow.
When selecting a flow sensor statutory regulations and standards, such as EN 1434, the operating
conditions, the manufacturer’s installation instructions and nationally applicable requirements shall all
be considered.
The nominal pressure level (PN/PS) of the flow sensor shall correspond to the pressure class at the
measuring point. In praxis the average pressure should be well below PN.
The permissible temperature range of the flow sensor shall comply with the temperature range of the
thermal conveying medium as well as the ambient temperature at the measuring point. Because of
temperature stress, the flow sensor should generally be installed in the outlet. This is the cooler pipe for
flow sensors in heat meters and this is the warmer pipe for cooling meters.
In combination with low temperature heating installations flow sensors are also installed in the inlet pipe
which is done to avoid measurement drop outs due to water loss in the installation.
8 Checking the flow sensor design after commissioning
8.1 General
The actual operating conditions of the heating circuit may deviate significantly from the calculated or
specified volumes.
Regular intermediate readings and plausibility checks shall be performed when evaluating the readings
to detect any deviations.
If minimum or maximum flow rates deviate significantly, the size of the flow sensor shall be considered.
The maximum flow value stored in the heat meter’s calculator can be used to check the actual flow level.
However, it should be remembered that the value was determined as an average over a thermal energy
meter-specific measuring (integration) period (e.g. one hour).
A plausibility check can also be performed based on the annual full usage hours using regional
temperature values.
If there are any doubts about the plausibility of the heat measurement, an on-site investigation is
recommended.
8.2 Operating conditions
To determine the maximum thermal energy power demand of the building, one needs to select the higher
value between the thermal energy output for generating hot water and the thermal energy output for
central heating or cooling. The higher value represents the output demand. It is generally not necessary
to add both values due to the usual priority control and if a priority control is not applied due to the short
hot water heating time. If a meter that stores the maximum value is commissioned, it can be used to check
the calculation against the measured values and it can be used to determine the appropriate flow range
when changing the meter.
The energy output demand for generating hot water can be calculated using the “Guidelines for hot water
preparation”. These guidelines provide an established European procedure developed by Euroheat &
Power (UNICHAL). The energy output demand can also be determined using the stored maximum values
if a thermal energy meter is available for generating hot water.
The following factors influence the choice of a meter:
— change in temperature difference;
— operating mode of the heat conveying medium circuit;
— quality of installation;
— speed of change (dynamics) in the temperature difference;
— actual required thermal energy demand and flow rate.
It is therefore important for the metering point operator to perform a plausibility check of the
measurement results.
8.3 Flow sensors
8.3.1 General
To optimise the application conditions for flow sensors, the recommendations below shall be considered,
regardless of the used sensor type.
8.3.2 Inlet and outlet pipes
A straight inlet pipe of at least 5 × diameter and an outlet pipe of 2 × diameter in the same dimensions as
the flow sensor is recommended to reduce any effect on the measurement deviation caused by the flow
profile. This recommendation enables most thermal energy meters on the market to be installed
correctly.
The inlet and outlet pipes should not contain any fittings that change the flow profile, e.g. flow limiters,
differential pressure regulators, dirt traps, filters, pipe bends, cross-sectional changes. Temperature
sensors are not allowed in the straight inlet pipe before the flow sensor but can be installed in the straight
outlet pipe after the flow sensor. Gaskets should not protrude into the settling section and connection
fittings should have the same nominal diameter as the flow sensors.
The straight inlet pipe shall be optimized for maximum length, as some disturbances are reduced
significantly over long distances. All structural design options shall be used to achieve straight inlet pipes.
If the measuring equipment was planned in the design stage, it is usually possible to design straight inlet
pipes of sufficient length.
To fulfil the legal metrology, the minimum inlet and outlet pipes shall comply with the specifications in
the approval documents.
In case of narrow installation conditions, which only leave space for an installation of the flow sensor
directly after a tube bend, it shall be ensured that the flow sensor selected is approved for a zero-inlet
pipe.
8.3.3 Influence of insufficient temperature mixing on measuring accuracy
Any kind of a strand formation caused by the combination of two flow circuits with different fluid
temperatures shall be eliminated sufficiently by having a long inlet pipe (at least 10 × diameters) or by
using mixing fittings. This applies to both flow measurement and temperature measurement.
8.3.4 Measurement deviations due to flow disturbances caused by swirl
Flow disturbances caused by swirl are difficult to reduce and should therefore be avoided with a suitable
pipe design. If the course of a pipe cannot be changed, a flow rectifier with a following inlet pipe shall be
used. Dirt traps also reduce flow distortions caused by swirl. At least at the change of the flow sensor the
dirt trap shall be cleaned. The increased pressure drop of these installations shall be considered.
Massively contaminated dirt traps can also cause heavy flow disturbances.
8.3.5 Measurement deviations due to pulsation
Pulsations may be caused by pumps and air pockets. The resulting measurement deviations can be
avoided by arranging the flow sensor with a certain minimum distance to pumps, high points in pipes or
other flow disturbance sources.
8.3.6 Measurement deviations due to the contamination of the thermal conveying medium
Any form of contamination of solid or gas particles in the thermal conveying medium will lead to
measurement deviations. A flow sensor shall not be installed at a position in a pipe system where air or
gas bubbles usually occur (e.g. at the highest points in the pipe circuit).
The thermal transfer medium changes over time due to the interaction between the fluid and the
materials in the system. It has a significant effect on the ageing behaviour of the flow sensor.
8.3.7 Types of flow sensor
a) Turbine flow sensor
This group of flow sensors includes several sensor types, e.g. impeller single-beam sensors, impeller
multi-beam sensors and Woltman sensors.
Inside single or multi-beam sensors the fluid flows against the impeller blades from one or more inlet
nozzles. Inside Woltman sensors the fluid flows axially against the rotor with helical blades, like turbine
wheel.
The transmission of the rotary motion can be magnetic, mechanical, inductive, capacitive, optical or
ultrasonic.
Some examples are shown in the following Figures 4, 5, 6 and 7:

Figure 4 — Single-beam sensor
Figure 5 — Woltman sensor for vertical or horizontal installation
Figure 6 — Multi-beam sensor
Figure 7 — Woltman sensor for horizontal installation only
b) Ultrasonic flow sensor
This type of flow sensor consists of a housing with integrated piezoelectric ultrasonic transducers that
are positioned opposite or staggered to each other. The piezoelectric ultrasonic transducers can operate
both as transmitters and as receivers.
The ultrasonic measuring technology is based on the runtime behaviour of ultrasonic signals in the fluid
flow. The sound is transmitted in the flowing water between the transducers at a velocity that is increased
or decreased by the amount of the flow velocity of the fluid flow, depending on whether it was measured
within or against the direction of the flow. There are several measuring principles for the application of
this technology (see Figures 8 and 9).
Figure 8 — Ultrasonic flow sensor

Figure 9 — Ultrasonic flow sensor
c) Magnetic-inductive flow sensor
The Magnetic-inductive flow sensor determines the flow velocity according to Faraday’s law of induction:
If an electrical conductor moves in a magnetic field in such a way that it intersects the magnetic field lines,
it will induce an electrical current. The flowing water forms the electrical conductor, requiring a minimum
conductivity of the medium. If a desalinated fluid (like e.g. well-treated district heating water) is used as
the thermal conveying medium, the specific electrical conductivity may be too low to use this measuring
technology.
The flow sensor consists of a pipe which is lined with insulating material in which two electrodes are
embedded directly opposite each other. Two magnetic coils generate a magnetic field in the pipe cross-
section. The induced voltage is measured at the electrodes and is proportional to the average flow
velocity. This voltage is fed to an amplifier, which generates a usable signal to determine the flow rate
(see Figure 10).
Figure 10 — Magnetic-inductive flow sensor
d) Fluidic oscillation flow sensor
The fundamental basis for a fluidic oscillator as static meter is the creation of a fluid jet by means of an
acceleration through a nozzle. Successively the jet is confronted with a splitter (bi-stable) or target
(instable). Either two feedback loops or a relaxation loop will make the jet oscillate with a geometry-
determined frequency. Its oscillating frequency is proportional with the volume flow passing through the
nozzle. This frequency can be detected by a variety of methods, i.e. by differential pressure (e.g. piezo-
electric) or by flow velocity (e.g. magnetic inductive, hot wire anemometry, ultrasonic). There are several
types of fluidic oscillators, depending on their geometric design. Examples can be found in the
Figure below. Both left ones concern a bounded jet using the Coanda wall attachment and the right one
uses a free jet avoiding wall attachment for fast switching.

With Coanda effect With Coanda, relaxation type Avoiding Coanda wall
attachment
Figure 11 — Three fluidic meter lay-outs; jet flow is upwards
The relaxation meter has an alternative loop, not connected to the jet. One of the advantages of fluidic
meters is its stability; as long as the geometrics don’t change the flow-frequency relation is conserved. A
fluidic oscillator can be used as a single flow sensor or for compound flow measurement using parallel
partial flow.
e) Other types of flow sensors
The flow rate can also be determined by other static measuring principles like differential pressure
meters, compressive force devices, pitot tubes, piston gauges, vortex meters, hot wire anemometers and
Coriolis meters, and non-static meters like oval-gear meters.
f) Clamp-on flow sensors
Clamp-on flow sensors cannot be approved as flow sensors for thermal energy meters because the overall
measurement uncertainty is too high. Only in-line flow sensors can be approved and verified for thermal
energy.
8.4 Temperature sensors
8.4.1 General
Temperature is usually measured using resistance temperature sensors according to EN 60751.
EN 60751 defines the error limit for the measuring resistor. These error limits are too high for thermal
energy measurements. Therefore, sensors shall be paired together to limit the measurement deviations
as a function of the temperature difference to the error limit of EN 1434-1.
For 2-wire sensors the maximum cable lengths depending on the lead cross section and the type of
temperature sensing element which is used shall not be exceeded. Table 2 provides the maximum lengths
of leads for Pt 100/Pt 500 temperature sensors.
Table 2 — Maximum lengths of leads for 2-wire Pt temperature sensors
Lead cross Max. length for Max. length for Max. additional absolute error caused by
section Pt 100 Pt 500 the cable
mm m m °C
Tcable = 20 °C Tcable = 100 °C
0,22 2,5 12,5
0,5 5 25
0,6 0,8
0,75 7,5 37,5
1,5 15 75
The meters maximum permitted sensor cable length in the meters type approval certificate shall be
checked.
It shall be ensured that the sensor cables outer diameter is compatible with the meter, to maintain the
meters ingress protection class.
Install the sensor cables for both inlet and outlet at the same temperature to avoid measurement error
on the temperature difference, caused by copper wires at different temperatures.
Long 2-wire cables shall be avoided if the absolute temperature in the meter is used for e.g. threshold
activation of additional energy or if the meter is used for cooling with a contractual maximum
temperature. In these applications a 4-wire cables installation and a calculator capable of 4-wire sensor
connection shall be considered.
The sensors shall be installed correctly to achieve accurate measurements. An incorrectly installed
temperature sensor can cause significant measurement deviations even if the temperature sensor works
properly.
The temperature sensors shall be installed in the inlet and outlet lines in the same way (symmetrical
installation), so that the same measuring conditions prevail at both measuring points. The temperature
sensors shall be suitable for the temperature range of the thermal conveying medium and for the pressure
conditions that occur at the measuring point.
8.4.2 Measurement deviations due to differential pressure and temperature difference
A high pressure drop between the temperature measuring points will cause a physical measurement
deviation. This deviation cannot be corrected because the pressure measurement (taken in connection
with the thermal energy meter) cannot be calibrated and leads therefore systematically to a lower
measured value.
The pressure drop between the temperature measuring points in the inlet and outlet shall not cause a
higher measurement deviation than 1/3 MPE for the energy calculation (see Table 3 for the link between
temperature difference, pressure drop and measurement deviation).
This effect can only be corrected by modifying the measuring point.
Table 3 — Systematic negative measurement deviation as a function of a different pressure in
inlet and outlet (pressure drop) and the temperature difference
Temperature difference in K
Diff in bar
3 5 10 20 30 40 50 60
0,5 0,2 0,2 0,1 0,1 0,1 0 0 0
1 0,5 0,4 0,3 0,2 0,1 0,1 0,1 0,1
2 0,9 0,7 0,5 0,3 0,2 0,2 0,1 0,1
3 1,4 1,1 0,8 0,5 0,3 0,2 0,2 0,2
...