IEC 61307:2026

IEC 61307 ®
Edition 4.0 2026-08
INTERNATIONAL
STANDARD
REDLINE VERSION
Industrial microwave heating installations - Test methods for the determination
of power output
ICS 25.180.10 ISBN 978-2-8327-1436-2
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CONTENTS
FOREWORD . 2
1 Scope . 4
2 Normative references . 4
3 Terms and definitions . 5
4 Methods of microwave power measurements . 9
4.1 General . 9
4.2 Available microwave power output .
4.2 Microwave Workload power . 10
4.3 Effective microwave power and efficiency . 11
5 Calorimetric power measurements of available microwave power . 11
5.1 General . 11
5.2 Direct liquid water power measurements . 11
5.3 Drying of substitute loads . 12
6 Determination of microwave workload power .
6 Dummy load power measurements . 13
7 Determination of effective microwave power with individual load items . 13
7.1 General . 13
7.2 Open container water test . 14
7.3 Tests using other liquids .
7.3 Tests with drying workload . 15
8 Electrical efficiency . 15
8.1 General . 15
8.2 Available microwave power output . 15
8.3 Electrical input . 16
9 Cold start, standby and holding power consumption . 16
9.1 General . 16
9.2 Some aspects . 16
Annex A (informative) Equipment class . 17
Bibliography . 18

Figure A.1 – Type A equipment . 17
Figure A.2 – Type B equipment . 17

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Industrial microwave heating installations -
Test methods for the determination of power output

FOREWORD
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
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This redline version of the official IEC Standard allows the user to identify the changes made
to the previous edition IEC 61307:2011. A vertical bar appears in the margin wherever a change
has been made. Additions are in green text, deletions are in strikethrough red text.

IEC 61307 has been prepared by IEC technical committee 27: Industrial electroheating and
electromagnetic processing. It is an International Standard.
This fourth edition cancels and replaces the third edition published in 2011. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) it covers not only the standby and hibernation modes, but also start-up, standby and holding
power consumption, as well as other aspects of operation flexibility;
b) more detailed descriptions of the aspects of microwave power, and handling of the A and B
types of equipment.
The text of this International Standard is based on the following documents:
Draft Report on voting
27/1243/FDIS 27/1247/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
1 Scope
This document specifies test methods for the determination of the available microwave output
power and the efficiency of frequency conversion from the electrical input, and of the available
and workload microwave output power in industrial microwave heating installations, as well as
operational flexibility.
This standard also specifies test methods for assessing the microwave power deposition in the
microwave workload – the microwave workload power, in microwave-only installations.
This document is in principle applicable to industrial microwave heating equipment and
installations in the frequency range from 300 MHz to 300 GHz but focussed on the microwave
ISM frequencies below 6 GHz.
This document relates to industrial microwave heating equipment operating under normal load
as intended by the manufacturer's specifications for normal operation.
This document does not apply to appliances for household and similar use (covered by
IEC 60335-2-25 [1] ), commercial use (covered by IEC 60335-2-90 [2] and IEC 60335-2-110 [3])
or laboratory use (covered by IEC 61010-2-010 [4]).
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.
IEC 60050-221:1990, International Electrotechnical Vocabulary – Chapter 221: Magnetic
materials and components
Amendment 1(1993)
Amendment 2 (1999)
Amendment 3 (2007)
IEC 60050-841:2004, International Electrotechnical Vocabulary (IEV) - Part 841: Industrial
electroheat
IEC 60050-726:1982, International Electrotechnical Vocabulary – Chapter 726: Transmission
lines and waveguides
IEC 60519-6, Safety in electroheat installations for electroheating and electromagnetic
processing - Part 6: Specifications for safety in industrial microwave heating equipment
Particular requirements for high frequency dielectric and microwave heating and processing
equipment
___________
Numbers in square brackets refer to the Bibliography.
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60519-6,
IEC 60050-841 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
3.1
microwave heating equipment
assembly of electrical and mechanical devices intended for the transfer of microwave energy to
the microwave load workload and comprising, in general, power supplies, microwave generators
or microwave generator assemblies with cooling arrangements and circulators if used,
microwave applicators or cavities with ventilation arrangements if used, interconnecting cables
and waveguides, control circuitry, and means for transporting the microwave load workload if
used
[IEC 60050-841:2004, 841-29-06, modified]
3.2
equipment class
group within a type of equipment, using the same principle for processing of the workload either
type A or type B
SEE: Annex A.
Note 1 to entry: An example of type is microwave equipment, and a class example is such equipment for wood
drying heating in a specified capacity interval using the ISM frequency 2,45 GHz.
3.3
type A equipment
equipment with the microwave generator (frequency converter part) independent
of, or separable from, the applicator to which the microwave power is provided
EXAMPLE A coaxial feeder, a waveguide junction.
Note 1 to entry: This allows the integration of a circulator as protection for the magnetron against reflected
microwave power as well as a tuner to minimize microwave reflection for proper tuning/matching.
Note 2 to entry: Any ancillary microwave generator components such as cathode heater transformers and the
generator cooling system are also separable.
3.4
type B equipment
equipment with the microwave generator (frequency converter part) being directly
coupled to the applicator, without a transmission line in between
3.5
electrical conversion efficiency of microwave heating equipment
quotient between the available microwave power output and the electrical input to the mains
frequency power supply or microwave generator assembly, at power settings for normal
intended operation
3.6
overall efficiency
efficiency of the complete system including all ancillary power consumption except for workload
transport
EXAMPLE Cooling fans or pumps and other cooling systems.
3.7
microwave generator
source used to produce electromagnetic energy in the frequency range from 300 MHz to
300 GHz
[IEC 60050-841:2004, 841-29-16]
Note 1 to entry: In the context of this document, the microwave generator is only the component where the
frequency conversion takes place. See 3.2.
3.8
microwave generator assembly
part of the microwave heating equipment comprising an apparatus producing microwave energy
and its associated transmission line output port
Note 1 to entry: The assembly includes the microwave generator, the power supply of the microwave generator and
its ancillary and control circuits. If a circulator is used, it is also included.
Note 2 to entry: Microwave heating equipment containing a microwave generator assembly was classified as type
A in earlier editions of IEC 61307; equipment where a transmission line output port is not available was classified as
type B.
3.9
microwave applicator
structure which applies the microwave energy to the load
[IEC 60050-841:2004, 841-29-11]
3.10
microwave cavity
space enclosed by inner metal walls and a door or an access opening and in which the
microwave load is placed
[IEC 60050-841:2004, 841-29-19, modified]
3.11
microwave enclosure
structure which is intended to confine the microwave energy to a defined region
EXAMPLE A cavity, door seals and waveguides.
[IEC 60050-841:2004, 841-29-20]
3.12
microwave workload
object to be treated by microwaves
NOTE Workload containers are not a part of the microwave workload but of the microwave load.
Note 1 to entry: The workload includes any container, holder or other device necessary for the processing and
which is directly or indirectly subjected to the output power. The processed object/material as such is also called the
load.
[IEC 60050-841:2004, 841-29-13]
3.13
normal load
nominal microwave load workload at full rated microwave output power as specified by the
manufacturer's documentation
3.14
normal operation
range of microwave output power with the normal loads in allowable working conditions of the
microwave heating equipment, as specified by the manufacturer's documentation
equipment operation under normal load conditions under nominal ambient conditions including
the nominal electrical supply network
3.15
intended operation
range of microwave output and allowable electrical input conditions, for intended workloads as
specified by the manufacturer
Note 1 to entry: Several setting combinations are typically specified.
3.16
performance
degree to which the intended functions, including energy or
power consumption and output as well as the result of the treatment of the workload are
accomplished
3.17
equipment capacity
measure of the production rate capability of equipment when in intended normal operation
EXAMPLE Flow, mass or volume.
Note 1 to entry: The equipment capacity does not refer to the volume of the available cavity space.
3.18
cold start-up
process by which the equipment is energised into hot standby operation from the cold state,
including all other start-up operations which enable the equipment to run under intended
operation
Note 1 to entry: This mode of operation applies to cases where there is a significant energy consumption needed
for obtaining a state of the equipment allowing the actual processing of the workload.
3.19
holding power
electric power consumption during which the workload is kept in the treatment chamber at a
specified temperature
Note 1 to entry: The temperature is typically maintained for a period of time, intended to equalize the workload
temperature.
Note 2 to entry: This mode of operation is not applicable for certain types of electroheating equipment.
3.20
standby (mode of) operation
condition allowing immediate normal operation
standby mode of operation
condition of zero output power between periods of continuous intended power operation with
power output
NOTE 1 This mode typically occurs immediately before and after normal operation.
Note 1 to entry: This mode of operation allows immediate start of full power operation.
Note 2 to entry: If the treatment of the workload requires non-ambient conditions such as elevated temperature,
this is maintained by non-microwave means.
Note 3 to entry: "immediate" means a time period consistent with normal loading, unloading or replacement of the
workload.
NOTE 4 The magnetron cathode heater circuit may be switched on in this mode of operation.
3.21
hot standby operation
mode of operation of the installation occurring immediately after normal operation
EXAMPLE Type A equipment with circulator protection.
Note 1 to entry: This mode of operation of the equipment is with its hot state remaining, without workload, and with
the means of operation ready for prompt normal operation.
3.22
calorimetric power meter
calorimeter power meter
calorimetric measurements
power meter which uses temperature rise in a medium as a means of measuring absorbed
power
method of measurement of power which uses the temperature rise in a medium as a means of
measuring absorbed power
Note 1 to entry: A different kind of calorimetric measurements uses the enthalpy of evaporation of water.
Note 2 to entry: The medium, typically water, is either the power-absorbing agent or has heat transferred to it from
a power-absorbing element.
[IEC 60050-726:1982, 726-21-10]
3.23
circulator
passive device having three or more ports in which the power entering any port is transmitted
to the next port according to a given order of sequence
Note 1 to entry: When an absorbing load is installed on the reflected power port of the circulator, the magnetron is
protected against reflected microwave power.
NOTE The typical forms are junction circulators [IEC 60050-221:1990, 221-05-14] of the T junction
[IEC 60050-726:1982, 726-17-12] or Y junction [IEC 60050-726:1982, 726-17-13].
[IEC 60050-221:1990, 221-05-11, modified]
3.24
insertion loss
loss resulting from the insertion of a network into a transmission system, the ratio of the power
delivered to that part of the system following the network, before insertion of the network, to the
power delivered to that same part after insertion of the network
Note 1 to entry: The insertion loss is generally expressed in decibels.
[IEC 60050-726:1982, 726-06-07]
3.25
isolator
two-port device having much greater attenuation in one direction of propagation
than in the opposite direction
Note 1 to entry: An isolator is often used to prevent return reflections along a transmission path.
3.26
microwave tuner
adjustable device which can match the applicator impedance to the magnetron, intended to
improve the efficiency of the microwave heating equipment
3.27
cross coupling
appearance of undesired microwave energy in a microwave generator
or the transmission line output port of a microwave generator assembly caused by one or several
other microwave generators or microwave generator assemblies
3.6
isolation (of a three-port circulator)
reverse attenuation between the main output port and main input port, with all ports being
impedance matched
NOTE 1 The isolation should not be confused with the reverse loss occurring between adjacent ports.
NOTE 2 This is a special case of cross coupling of a circulator [IEC 60050-726:1982, 726-16-06].
3.7
means of access
all structural features of the microwave heating equipment which can be opened or removed
without the use of a tool to provide access to the interior of the microwave applicator or
microwave cavity
3.14
microwave load
objects introduced into the applicator or cavity, or put in the intended position near an open
applicator
[IEC 60050-841:2004, 841-29-12]
3.15
microwave transparency
property of a material having negligible absorption and reflection of microwaves
NOTE The relative permittivity of a microwave transparent material is usually less than 7 and the loss factor is
usually less than 0,015. However, if the microwave workload has a low loss factor, more stringent requirements
apply.
[IEC 60050-841:2004, 841-29-14, modified]
4 Methods of microwave power measurements
4.1 General
Three different methods are described. All tests are to be carried out under conditions which
do not exceed intended operation. Several methods are described in Subclauses 5.2 and 5.3,
and Clause 7. Their applicability depends on the microwave frequency and power level, and if
the equipment comprises a separate microwave generator assembly.
There are two basic equipment layouts: types A and B, as defined in 3.3 and 3.4, and illustrated
in Figure A.1 and Figure A.2 in Annex A. The separation of the microwave oscillator from the
microwave applicator part in type A equipment allows more detailed investigations of various
system properties than what is possible with type B equipment.
– The available microwave power output is the maximum rated from the generator.
Subclause 5.2 is applicable for most types of equipment either type A or type B. For type A
equipment with microwave power output less than 1 kW, dummy loads can be used, and
this is described in Clause 6. Measurements of the available microwave power in type B
drying processes are more complicated and are described in 5.3.
– The effective microwave power is dealt with in Clause 7 and provides a more practical value.
Workloads which heat in a similar way to those used in the actual processing under practical
conditions are used.
– The rated/available microwave power and its test conditions shall be described in the
operation manual/instruction for use.
NOTE 1 Since the wavelength of frequencies above about 20 GHz is very short, the power deposition may be is
then typially of the irradiation type with a short penetration depth. With frequencies in the low end of the microwave
band at 300 MHz, water may is not be useable with the calorimetric method, and some of the methods of measuring
microwave power deposition in this document may are generally not be applicable, since properties of microwave
workloads will then be highly variable during the heating process. In the low end of the microwave band at 300 MHz,
the microwave absorption capability of loads may be highly variable during the heating process, large load masses
may be needed, and representative artificial liquid loads for calorimetry may be difficult to use. In general, large load
masses are often needed, and there are quite few representative artificial liquid loads for calorimetry.
NOTE 2 There are variabilities in the microwave absorption capability of microwave loads, and in particular the
unevenness of heating of these. Therefore, it is crucial to treat with care the microwave workload power data or the
effective microwave power data with a substitute liquid load obtained according to this document should be treated
with care. Power data is, however, important and objective factors related to the overall energy utilisation efficiency
are by that also a performance factor.
NOTE 3 A method for measuring the microwave power output in household and microwave ovens is specified in
IEC 60705 [5]. It uses a large water load, with compensation of heat capacity of the container and of heat exchange
with ambient. Technically, the method gives what is defined as the available microwave power output in this
document.
4.2 Available microwave power output
Measurements at the microwave generator assembly output port give the available microwave
power output (see Clause 5).
4.2 Microwave Workload power
Calorimetric measurements in a normal load, including the power losses in any containers for
the microwave workload, give the microwave workload power (see Clause 6).
This is the amount of power required to achieve an aimed enthalpy change in the microwave
workload within a fixed period of time. It depends on the type of microwave workload, the change
of its complex permittivity with temperature, as well as any workload containers or supports,
and the design of the microwave applicator or cavity.
The workload power is the power delivered to the normal workload under normal operation.
However, there are typically difficulties in assessing the actual absorbed energy in the workload
under practical dynamic conditions, so calorimetric measurements are instead used, under
conditions allowing a higher accuracy in the determination of power absorption.
The available microwave power output is always larger than the microwave workload power,
due to some or all of the following power loss mechanisms:
– impedance mismatching of the microwave generator;
– microwave enclosure cavity or applicator metal surface losses;
– absorption by imperfect microwave transparency of containers for the workload and any
other ancillary objects in the microwave enclosure;
– microwave leakage out of the microwave enclosure;
– power losses due to cross coupling.
NOTE 1 Measuring the forward microwave power and the reflected microwave power enables the calculation of the
absorbed microwave power for the workload by simple subtraction of the reflected power from the forward power by
using an isolator or a circulator.
NOTE 2 Detectors in an isolator or in the input (first) and third arm in a circulator are then used.
4.3 Effective microwave power and efficiency
Typically, actual practical microwave workloads are not well suited for calorimetric accurate
measurements of the absorbed power. Liquid substitutes are then used in calorimetric
measurements are then used and give the effective microwave power dealt with in Clause 7
(see Clause 7 and Clause 8).
Drying processes are dynamic, due to the microwave properties of the workload varying during
the process. Heating-up and evaporative weight losses can then be used as an alternative
calorimetric measurement, and calculated as absorbed energy over time, using the enthalpy
properties of the water being lost in the process and the heating-up of the dry parts of the
workload or substitute.
5 Calorimetric power measurements of available microwave power
5.1 General
Only the principles are outlined in this document. The applied measurement instrumentation
and use shall conform to known engineering techniques. Water is the directly or indirectly power-
absorbing substance.
5.2 Direct liquid water power measurements
It is important that any directly power-absorbing water has a microwave absorption capability
and load geometry which provides a good and essentially temperature independent impedance
matching over the actually used temperature interval. A sodium chloride solution with specific
conductivity between 200 µS/cm and 600 µS/cm shall be used for the direct absorption at
frequencies below 900 MHz.
The calorimetric power meter typically consists of a waveguide section, equipped with a
microwave transparent tube through which the water can flow. The water shall be thoroughly
mixed. The recommended water flow rate is about 1 l/min for each kilowatt but not less than
0,5 l/min. The difference between the outlet and inlet temperature shall be at least 10 K.
The inlet temperature of the water shall not exceed 35 °C, and the outlet temperature shall not
exceed 60 °C. However, for microwave power levels less than 3 kW, these temperatures should
be on both sides of the ambient temperature, to reduce heat loss errors.
Under operating conditions, the voltage standing wave ratio (VSWR) as measured by a network
analyser with a matched waveguide transition or an equivalent measurement device replacing
the microwave generator assembly and within the water temperature interval specified above,
shall not exceed 1,25.
If a circulator is used, its isolation shall be greater than 20 dB and the impedance matching of
the circulator with dissipative termination is to comply with this subclause.
The water flow shall be monitored, for instance by means of flow interlock switches, to avoid
the formation of steam which may can lead to eruption.
The power dissipated in the water is measured directly or compared with a calibrated heated
water standard.
The measurement shall be carried out only when the flow rate is stable, and both the microwave
generator and calorimetric load are operating under stable conditions. It is necessary to use
high-accuracy thermometers and flowmeters to ensure that the inaccuracy of power output
measurement is less than 5 %.
The available microwave power output P is calculated from Formula (1):
4 187⋅ QT⋅∆
P= (1)
where
P is the available microwave power output, in W;
-1 -1
Q is the water flow rate, in kg/min; the factor 4 187 is its specific heat in Jkg K , and 60 is
a factor resulting from units applied;
∆T is the temperature difference in K between the water outlet and inlet temperature.
NOTE If the microwave generator assembly contains a circulator with a dissipative termination in the third arm
protecting the microwave generator (e.g. by a water termination), this may can be used as a calorimetric power meter
by short-circuiting the load port second arm. It is then to be noted that twice the insertion loss applies for this
measurement, but not the circulator insertion loss is included in the evaluation for determination of the available
microwave power output.
5.3 Drying of substitute loads
A good and essentially temperature independent impedance matching over the actually used
temperature interval is not assumed. This means that the whole intended drying process shall
typically be reasonably representative. Test conditions then become different in continuous and
batch processing. The following should be observed:
– the choice of test load should be such that its microwave absorption capability during the
processing is reasonably similar to that of the normal load;
– variable microwave output power densities applied under normal operation shall also be
applied to the test load during the processing of the individual load items;
– influences on energy transfer to the test load by the temperature and humidity of the air in
the processing region;
– power losses due to cross coupling.
NOTE It is assumed that he starting conditions are representative of the normal load conditions.
6 Determination of microwave workload power
This test is applicable only if the normal load is well specified with regard to specific heat and
temperature rise in the process. Furthermore, it shall be possible to accurately measure the
average temperature rise after processing. If the set-up is suspected to provide an inaccuracy
of more than 5 % of the final result, the method described in Clause 5 or Clause 7 is instead
used.
NOTE Typically, accurate tests according to this clause can be made only in continuous processing of pumpable
workloads. These loads are representative only if their microwave properties are similar to those of the normal load.
The input temperature T (°C) of the microwave workload is measured. During steady-state
in
processing, a suitable length of processed microwave workload exiting the microwave heating
equipment during a predetermined time t (s) is quickly taken out as sample and thermal
insulation is provided. Temperature equilibration is then accomplished by either forced
convection (stirring or kneading of the sample) or by internal heat conduction, after which the
output temperature Tout and the mass m of the sample are measured. Its specific heat c has
been pre-determined.
The microwave workload power P is then calculated from the following equation:
W
(T − T ) ⋅ c⋅ m
out in
P = (2)
w
t
where
P is the microwave workload power, in W;
W
T is the output temperature, in °C;

out
T is the input temperature, in °C;

in
c is the specific heat of the workload, in J/(kg · K);
m is the mass of the sample in kg;
t is the sampling time, in s, during which the mass m of the sample is taken out from the
microwave heating equipment.
6 Dummy load power measurements
This is a special test which avoids the accuracy-reducing use of typical practical workloads, as
well as applicators or cavities for these measurements. It should not be confused with the
effective microwave power tests.
The dummy load is an artificial item with known thermal properties, designed to accurately
record the rate of increase of the absorbed or available output energy. Alternatively, measuring
the internal field strength in the dummy load device using such a capability may be used. The
absorbing part of the dummy load is typically matched by a microwave tuner to a low-reactance
termination, cooled by natural air convection, by forced air or by water. It is generally connected
to the microwave generator or generator assembly by a 50 Ω coaxial feeder, or by a TE10
waveguide. At low power levels, natural air convection is applied and at higher power levels up
to about 2 kW of forced air cooling can be applied.
NOTE Applicable dummy loads with a two-port design are commercially available, providing a calibrated insertion
loss at levels of −30 dB to −60 dB, suitable for the use of a commercially available power meter at its output port.
It is necessary to use high-accuracy components and instruments, to ensure that the inaccuracy
of power output measurement is less than 5 %.
7 Determination of effective microwave power with individual load items
7.1 General
The tests in Clause 7 are applicable in cases where the normal load cannot be well specified
with regard to specific heat, consists of individual items, or the process is for other reasons not
suitable for power determinations according to Clause 5. The normal load is replaced by an
artificial load, intended to have dielectric properties and geometries which result in reasonably
similar microwave absorption properties as in the normal load.
In the case of uncertainties with regard to the representativity suitability and equivalence of the
chosen artificial load, a network analyser is to be used to determine the impedance mismatches
with the normal load or a suitable substitute used as microwave workload and the artificial load,
respectively. The resulting calculated deviation in reflected power should not exceed 10 % of
the transmitted power, unless the microwave absorption capability of the normal load is very
difficult to characterise or varies significantly during processing. The estimated inaccuracy shall
be presented with the calculated effective microwave power.
Measuring devices including workload containers shall not be affected by electromagnetic fields,
unless explicitly specified or accepted by the manufacturer. The choice of measurement method
and the results shall be described in a document, with a reference to this document.
An indication shall also be given in the instructions for use that changes in the loading
conditions during the intended use typically have an influence on the efficiency of the microwave
heating equipment.
7.2 Open container water test
NOTE 1 A water load test is used for household and commercial microwave ovens, using 1 000 g of cold water in
a room temperature container, with the water being heated by only about 20 K. The power is calculated using
Formula (1) in 5.2, and is labelled microwave power output.
The water shall be placed in thin wall open containers, manufactured from a microwave
transparent material.
The amount of water shall be at least 0,5 l for each kilowatt of microwave generator power to
which it is exposed, and the height of the water column shall be at least 25 mm. Containers
should be distributed with a spacing such that at least 40 % of the available area within the
microwave enclosure is covered, unless specific patterns are specified by the manufacturer.
Due to possible effects of cross coupling between multiple microwave generators or microwave
generator assemblies, the test shall be made with a number of simultaneously operating
microwave generators or microwave generator assemblies and such large areas covered by the
containers that any such influence is included in the test result.
NOTE 2 Batch equipment and equipment considered to be of multimode or irradiation character and intended for
processing of loads significantly higher than 50 mm, may can be tested with single or multiple containers each taking
several litres.
As the water temperature increases by approximately 14 K per minute per litre of water for each
kilowatt of dissipated power in it, the processing time for the test is typically significantly shorter
than with the normal load. This is to avoid heat losses to the ambient surroundings, in particular
by evaporation. It shall be noted that if stable conditions may are not be obtained, any possibly
errors resulting due to such losses shall be recorded.
The heat capacity of those parts of the containers which are heated by the water shall be used
for corrections in the calculations of effective microwave power, using the energy deposited in
both.
7.3 Tests using other liquids
If the microwave heating equipment is intended for processing normal loads with a low moisture
content such as wood, some types of ceramics, resins or paper, or normal loads having a small
or inhomogeneous particulate structure, the dielectric properties of water may not be sufficiently
representative.
NOTE 1 The real permittivity of water is very high and its loss factor may be too low at frequencies below or even
at the ISM frequency 2 450 MHz, to be representative for the power absorption characteristics of the normal load.
Adding sodium chloride may result in edge overheating and subsequent power losses by evaporation at 2 450 MHz.
Additionally, the high real permittivity of water may result in a reduction of power absorption by stronger wave
reflections at the surface than for microwave workloads with lower real permittivity and a more uneven top surface.
Particularly in microwave heating equipment with separated multiple microwave applicators each having one or some
few microwave generators or microwave generator assemblies delivering some few kilowatts of available microwave
power output, and intended for drying or similar treatment of small microwave workloads, it is recommended to use
liquids having a low real permittivity and good microwave absorption capability.
NOTE 2 Glycerol may be used. Technical glycerol contains about 15 % water and may be acceptable, but its thermal
data should then be checked
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