SIST EN ISO 7726:2002

SLOVENSKI STANDARD
01-september-2002
1DGRPHãþD
SIST EN 27726:2001
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Ergonomics of the thermal environment - Instruments for measuring physical quantities
(ISO 7726:1998)
Umgebungsklima - Instrumente zur Messung physikalischer Größen (ISO 7726:1998)
Ergonomie des ambiances thermiques - Appareils de mesure des grandeurs physiques
(ISO 7726:1998)
Ta slovenski standard je istoveten z: EN ISO 7726:2001
ICS:
13.180 Ergonomija Ergonomics
17.020 Meroslovje in merjenje na Metrology and measurement
splošno in general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN ISO 7726
NORME EUROPÉENNE
EUROPÄISCHE NORM
July 2001
ICS 13.180 Supersedes EN 27726:1993
English version
Ergonomics of the thermal environment - Instruments for
measuring physical quantities (ISO 7726:1998)
Ergonomie des ambiances thermiques - Appareils de Umgebungsklima - Instrumente zur Messung
mesure des grandeurs physiques (ISO 7726:1998) physikalischer Größen (ISO 7726:1998)
This European Standard was approved by CEN on 10 May 2001.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the Management Centre or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the Management Centre has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,
Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2001 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 7726:2001 E
worldwide for CEN national Members.

Foreword
The text of the International Standard from Technical Committee ISO/TC 159 "Ergonomics" of
the International Organization for Standardization (ISO) has been taken over as an European
Standard by Technical Committee CEN/TC 122 "Ergonomics", the secretariat of which is held
by DIN.
This European Standard replaces EN 27726:1993.
This European Standard shall be given the status of a national standard, either by publication
of an identical text or by endorsement, at the latest by January 2002, and conflicting national
standards shall be withdrawn at the latest by January 2002.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of
the following countries are bound to implement this European Standard: Austria, Belgium,
Czech Republic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy,
Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and the United
Kingdom.
Endorsement notice
The text of the International Standard ISO 7726:1998 has been approved by CEN as a
European Standard without any modification.
NOTE: Normative references to International Standards are listed in annex ZA (normative).
Annex ZA (normative)
Normative references to international publications
with their relevant European publications
This European Standard incorporates by dated or undated reference, provisions from other
publications. These normative references are cited at the appropriate places in the text and the
publications are listed hereafter. For dated references, subsequent amendments to or revisions
of any of these publications apply to this European Standard only when incorporated in it by
amendment or revision. For undated references the latest edition of the publication referred to
applies (including amendments).
NOTE Where an International Publication has been modified by common modifications,
indicated by (mod.), the relevant EN/HD applies.
Publication Year Title EN/HD Year
ISO 7730 1994 Moderate thermal environments - EN ISO 7730 1995
Determination of the PMV and PPD
indices and specification of the
conditions for thermal comfort
INTERNATIONAL ISO
STANDARD 7726
Second edition
1998-11-01
Ergonomics of the thermal environment —
Instruments for measuring physical
quantities
Ergonomie des ambiances thermiques — Appareils de mesure des
grandeurs physiques
A
Reference number
ISO 7726:1998(E)
ISO 7726:1998(E)
Contents
Page
1 Scope. 1
2 Normative reference . 1
3 General . 1
4 Measuring instruments . 2
5 Specifications relating to measuring methods . 5
Annex A  Measurement of air temperature. 12
Measurement of the mean radiant temperature.
Annex B  14
Annex C  Measurement of plane radiant temperature . 28
Measurement of the absolute humidity of the air .
Annex D  35
Annex E  Measurement of air velocity. 45
Annex F  Measurement of surface temperature . 48
Annex G  Measurement of operative temperature. 49
Annex H  Bibliography. 51
©  ISO 1998
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced
or utilized in any form or by any means, electronic or mechanical, including photocopying and
microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
ii
ISO 7726:1998(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide
federation of national standards bodies (ISO member bodies). The work of
preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which
a technical committee has been established has the right to be represented
on that committee. International organizations, governmental and non-
governmental, in liaison with ISO, also take part in the work. ISO
collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.
Draft International Standards adopted by the technical committees are
circulated to the member bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the member bodies casting
a vote.
International Standard ISO 7726 was prepared by Technical Committee
ISO/TC 159, Ergonomics, Subcommittee SC 5, Ergonomics of the physical
environment.
This second edition cancels and replaces the first edition (ISO 7726:1985),
of which it constitutes a technical revision.
Annexes A to H of this International Standard are for information only.
iii
ISO 7726:1998(E)
Introduction
This document is one of a series of International Standards intended for
use in the study of thermal environments.
This series of International Standards deals in particular with
— the finalization of definitions for the terms to be used in the methods of
measurement, testing or interpretation, taking into account standards
already in existence or in the process of being drafted;
— the laying down of specifications relating to the methods for measuring
the physical quantities which characterize thermal environments;
— the selection of one or more methods for interpreting the parameters;
— the specification of recommended values or limits of exposure for the
thermal environments coming within the comfort range and for extreme
environments (both hot and cold);
— the specification of methods for measuring the efficiency of devices or
processes for personal or collective protection from heat or cold.
Any measuring instrument which achieves the accuracy indicated in this
International Standard, or even better improves on, may be used.
The description or listing of certain instruments in the annexes can only
signify that they are "recommended", since characteristics of these
instruments may vary according to the measuring principle, their
construction and the way in which they are used. It is up to users to
compare the quality of the instruments available on the market at any given
moment and to check that they conform to the specifications contained in
this International Standard.
iv
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INTERNATIONAL STANDARD  ISO ISO 7726:1998(E)
Ergonomics of the thermal environment — Instruments for
measuring physical quantities
1 Scope
This International Standard specifies the minimum characteristics of instruments for measuring physical quantities
characterizing an environment as well as the methods for measuring the physical quantities of this environment.
It does not aim to define an overall index of comfort or thermal stress but simply to standardize the process of
recording information leading to the determination of such indices. Other International Standards give details of the
methods making use of the information obtained in accordance with this standard.
This International Standard is used as a reference when establishing
a) specifications for manufacturers and users of instruments for measuring the physical quantities of the
environment;
b) a written contract between two parties for the measurement of these quantities.
It applies to the influence of hot, moderate, comfortable or cold environments on people.
2 Normative reference
The following standard contains provisions which, through reference in this text, constitute provisions of this
International Standard. At the time of publication, the edition indicated was valid. All standards are subject to
revision, and parties to agreements based on this International Standard are encouraged to investigate the
possibility of applying the most recent edition of the standard indicated below. Members of ISO and IEC maintain
registers of currently valid International Standards.
ISO 7730:1994, Moderate thermal environments — Determination of the PMV and PPD indices and specification of
the conditions for thermal comfort.
3 General
3.1 Comfort standard and stress standard
The specifications and methods contained in this International Standard have been divided into two classes
according to the extent of the thermal annoyance to be assessed.
The type C specifications and methods relate to measurements carried out in moderate environments approaching
comfort conditions (comfort standard).
The type S specifications and methods relate to measurements carried out in environments subject to a greater
thermal stress or even environments of extreme thermal stress (heat stress standard).
The specifications and methods described for each of these classes have been determined bearing in mind the
practical possibilities of in situ measurements and the performances of measuring instruments available at present.
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3.2 Physical quantities characterizing the environment
3.2.1 Introduction
The determination of overall indices of comfort or thermal stress requires knowledge of physical quantities
connected with the environment. These quantities can be divided into two categories according to their degree of
dependence on the environment.
3.2.2 Basic physical quantities
Each of the basic physical quantities characterizes one of the factors of the environment independently of the
others. They are often used to define the indices of comfort or thermal stress based on the rationalization of the
establishment of the thermal balance of a person placed in a given thermal environment. These quantities are as
follows:
a) air temperature, expressed in kelvins ( ) or in degrees Celsius ( );
T t
a a
b) mean radiant temperature expressed in kelvins T , or in degrees Celsius t plane radiant temperature
() ()
r r
expressed in kelvins (T ) or in degrees Celsius (t ) direct radiation expressed in watts per square metre;
pr pr
c) absolute humidity of the air, expressed by partial vapour pressure (p ) in kilopascals;
a
d) air velocity (V ), expressed in metres per second;
a
e) surface temperature, expressed in kelvins (T ), or in degrees Celsius (t ).
s s
The connections between these quantities and the various gains and losses of heat in relation to the human body
are shown in table 1. Table 1 also gives four other quantities which, because they are usually estimated from data
tables rather than measured, are not included in the remainder of this International Standard.
NOTE —  The concept of mean radiant temperature allows the study of radiative exchanges between man and his
environment. It presupposes that the effects on man of the actual environment which is generally heterogeneous and the virtual
environment which is defined as homogeneous are identical. When this hypothesis is not valid, in particular in the case of
asymmetric radiation, the radiation exchanges arising from thermally different regions and the extent of their effect on man
should also be assessed using the concept of plane radiant temperature.
3.2.3 Derived physical quantities
The derived physical quantities characterize a group of factors of the environment, weighted according to the
characteristics of the sensors used. They are often used to define an empirical index of comfort or thermal stress
without having recourse to a rational method based on estimates of the various forms of heat exchanges between
the human body and the thermal environments, and of the resulting thermal balance and physiological strain. Some
derived quantities are described in the specific standards as they apply and where measuring requirements are
included.
4 Measuring instruments
4.1 Measured quantities
4.1.1  The air temperature is the temperature of the air around the human body (see annex A).
4.1.2  The mean radiant temperature is the uniform temperature of an imaginary enclosure in which radiant heat
transfer from the human body is equal to the radiant heat transfer in the actual non-uniform enclosure.
The mean radiant temperature can be measured by instruments which allow the generally heterogeneous radiation
from the walls of an actual enclosure to be "integrated" into a mean value (see annex B).
The black globe thermometer is a device frequently used in order to derive an approximate value of the mean
radiant temperature from the observed simultaneous values of the globe temperature, t , and the temperature and
g
the velocity of the air surrounding the globe.
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The accuracy of measurement of the mean radiant temperature obtained using this appliance varies considerably
according to the type of environment being considered and the accuracy of measurement of the temperatures of the
globe and the air and the velocity of the air. The actual measuring accuracy shall be indicated wherever it exceeds
the tolerances specified in this International Standard.
The mean radiant temperature is defined in relation to the human body. The spherical shape of the globe
thermometer can give a reasonable approximation of the shape of the body in the case of a seated person. An
ellipsoid-shaped sensor gives a closer approximation to the human shape both in the upright position and the
seated position.
The mean radiant temperature can also be calculated from measured values of the temperature of the surrounding
walls and the size of these walls and their position in relation to a person (calculation of geometrical shape factors).
(See annex B.)
The mean radiant temperature may also be estimated for the plane radiant temperature in six opposite directions
weighted according to the projected area factors for a person. Similarly, it can be estimated from the measurement
of the radiant flux from different directions.
Any other measuring device or calculation method which allows the mean radiant temperature to be determined
with the accuracy specified in the following subclauses may be used.
4.1.3  The plane radiant temperature is the uniform temperature of an enclosure where the radiance on one side of
a small plane element is the same as in the non-uniform actual environment.
The so-called "net" radiometer is an instrument which is often used to measure this quantity (see annex C). With
this it is possible to determine the plane radiant temperature from the net radiation exchanged between the
environment and the surface element and the surface temperature of the radiometer.
A radiometer with a sensor consisting of a reflective disc (polished) and an absorbent disc (painted black) can also
be used.
The plane radiant temperature can also be calculated from the surface temperatures of the environment and the
shape factors between the surfaces and the plane element (see annex C).
The radiant temperature asymmetry is the difference between the plane radiant temperature of the two opposite
sides of a small plane element (see definition of the plane radiant temperature).
The concept of radiant temperature asymmetry is used when the mean radiant temperature does not completely
describe the radiative environment, for instance when the radiation is coming from opposite parts of the space with
appreciable thermal heterogeneities.
The asymmetric radiant field is defined in relation to the position of the plane element used as a reference. It is,
however, necessary to specify exactly the position of the latter by means of the direction of the normal to this
element.
The radiant temperature asymmetry is measured or calculated from the measured value of the plane radiant
temperature in the two opposing directions.
Any other device or method which allows the radiant temperature asymmetry or the plane radiant temperature to be
measured or calculated with the same accuracy as indicated below may be used.
4.1.4  The absolute humidity of the air characterizes any quantity related to the actual amount of water vapour
contained in the air as opposed to quantities such as the relative humidity or the saturation level, which gives the
amount of water vapour in the air in relation to the maximum amount that it can contain at a given temperature and
pressure.
With regard to exchanges by evaporation between a person and the environment, it is the absolute humidity of the
air which shall be taken into account. This is often expressed in the form of partial pressure of water vapour.
The partial pressure of water vapour of a mixture of humid air is the pressure which the water vapour contained in
this mixture would exert if it alone occupied the volume occupied by the humid air at the same temperature.
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The absolute humidity can be determined directly (dew-point instruments, electrolytic instruments) or indirectly by
the measurement of several quantities simultaneously (relative humidity and temperature of the air; psychrometric
wet temperature and temperature of the air) (see annex D).
The psychrometer is an appliance which is frequently used for measuring humidity. It allows the absolute humidity
of the air to be determined from a measured value of the air temperature (t ) and the psychometric wet temperature
a
(t ). The accuracy of measurement is likely to be in accordance with the specifications of this International Standard
w
only if the appliance is well designed and the precautions to be taken during use closely adhered to.
Any device which allows the absolute humidity of the air to be measured with the accuracy indicated in the following
subclauses may be used.
4.1.5  The air velocity is a quantity defined by its magnitude and direction. The quantity to be considered in the
case of thermal environments is the speed of the air, i.e. the magnitude of the velocity vector of the flow at the
measuring point considered (see annex E).
The air velocity, V , at any point in a space fluctuates with time and it is recommended that the velocity fluctuations
a
be recorded. An air flow can be described by the mean velocity, V , which is defined as the average of the velocity
a
over an interval of time (measuring period) and by the standard deviation of the velocity, SD, given by the equation:
n
SD =  V - V
()aa
�
i
n -1
i =
where
V is the velocity at the time "i" of the measuring period.
a
i
The turbulence intensity, TU, of the airflow is defined as the standard deviation divided by the mean velocity and is
usually expressed in percent,
SD
TU=· 100
V
a
Surface temperature is the temperature of a given surface. This is used to evaluate the radiant heat
4.1.6
exchange between the human body by means of the mean radiant and/or the plane radiant temperature. It is also
used to evaluate the effect of direct contact between the body and a given surface. The surface temperature can be
measured by the method given in annex F, including:
— contact thermometer, where the sensor is in direct contact with the surface. The sensor may change the heat
flow at the measuring point and then influence the result.
— infrared sensor, where the radiant heat flux from the surface is measured and converted to a temperature. This
may be influenced by the emissivity of surface.
4.2 Characteristics of measuring instruments
4.2.1 Characteristics of instruments for measuring the basic quantities
The measuring ranges, measuring accuracy and 90 % response times of the sensors for each of the basic
quantities are summarized in table 2. These characteristics shall be considered to be minimum requirements.
According to needs and technical manufacturing possibilities, it is always possible to specify more exact
characteristics. Thus, for certain quantities, very precise thermal stress measurements may require the use of
appliances with measuring ranges in class S and accuracy of class C.
For the purposes of this International Standard, the time constant of a sensor is considered to be numerically equal
to the time taken for the output of the sensor, in response to a step change in the environmental quantity being
measured, to reach 63 % of its final change in steady-state value without overshoot. The response time, which is in
practice the time after which the quantity being measured (for example: temperature of the thermometer) can be
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considered to be sufficiently close to the exact figure for the quantity to be measured (for example: temperature of
the air), can be calculated from the time constant. A 90 % response time is achieved after a period equal to 2,3
times the time constant. It is necessary to wait, as a minimum, for a time equivalent to the response time before a
measurement is taken.
As the time constant and hence the response time of a sensor does not depend solely on the sensor (mass, surface
area, presence of a protective shield) but also on the environment, and hence on factors connected with a given
measurement (air velocity, radiation, etc.), it is necessary to indicate the conditions under which these values were
obtained. The standard environmental conditions are specified in table 3 (classes C and S). They shall be used as a
reference except where this contradicts the principle for measuring the quantities under consideration.
In addition, the accuracy of measurement for air temperatures, mean radiant temperature, radiant temperature
asymmetry, air velocity and humidity also depends on the effect of other quantities. Consequently, the accuracy
specified in table 2 shall be achieved for the environmental conditions specified in the table.
4.2.2 Characteristics of integrating types of measuring instruments
Any measuring instrument integrating the measurement of several variables shall have a measuring interval, a
response time and an accuracy equal to or better than those of the corresponding individual variables.
5 Specifications relating to measuring methods
5.1 General
The methods for measuring the physical characteristics of the environment shall take account of the fact that these
characteristics vary in location and time.
The thermal environment may vary with the horizontal location, and then account has to be taken of how long a time
a person is working at the different locations. The environment may also vary in the vertical direction, as shown in
5.2.
5.2 Specifications relating to variations in the physical quantities within the space surrounding
the subject
An environment may be considered to be "homogeneous" from the bio-climatical point of view if, at a given moment,
air temperature, radiation, air velocity and humidity can be considered to be practically uniform around the subject,
i.e. when the deviations between each of these quantities and their mean spatial value calculated as a mean of the
locations does not exceed the values obtained by multiplying the required measuring accuracy from table 2 by the
corresponding factor X listed in table 4. This condition is frequently met in the case of air temperature, air velocity
and humidity, but more rarely in the case of radiation.
When the environment is too heterogeneous, the physical quantities shall be measured at several locations at or
around the subject and account taken of the partial results obtained in order to determine the mean value of the
quantities to be considered in assessing the comfort or the thermal stress. Previous analyses of the thermal stress
of the work places being studied or of work places of a similar type may provide information which is of interest in
determining whether certain of the quantities are distributed in a homogeneous way. It is usual in the case of poorly
defined rooms or work places to consider only a limited zone of occupancy where the criteria of comfort or thermal
stress shall be respected. In case of dispute in the interpretation of data, measurements carried out presuming the
environment to be heterogeneous shall be used as a reference.
Table 5 shows the heights to be used for measuring the basic quantities and the weighting coefficients to be used
for calculating the mean values for these quantities according to the type of environment considered and the class
of measurement specifications.
The heights to be used for the derived quantities shall preferably be chosen in conformity with the information
supplied in table 5. Plane radiant temperature, mean radiant temperature and absolute humidity are normally only
measured at the centre height. Reference, however, shall be made to the general standard which defines the stress
indices or thermal comfort indices and which takes precedence over this International Standard.
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The different sensors shall be placed at the heights indicated in table 5 where the person normally carries out his
activity. When it is impossible to interrupt the activity in progress, it is necessary to place the sensors in positions
such as that the thermal exchanges are more or less identical to those to which the person is exposed (this
measurement detail shall be mentioned in the results).
5.3 Specifications relating to the variations in the physical quantities with time
The physical quantities in the space surrounding the person can change as a function of time, for the following two
reasons:
a) for a given activity, the quantities can vary as a function of external incidents such as those which accompany a
manufacturing process in the case of an industrial activity;
b) the quantities can also vary as a result of the movements of the person in different environments (for example,
a warm environment close to a machine and a comfortable rest environment).
An environment is said to be stationary in relation to the subject when the physical quantities used to describe the
level of exposure are practically independent of the time, i.e. for instance when the fluctuations in these parameters
in relation to their mean temporal value do not exceed the values obtained by multiplying the required measuring
accuracy from table 2 by the corresponding factor X listed in table 4.
It should be noted that the other quantities used to describe the level of exposure to heat (metabolism, energy
efficiency, insulation of clothing) can also depend on time.
When an environment cannot be considered as stationary in relation to the subject, note should be taken of the
main variations in its physical quantities as a function of time (this information will be used in other standards in this
series in order to determine an overall comfort or thermal stress index). The measuring time and interpretation of
the data will depend on which comfort or thermal stress index is being used. This information shall be found by
reference to the appropriate standards.
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Table 3 — Standard environmental conditions for the determination of time constants of sensors
Quantities
of the
standard
environment
V
a
Measurement t p
t
a a
r
of the
response time
of sensors for
Air temperature = t Any , 0,15 m/s
a
Mean radiant temperature = t Any , 0,15 m/s
r
Absolute humidity = 20 °C = t To be specified according to the
a
measuring method
Air velocity = 20 °C = t Any
a
Plane radiant temperature = 20 °C = t Any , 0,15 m/s
a
Surface temperature = 20 °C = t Any , 0,15 m/s
a
Table 4 — Criteria for a homogeneous and steady-state environment
Class C (comfort) Class S (thermal stress)
Quantity
Factor X Factor X
Air temperature 3 4
Mean radiant temperature 2 2
Radiant temperature asymmetry 2 3
Mean air velocity 2 3
Vapour pressure 2 3
NOTE —  Deviation between each individual quantity and their mean value shall be less than that obtained by multiplying
the required measuring accuracy (table 2) by the appropriate factor from this table.
X
Table 5 — Measuring heights for the physical quantities of an environment
Weighting coefficients for measurements for calculation mean Recommended heights
values (for guidance only)
Locations of the
sensors
Homogeneous environment Heterogeneous environment
Sitting Standing
Class C Class S Class C Class S
Head level 1 1 1,1 m 1,7 m
Abdomen level 1112 0,6 m 1,1 m
Ankle level 1 1 0,1 m 0,1 m
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Annex A
(informative)
Measurement of air temperature
A.1  Introduction
The air temperature shall be taken into account when determining heat transfer by convection at the level of the
person. The measurement of this quantity, while often considered simple, can in fact lead to considerable errors if a
number of precautions are not taken.
A.2  Principle for measuring a temperature
Temperature is obtained by measuring physical quantities which are its continuous functions: lengths of solids,
volumes of liquids, electrical resistance, electromotive force.
Whatever the physical quantity measured, a sensor can only measure the temperature at which it finds itself and
this temperature may differ from the temperature of the fluid (air for instance) to be measured.
A.3  Precautions to be taken when using a temperature probe
A.3.1  Reduction of the effect of radiation
Care should be taken to prevent the probe from being subjected to radiation from neighbouring heat sources as the
temperature measured in such a case would not be the actual temperature of the air but a temperature intermediate
between the air temperature, and the mean radiant temperature.
Various means of reducing the effect of radiation on the probe are available, such as the following:
a) Reduction of the emission factor of the sensor, by the use of a polished sensor when the latter is made of metal
or a sensor covered with a reflective paint when it is of the insulating type.
b) Reduction in the difference in temperature between the sensor and the adjacent walls. Since it is not possible to
modify the temperature of the walls of the enclosure, one or more reflective screens are used, arranged
between the sensor and the enclosure. Thus the sensor "views" a wall, the temperature of which gradually
approaches that of the sensor as the number of screens increases. This method of protecting the sensor is
effective and easy to install.
The screens can in practice be made from thin (0,1 mm or 0,2 mm) sheets of reflective metal (for example
aluminium). When the screens are used on their own, i.e. without forced ventilation, the inner screen shall be
separated from the sensor by an air space large enough to allow air to circulate inside by natural convection.
c) Increasing the coefficient of heat transfer by convection, by an increase in the air velocity around the sensor by
forced ventilation (mechanical or electrical ventilator) and by a reduction in the size of the sensor (thermistor,
thermocouple).
Figure A.1 shows the relation between the air velocity, sensor size and relative influence of air and radiant
temperature on an unshielded air temperature sensor. The measured temperature can be expressed as
X,t 1 (1 2 X),t where X is the relative influence of air temperature. Figure A.1 shows a significant influence of both
a r
sensor size (diameter) and air velocity. The figure is based on the heat exchange calculations for a sphere (see
annex B). It is assumed that the emissivity of the sensor is 0,95.
EXAMPLE:
If the sensor is 1 mm in diameter and air velocity = 0,15 m/s, the temperature of the sensor
= 0,85 t 1 0,15 t .
a r
The figure is only for information purposes and should not be used to correct a measurement.
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Figure A.1 — The relative influence of air temperature on
sensor temperature for different air velocities and sensor diameters
(For larger diameters see figure B.1)
Certain devices use the three means of protection mentioned above simultaneously, which results in small
measuring errors.
A.3.2  Thermal inertia of the sensor
A thermometer placed in a given environment does not indicate the air temperature instantaneously. It requires a
certain period to reach equilibrium.
A measurement should not be made before a period has elapsed equal to at least 1,5 times the response time
(90 %) of the probe.
A thermometer will respond more rapidly:
— the smaller and lighter the temperature sensor is and the lower its specific heat capacity;
— the better the thermal exchanges with the environment. With regard to this, increasing the coefficient of heat
transfer by convection at the level of the sensor, already an advantage as far as the established conditions are
concerned, also improves the response of the thermometer during transitional conditions.
A.4  Types of temperature sensor
a) Expansion thermometers:
1)  liquid expansion thermometer (mercury);
2)  solid expansion thermometer.
b) Electrical thermometers:
1) variable resistance thermometer
—  platinum resistor;
—  thermistor;
2) thermometer based on the generation of an electromotive force (thermocouple).
c) Thermomanometers (variation in the pressure of a liquid as a function of temperature).
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Annex B
(informative)
Measurement of the mean radiant temperature
B.1  Introduction
The net amount of radiant heat lost or received by the human body is the algebraic sum of all radiant fluxes
exchanged by its exposed parts with the various surrounding heat sources. Each of these fluxes can be calculated
knowing the dimensions, locations and thermal characteristics (surface temperature and emissivity) of the source
and of the exposed body or clothing part. This method however, soon becomes complex and time consuming to put
into effect once the number of sources becomes large or the sources have elaborate shapes.
The aim of this annex is
— to describe a method for determining the mean radiant temperature from the measurement of the temperature
of the black globe and the air temperature and air velocity at the level of this globe;
— to summarize other methods for measuring the mean radiant temperature;
— to indicate the principle for calculating the mean radiant temperature using angle factors.
The black-globe thermometer will be used in this annex as an instrument for measuring a physical value, namely
the mean radiant temperature.
B.2  Measurement of the mean radiant temperature using the black globe
B.2.1  Description of the black-globe thermometer
The black-globe thermometer consists of a black globe in the centre of which is placed a temperature sensor such
as the bulb of a mercury thermometer, a thermocouple or a resistance probe.
The globe can in theory have any diameter but as the formulae used in the calculation of the mean radiant
temperature depend on the diameter of the globe, a diameter of 0,15 m, specified for use with these formulae, is
generally recommended.
It should be noted that the smaller the diameter of the globe, the greater the effect of the air temperature and air
velocity, thus causing a reduction in the accuracy of the measurement of the mean radiant temperature.
So that the external surface of the globe absorbs the radiation from the walls of the enclosure, the surface of the
globe shall be darkened, either by means of an electro-chemical coating or, more generally, by means of a layer of
matt black paint.
B.2.2  Principle of the measurement
The black globe shall be placed in the actual enclosure where the mean radiant temperature , is to be measured.
T
r
The globe tends towards a thermal balance under the effect of the exchanges due to the radiation coming from the
different heat sources of the enclosure and under the effect of the exchanges by convection.
The temperature of the globe at the thermal balance allows T to be determined.
r
The temperature sensor placed inside the globe allows the mean temperature of the latter to be measured. In fact,
the temperature of the inner surface of the globe (thin) and the temperature of the air outside the globe (closed
space) are practically equal to the mean external temperature of the globe.
NOTE —  Throughout the remaining part of this International Standard, the expressions temperature of the globe and
temperature of the sensor placed inside the globe will be identical.
©
ISO
ISO 7726:1998(E)
The balance of the thermal exchanges between the globe and the environment is given by the equation
q 1 q = 0 (1)
r c
where
q is the heat exchange by radiation between the walls of the enclosure and the globe,
r
in watts per square metre;
q is the heat exchange by convection between the air and the globe, in watts per square metre.
c
The heat transfer by radiation between the walls of the enclosure, characterized by the mean radiant temperature,
and the globe is expressed as follows:
qTes -T (2)
()
rg= r g
where
e is the emissivity of the black globe (without dimension);
g
s is the Stefan-Boltzmann constant, in watts per square metre kelvin to the fourth power;
28 2 4
[s = 5,67 × 10 W/(m · K )];
T is the mean radiant temperature, in kelvins;
r
T is the temperature of the black globe, in kelvins.
g
The heat transfer by convection between the air contained in the enclosure and the globe is given by the equation:
q = h (T 2 T ) (3)
c cg a g
where
h is the coefficient of heat transfer by convection at the level of the globe, in watts per square metre kelvin.
cg
In the case of natural convection
14/
DT
� �
h = 14, � �
cg
Ł ł
D
and in the case of forced convection
06,
V
a
63,
h =
cg
04,
D
where
D is the diameter of the globe, in metres;
V is the air velocity at the level of the globe, in metres per second.
a
In a type C environment, the coefficient of heat transfer by convection to be adopted is the one giving the highest
value. In a type S environment, it is possible either to adopt the same method as previously or, more simply, to
adopt the coefficient of heat transfer in forced convection directly.
The thermal balance of the black globe is expressed as follows:
esTT-+h T-T=0 (4)
gr()gcg()a g
©
ISO
ISO 7726:1998(E)
The mean radiant temperature is given by
h
cg
TT=+4 TT- (5)
rg ()ga
es
g
By natural convection, one obtains:
14/
14/
Ø ø
8� �
tt-
02, 5·10 ga
Œ œ
� �
tt=+ 273+ ·-tt - 273 (6)
() ()
rg ga
Œ œ
� �
e D
g
Ł ł
Œ œ
º ß
In the case of the standard globe = 0,15 m, e = 0,95 (matt black paint) and equation (6) becomes
D
g
14/
4 14/
Ø 8 ø
t=+t 273+ 0,4· 10 tt- ·tt- - 273 (7)
r()g ga ()ga
Œ œ
º ß
By forced convection, one obtains
14/
80,6
Ø ø
11,··10 V
a
tt=+Œ 273+ tt- œ - 273 (8)
() ()
rg ga
04,
Œ e·D œ
g
º ß
or for the standard globe
14/
Ø 80,6 ø
tt=+ 273 + 2,5· 10·v t-t - 273 (9)
rg() a(g a)
Œ œ
º ß
In practice, it is this expression which will be most frequently used to calculate the mean radiant temperature. It is
valid only for a standard globe by forced convection.
The relative influence of air temperature and mean radiant temperature on a globe is shown in figure B.1.
EXAMPLE:
For a 100 mm globe at an air velocity of 0,35 m/s, the globe temperature, t = 0,6 t 1 0,4 t .
g a
r
Figure B.1 — Relative influence of air temperature, t , and mean radiant temperature, t , on the globe
a
r
temperature for different air velocities and globe diameters
©
ISO
ISO 7726:1998(E)
EXAMPLES:
The following results were obtained in an environment using a standard globe:
t = 55 °C
g
t = 30 °C
a
V = 0,3 m/s
a
The coefficient of exchange at the level of the globe is calculated as follows:
— in natural convection
02, 5
14/
DT 55- 30
� �
�� 2
h= 14,,��= 14 =�5W/mK
� �
cg ()
Łł
D Ł 01, 5ł
— in forced convection
06,
06,
��
03,
V ()
a 2
h= 63,,��=·63 = 65, W/mK�
()
cg
��
04, 04,
D 01, 5
Łł ()
The coefficient of exchange in forced convection will therefore be used.
The mean radiant temperature is calculated according to equation (9):
80,/6 14
t=+55 273+ 2,5· 10·V 55- 30 - 273
() ( )
ra[]
t = 74,7 °C
r
If the measurement is carried out with a globe with the following characteristics:
D = 0,1 m
e = 0,95
g
the temperature measured for the black globe is 53,2 °C.
The mean radiant temperature is then calculated according to equation (8):
14/
06,
 
11, 0,3
()
 
t=+()53,2 273+ ()53,,2− 30 −=273 74 7°C
r
04,
 
09, 5 0,10
()
 
 
The figure for the mean radiant temperature characteristic of the environment considered is thus obtained.
B.2.3  Precautions to be taken when using
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