SIST EN 16242:2013

SLOVENSKI STANDARD
oSIST prEN 16242:2011
01-april-2011
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Conservation of cultural property - Procedures and instruments for measuring humidity in
the air and moisture exchanges between air and cultural property
Erhaltung des kulturellen Erbes - Verfahren und Geräte zur Messung der Luftfeuchte und
des Feuchtigkeitsaustausches zwischen Luft und Kulturgut
Conservation des biens culturels - Modes opératoires et instruments de mesure de
l'humidité de l'air et des échanges d'humidité entre l'air et les biens culturels
Ta slovenski standard je istoveten z: prEN 16242
ICS:
97.195 Umetniški in obrtniški izdelki Items of art and handicrafts
oSIST prEN 16242:2011 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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oSIST prEN 16242:2011

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oSIST prEN 16242:2011


EUROPEAN STANDARD
DRAFT
prEN 16242
NORME EUROPÉENNE

EUROPÄISCHE NORM

February 2011
ICS 97.195
English Version
Conservation of cultural property - Procedures and instruments
for measuring humidity in the air and moisture exchanges
between air and cultural property
Conservation des biens culturels - Modes opératoires et Erhaltung des kulturellen Erbes - Verfahren und Geräte zur
instruments de mesure de l'humidité de l'air et des Messung der Luftfeuchte und des
échanges d'humidité entre l'air et les biens culturels Feuchtigkeitsaustausches zwischen Luft und Kulturgut
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee CEN/TC 346.

If this draft becomes a European Standard, 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.

This draft European Standard was established by CEN 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 CEN-CENELEC Management
Centre has the same status as the official versions.

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, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to
provide supporting documentation.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without notice and
shall not be referred to as a European Standard.


EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2011 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 16242:2011: E
worldwide for CEN national Members.

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Contents Page
Foreword .3
Introduction .3
1 Scope .4
2 Normative references .4
3 Terms and definitions .5
4 Symbols and abbreviations .7
5 Quantities characterising humidity in air .7
6 Recommendations relating to measuring methods .8
7 Recommendations relating to measuring instruments .9
8 Instrument calibration . 10
9 Main features of the hygrometers . 10
9.1 Electronic psychrometer . 10
9.2 Electronic hygrometer with a capacitive sensor . 11
9.3 Electronic hygrometer with a resistive sensor . 11
9.4 Chilled-mirror dew-point hygrometer . 11
9.5 Hair hygrometer/hygrograph . 12
Annex A (informative) Equations for computing relative humidity and related variables . 13
Annex B (informative) Examples of environmental diagnostics . 16
Annex C (informative) Instrumental errors . 20
C.1 Propagated uncertainty due to a 0.1°C temperature measurement uncertainty in the
psychrometer reading . 20
C.2 Error due to a thermal inertia of a case, a probe or a shield . 21
C.3 Typical non-linearity and hysteresis of the hair hygrometer . 22
Bibliography . 25

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Foreword
This document (prEN 16242:2011) has been prepared by Technical Committee CEN/TC 346 “Conservation of
cultural property”, the secretariat of which is held by UNI.
This document is currently submitted to the CEN Enquiry.
Introduction
Humidity plays a key role in the conservation of cultural heritage because most materials and/or deterioration
mechanisms are directly or indirectly affected by humidity levels or changes. This European Standard is a
guide intended to assist in providing an acceptable environment to cultural heritage objects. Humidity in air,
expressed in a number of ways, is an important aspect of that environment. Therefore, the control of levels
and variability of humidity reduces the risk of deterioration and is an important preventive measure, minimising
the need for future conservation interventions.
This European Standard is a guide to specifying adequate procedures for measuring humidity in air and the
minimum characteristics of instruments for such measurements so that they are carried out to an appropriate
level of accuracy. Although standards exist for measuring humidity in air in other fields like meteorology or
ergonomics of thermal environments, this standard focuses on the specific requirements of cultural objects.
This document is one of the series of European Standards intended for use in the study of environments for
cultural property.
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1 Scope
This Standard specifies procedures and instruments for the measurement of relative humidity (RH) in air in
outdoor or indoor environments. It indicates how RH can be directly measured or how it can be calculated
from air temperature, wet-bulb temperature and dew-point temperature. This standard contains
recommendations for accurate measurements of ambient conditions and moisture exchanges between air and
artworks. It is addressed to anyone in charge of environmental diagnostics, preservation, conservation or
maintenance of buildings, collections or single objects.
2 Normative references
The following referenced documents are indispensable for the application 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 15757, Conservation of Cultural Property — Specifications for temperature and relative humidity to limit
climate-induced mechanical damage in organic hygroscopic materials
EN 15758, Conservation of Cultural Property — Procedures and instruments for measuring temperatures of
the air and the surfaces of objects
EN ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories
ISO 4677-1, Atmospheres for conditioning and testing — Determination of relative humidity — Part 1:
Aspirated psychrometer method
ISO 7726, Ergonomics of the thermal environment — Instruments for measuring physical quantities
ISO/IEC Guide 1998, Guide to the expression of uncertainty in measurement (GUM)
ASTM D4230-02, Standard Test Method of Measuring Humidity with Cooled-Surface Condensation (Dew-
Point) Hygrometer
ASTM E337-02, Standard Test Method for Measuring Humidity with a Psychrometer (the Measurement of
Wet- and Dry-Bulb Temperatures)
DIN 50012-1, Climates and their technical application — Methods of measuring humidity — General
DIN 50012-2, Climates and their technical application — Methods of measuring humidity — Psychrometers
NF X15-117, Measurement of air moisture — Mechanical hygrometers
NF X15-119, Measurement of air moisture — Salt solution humid air generators for the calibration of
hygrometers
NF X20-521, Gas analysis. Determination of the water dew point of natural gas — Cooled surface
condensation hygrometers

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3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
absolute humidity
AH
m
v
volume density of water vapour, i.e. the mass of vapour contained in the unit volume of moist AH = ,
V
3
expressed in g/m
3.2
atmospheric (or barometric) pressure
p
pressure of the air column above the measuring point, expressed in hPa (hectopascal)
3.3
barometer
instrument measuring atmospheric pressure
3.4
dew-point hygrometer
instrument measuring the temperature at which a cooled parcel of air becomes saturated with water vapour
3.5
dew-point temperature
DP
temperature to which a parcel of moist air must be cooled at constant pressure and constant water vapour
content, in order to become saturated with water vapour in equilibrium with water i.e. at which water vapour
begins to condensate, expressed in degrees Celsius (°C)
3.6
dry air
atmospheric air without water vapour
3.7
dry-bulb temperature
T, t
actual air temperature. In a psychrometer, the temperature reached by the thermometer having the dry bulb
NOTE Capital T is used when the measurement is expressed in Kelvin (K); lowercase t when expressed in degrees
Celsius (°C).
3.8
equilibrium moisture content
EMC
moisture content at which a material neither loses nor gains moisture from the surrounding atmosphere at
given relative humidity and temperature levels, expressed in g/kg as the ratio of the mass of water m
fw
m
fw
contained in the material the dry mass of the same material m , i.e.: w =
dm
m
dm
3.9
frost-point temperature
temperature to which a parcel of moist air must be cooled at constant pressure and constant water vapour
content, in order to become saturated with water vapour in equilibrium with ice i.e. at which water vapour
begins to freeze forming ice crystals, expressed in degrees Celsius (°C)
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3.10
hygrograph
instrument measuring relative humidity (see hygrometer) but providing an apparatus to record humidity over
time. Generally, a mechanically- or electrically-driven drum supporting a strip chart with RH graduation where
an ink pen traces a time plot of the ambient humidity
3.11
hygrometer
instrument measuring relative humidity. It generally comprises a sensor, which is set in equilibrium with the
air, and a system which transforms the signal from the sensor into an output reading expressed in percent
(%rh)
3.12
mixing ratio or humidity mixing ratio
MR
m
v
ratio of the mass of water vapour m to the mass of dry air m , i.e. MR = , expressed in g/kg
v a
m
a
3.13
moist air
actual mixture of dry air and water vapour
3.14
psychrometer
instrument measuring the dry- and wet-bulb temperatures and calculating the relative humidity. It consists of
two identical thermometers, one of which is sheathed in wet wicking, and a fan to ensure their ventilation at a
constant velocity in order to reach equilibrium with air. The reading output is expressed in degrees Celsius
(°C)
3.15
relative humidity
RH
e
ratio of the actual water vapour partial pressure to the saturation vapour pressure RH = ×100 ,
e (t)
sat
expressed in percent (%rh)
3.16
measuring range
interval of values that are intended to be measured, or that are potentially measurable, or that have been
measured, specified by their upper and lower limits
3.17
repeatability
ability of the measuring instrument to reproduce the same output when successively measuring the same
value of the measure and under the same conditions
3.18
resolution
smallest difference between indications of a displaying device that can be meaningfully distinguished
3.19
response time
time interval between the instant when the measure is subjected to a specified abrupt change and the instant
when the response of the measuring instrument reaches and remains within specific limits around its final
steady value. The response time of RH sensors is typically expressed as the time needed to reach 63.2% of
the final values and in this case is called the time constant
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3.20
saturation vapour pressure
e (t)
sat
maximum pressure of the water vapour in equilibrium with plane surface of pure water. Expressed in hPa
(hectopascal)
3.21
sensor
device that senses either an absolute value or a change in a physical quantity and converts them into a useful
signal for an information-gathering system
3.22
thermometer
instrument for measuring temperature. It comprises: a sensor which is placed in thermal equilibrium with air (if
it measures air temperature) or the surface, sometimes a probe that contains and protects the sensor, and a
system that transforms the input from the sensor into an output expressed in °C
3.23
time constant
time needed to reach 1/e = 63.2% of the final output value when the air, or the surface temperature, is
subjected to a specified abrupt change to a final steady value. See also response time.
3.24
time stability
ability to keep constant the characteristics in the course of time. It is often expressed in terms of a percent
change of the response per year (%/yr)
3.25
surface temperature
t
s
temperature of a given surface of an object. It can be measured with contact thermometers, total radiation
thermometers or infrared thermometers. Surface temperature is generally different from air temperature,
expressed in degrees Celsius (°C)
3.26
uncertainty (of measurement)
parameter that characterizes the dispersion of the values that could reasonably be attributed to the measure
3.27
wet-bulb temperature
t
w
in a psychrometer, the temperature reached by a thermometer sheathed in wet wicking, expressed in degrees
Celsius (°C)
4 Symbols and abbreviations
No symbols and abbreviations apply to this European standard.
5 Quantities characterising humidity in air
Air humidity is expressed in a number of ways. In this Standard we refer to four key quantities characterising
humidity in air for the purposes of environmental diagnostics to preserve cultural heritage: relative humidity,
humidity mixing ratio, absolute humidity and dew-point temperature.
Relative humidity (RH) is the ratio of the actual partial pressure of water vapour to the saturation vapour
pressure at the same temperature, and represents the degree of saturation the water vapour has reached.
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Relative humidity is responsible for, or related to, many deterioration mechanisms affecting cultural property
preservation. Dry environments become dusty and electrostatic deposition is enhanced. Humid environments
increase chemical reactivity of gaseous pollutants. Hygroscopic materials, such as wood, paper, textiles,
leather or bone, absorb and release moisture in response to changes in RH, reaching eventually at a given
temperature and RH a constant level of moisture termed Equilibrium Moisture Content (EMC). The variations
in EMC produce dimensional changes of the materials, i.e. expansion when EMC is increasing and shrinkage
when decreasing, which may lead to high levels of stress and physical damage as fracture and deformation.
High EMC favours mould growth, as well as hydrolysis, oxidation, corrosion or other chemical reactions.
In the case of photosensitive surfaces, RH has a synergic effect with light, temperature, pollution and other
environmental factors in accelerating fading, discoloration and embitterment.
The humidity mixing ratio (MR) is the ratio of the mass of water vapour to the mass of dry air. It is
independent of air temperature, volume and pressure and represents how much air is rich in vapour. The
parameter is used to distinguish if water molecules are added to or removed from the atmosphere, e.g. to
monitor evaporation, condensation, or mixing of two air masses. It is useful for environmental diagnostic
purposes, to provide evidence of the action of HVAC systems or air-surface interactions. By measuring this
parameter along a horizontal cross- section of a room, it is possible to reveal external air penetrating through
openings, or moisture released by visitors, or when and where masonry is evaporating (higher MR close to the
wall) or adsorbing moisture (lower MR to the wall).
Absolute humidity (AH) is the amount (mass) of water vapour contained in a given volume, e.g. a building, a
room or a showcase. It depends on air pressure and temperature and it is useful in assessing the maximum
quantity of water vapour that a given volume can contain at specified temperature conditions. When AH
exceeds the air saturation level, the excess will condense. From the knowledge of the volume of a closed
space, it is possible to calculate how much water will condense on objects and masonry. Such information can
be used to determine, e.g., the maximum allowable number of visitors in a closed room, in order to avoid high
humidity levels.
Dew-point temperature (DP) is the temperature to which moist air must be cooled at constant pressure and
constant water vapour content, in order for saturation to occur. When the parameter is compared with the
surface temperature (TS) of a structure or an object, the potential risk of water vapour condensation on that
surface can be evaluated, i.e. condensation will occurs if TS is below DP and does not occur if T is above DP.
Equations to calculate the above quantities are reported in Annex A. Examples of environmental diagnostics
using these quantities are reported in Annex B.
6 Recommendations relating to measuring methods
An accurate determination of relative humidity (RH) requires particular care because the measurement
depends on the temperatures of the air and the instrument, which should be in equilibrium with each other and
the equilibrium content of water vapour in the air.
The measurement of air humidity for the conservation of cultural property (EN 15757) must be conceived in
the frame of a specific monitoring campaign that considers not only the value of this quantity but also the
thermal field in the surrounding environment as well as close to the artefact.
The locations of the measuring points must be selected in such a way that they are representative of the
environment under investigation. Each room generally shows variations of temperature and RH from point to
point, therefore temperature and RH of the air that interacts with the object should be measured at a close
distance to the target surface. RH should be measured also in free air, i.e. in a location not affected by the
target (preferably, at a one metre distance or in the middle of the room). From these two measurements it is
possible to establish whether the surface is exchanging moisture with air or not.
If the target surface temperature is different from the air temperature, the air layer in contact with the surface
reaches a different RH which is difficult to measure. The actual RH at the interface between air and surface
should be calculated from the actual surface temperature and the humidity mixing ratio of the air in the
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proximity, the latter to be derived from the values of air temperature and RH or wet-bulb or dew-point
temperatures.
Measurements in locations affected by disturbing factors like heaters, ventilation grilles, windows or doors, or
surfaces having a different temperature should be avoided. The measuring instruments should be placed at
the level of the object if air stratification is present.
The effect of temperature on relative humidity is particularly important and must be taken into account in order
to avoid measurement errors equal to or larger than the estimated measurement uncertainty. A source of error
in humidity measurements can be the temperature difference between the air and the instrument.
The response time of an RH hygrometer must be considered. Meaningful results can be obtained after the
sensor attains the equilibrium with the given temperature and relative humidity, which requires a time period of
approximately twice the response time. If the probe is likely to be exposed to solar radiation, intense light
illumination or infrared radiation from heaters, it should be shielded.
Relative humidity can be measured both directly, by means of sensors whose output is proportional to this
quantity, and indirectly, through the measurement of the air temperature and the dew-point or wet-bulb
temperatures. In the latter case, one has to take atmospheric pressure into account, in particular for
measurements performed at high altitudes in the mountains.
7 Recommendations relating to measuring instruments
Measuring ranges, uncertainty, repeatability, stability, resolution, uncertainty and response time of the
instruments for measuring air humidity are summarised in Table 1. Please note that the following
characteristic should apply to the instruments, not to sensors considered separately. These characteristics
should be considered as minimum requirements. The description or listing of certain instruments only means
that they are recommended. Any measuring system which meets or exceeds the requirements of this
European Standard can be used. It is up to users to analyze the performance of instruments available on the
market and verify if they conform to this Standard.
Table 1 — Characteristics of measuring instruments
Capacitive
Dew-point Electronic Resistive electronic
electronic
hygrometer psychrometer hygrometer
hygrometer
5 % - 95 % 5 % - 95 % 5 % - 95 %
Measuring range -20 °C – 50 °C
(10 °C ≤ t ≤ 50 °C) (-10 °C≤ t ≤ 50 °C) (-10 °C≤ t ≤ 50 °C)
Uncertainty (*) 0.5 °C 3 % 3 % 3 %
Repeatability 0.1 °C 0.1 °C 0.2% 0.2%
Resolution 0.1 °C 0.1 °C 1% 1%
required: <2 min required: <5 min required: <5 min
Not relevant
Response time desirable: <1 min desirable: <2 min desirable: <2 min


Stability
≤ 0.2 °C/year ≤ 2 %/year ≤ 2 %/year ≤ 2 %/year

(*) including display resolution and short-term repeatability.
The ‘desirable’ response time would be of a considerable benefit for spot readings or continuous monitoring of
short-term effects.
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Inaccurate measurements performed either by unqualified personnel, unaware of adequate measuring
procedures, or done with uncalibrated, low quality instruments should be rejected because they may suggest
inappropriate remedial actions which might have a negative impact on preservation.
8 Instrument calibration
Instruments should be calibrated periodically so that they achieve a high level of accuracy of measurement for
humidity control in providing a proper environment for cultural property.
In order to achieve this objective one of the following conditions must be met:
 all measuring instruments must be provided with a calibration certificate issued by an accredited
laboratory in compliance with EN ISO/IEC 17025, or
 at least one reference instrument must be provided with a calibration certificate issued by an accredited
laboratory in compliance with EN ISO/IEC 17025. Such an instrument will be used for calibration by
comparison of the instruments, using validated procedures, for measurements in the field. In this case,
the class of accuracy of the reference instrument must be higher than that of the instrument under
calibration.
The calibration certificate must report, for each humidity value, the correction to the indication of the
instrument under calibration and the associated uncertainty in agreement with the ISO/IEC Guide 98. In
addition, it is useful to do frequent checks to verify if instruments depart from their calibration. Checks can be
done with a primary instrument, or saturated salt solutions, or humid air generators (NF X15-119).
9 Main features of the hygrometers
9.1 Electronic psychrometer
Operating principle: The psychrometer (ASTM E337-02; DIN 50012-2; ISO 4677-1; ISO 7726) consists of
two thermometers and a fan to ensure their ventilation at a sufficient velocity. The first thermometer is an
ordinary thermometer measuring air temperature. This is referred to as ‘dry-bulb’ temperature as opposed to
‘wet-bulb’ temperature indicated by the second thermometer. The latter has a bulb sheathed with wet wicking.
Evaporation from the wet bulb lowers its temperature, i.e. the wet-bulb thermometer becomes colder than the
dry-bulb thermometer, and the temperature difference is proportional to the degree of saturation of humid air.
Recommended use: When appropriately used, this instrument provides accurate humidity measurements. It
can also be used for a simple on-site check of other hygrometers. Its use requires trained personnel.
Main limitations: When the ambient temperature drops below 0°C, water in the reservoir freezes. This
instrument cannot be used at wet-bulb temperatures below 0°C because the wet wicking freezes. At low RH,
the wicking may freeze at ambient temperature up to 10°C. The water reservoir capacity imposes a limit on
the duration of unattended monitoring. The ageing of the fan should be periodically checked. The fan and the
air flow it generates may slightly perturb the air parameters. Relative humidity is calculated with formulae
which include atmospheric pressure. The latter has to be independently measured when it departs too much
from the sea level pressure (e.g. in mountain sites). However, in non-elevated locations, under usual
barometric pressure variability, in the absence of a barometric reading, it is possible to substitute 1000 hPa to
the pressure readings with an acceptable approximation.
Key factors:
 The instrument operates in a wide RH range from 20 % to 100 %.
 Air flow speed should be kept constant in the range from 2.5 to 6 m/s. The fan should be operated well in
advance so that the both sensors and the wet wicking reach an equilibrium. This performance is
adequately provided by an electronic psychrometer with an electrically-driven fan. The old-type
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mechanical psychrometer with spring-operated fan changes the ventilation speed with the spring winding
and may provide incorrect readings. The use of old-type mechanical instruments is not recommended.
 Temperature sensors should comply with the EN 15758 and should have a resolution equal to or lower
than 0.1°C; temperature sensors must be calibrated and, when both are dry, they should read the same
temperature.
 Temperature sensors should be shielded against radiation with a metal shield.
 Wet wicking should be clean; this is particularly relevant
...