IEC 60068-3-11:2007
(Main)Environmental testing - Part 3-11: Supporting documentation and guidance - Calculation of uncertainty of conditions in climatic test chambers
Environmental testing - Part 3-11: Supporting documentation and guidance - Calculation of uncertainty of conditions in climatic test chambers
Demonstrates how to estimate the uncertainty of steady-state temperature and humidity conditions in temperature and humidity chambers. Since this is inextricably linked to the methods of measurement, these are also described. This standard is equally applicable to all environmental enclosures, including rooms or laboratories. The methods used apply both to temperature chambers and combined temperature and humidity chambers. This standard is meant to help everyone using climatic test chambers. Those already familiar with uncertainty of measurement will find it useful for guidance on typical sources of uncertainty and how they should be quantified and combined. It is also intended to assist the first-time or occasional user who has little or no knowledge of the subject. To discuss uncertainty, it is important first to understand what is being measured or characterized. The calibration or characterization of the performance of a chamber is concerned with the humidity and temperature of the air in the chamber, as experienced by the item under test, at a given set point. This should not be confused with characterizing or calibrating the chamber sensor, which is a separate matter.
Essais d'environnement - Partie 3-11: Documentation d'accompagnement et guide - Calcul de l'incertitude des conditions en chambres d'essais climatiques
Montre comment estimer l'incertitude des conditions de température et d'humidité à l'état stable dans une chambre d'essais de température et d'humidité. Sachant que cela est inextricablement lié aux méthodes de mesures, celles-ci sont également décrites. La présente norme est également applicable à tout type d'enceintes d'environnement, et même les salles ou laboratoires. Les méthodes sont usuellement appliquées aux chambres d'essais régulées en température et aux chambres d'essais régulées en température et humidité. La présente norme cherche à aider toute personne utilisant des chambres d'essais climatiques. Les personnes étant déjà familières avec l'incertitude de mesure la trouveront utile pour les conseils sur les composantes types d'incertitudes et sur la manière dont il convient de les quantifier et de les combiner. Elle est également destinée à aider le nouvel utilisateur ou l'utilisateur occasionnel qui n'a qu'une faible connaissance du sujet ou pas de connaissance du tout. Afin d'aborder les incertitudes il est important tout d'abord de comprendre ce qui est mesuré ou caractérisé. L'étalonnage ou la caractérisation de la performance d'une chambre d'essais fait appel à la détermination de l'humidité et de la température de l'air de la chambre, vu par l'objet en essai, à une valeur de consigne donnée. Il convient que cela ne soit pas confondu avec le calibrage ou l'étalonnage du capteur de la chambre d'essai, qui est une opération différente.
General Information
Standards Content (Sample)
INTERNATIONAL IEC
STANDARD
CEI
60068-3-11
NORME
First edition
INTERNATIONALE
Première édition
2007-05
Environmental testing –
Part 3-11:
Supporting documentation and guidance –
Calculation of uncertainty of conditions
in climatic test chambers
Essais d’environnement –
Partie 3-11:
Documentation d’accompagnement et guide –
Calcul de l’incertitude des conditions
en chambres d’essais climatiques
Reference number
Numéro de référence
IEC/CEI 60068-3-11:2007
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INTERNATIONAL IEC
STANDARD
CEI
60068-3-11
NORME
First edition
INTERNATIONALE
Première édition
2007-05
Environmental testing –
Part 3-11:
Supporting documentation and guidance –
Calculation of uncertainty of conditions
in climatic test chambers
Essais d’environnement –
Partie 3-11:
Documentation d’accompagnement et guide –
Calcul de l’incertitude des conditions
en chambres d’essais climatiques
PRICE CODE
V
CODE PRIX
Commission Electrotechnique Internationale
International Electrotechnical Commission
МеждународнаяЭлектротехническаяКомиссия
For price, see current catalogue
Pour prix, voir catalogue en vigueur
– 2 – 60068-3-11 © IEC:2007
CONTENTS
FOREWORD.4
INTRODUCTION.6
1 Scope.7
2 Normative references .7
3 Terms and definitions .8
4 Concept of uncertainty.11
4.1 Uncertainty, error and “true value”.11
4.2 Statements of uncertainty.12
4.3 Combining uncertainties .13
5 Tolerance .13
6 Humidity and temperature measurement .13
7 Methods for determining climatic test chamber uncertainties .14
7.1 Empty chamber .16
7.2 Typical load.16
7.3 Measurement of conditions in the chamber during the test .17
7.4 Conditions to measure.17
7.5 Measurements required.18
7.6 Sources of uncertainty.19
7.7 Essential contributions of uncertainty .20
8 Estimation of uncertainty components and their combination .24
9 Overall uncertainty of temperature measurement.24
9.1 General .24
9.2 Further considerations.26
10 Overall uncertainty of relative humidity measurement .26
10.1 Uncertainty of temperature measurement at each sensor point.27
10.2 Uncertainty of the relative humidity measurement.27
11 Anomalous data and presentation of results .30
11.1 Average case analysis.30
11.2 Worst case analysis .30
Annex A (informative) Measurement data sets – Loaded chamber .32
Bibliography.34
60068-3-11 © IEC:2007 – 3 –
Figure 1 – Approaches to calibration method and uncertainty calculation.15
Figure 2 – Illustration of the fluctuation of a temperature sensor .23
Table 1 – Combination of temperature uncertainties .24
Table 2 – Combination of temperature uncertainties at each point .27
Table 3 – Combination of humidity uncertainties .28
Table A.1 – Typical temperature measurement data set and it’s analysis and refs .32
Table A.2 – Humidity measurements analysis based on Table A.1 temperatures.33
– 4 – 60068-3-11 © IEC:2007
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ENVIRONMENTAL TESTING –
Part 3-11: Supporting documentation and guidance –
Calculation of uncertainty of conditions in climatic test chambers
FOREWORD
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9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60068-3-11 has been prepared by IEC technical committee 104:
Environmental conditions, classification and methods of test.
The text of this standard is based on the following documents:
FDIS Report on voting
104/409/FDIS 104/415/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
60068-3-11 © IEC:2007 – 5 –
A list of all parts in the IEC 60068 series, under the general title Environmental testing can be
found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – 60068-3-11 © IEC:2007
INTRODUCTION
This part of IEC 60068 provides guidance for analysing uncertainties of temperature and
humidity in climatic test chambers. It has been written for technicians, engineers and
managers in environmental testing, and for anyone who needs to understand the results of
environmental tests.
The performance of climatic test chambers is a key concern in environmental test engineer-
ing. To comply with any test specification, the performance of the chamber needs to be
characterized to decide whether the generated conditions fall within the specified limits. This
characterization can be a difficult task, and the analysis of uncertainties in chamber
performance is often surrounded by confusion. This publication is intended to ease that
process.
In what follows, the concept of uncertainty of measurement is introduced first and then the
significance of tolerance discussed. Aspects of humidity and temperature measurement are
considered, followed by methods for determining and combining uncertainties. The cases of
both calibrating an empty chamber and of measuring conditions in a loaded chamber are
considered. Finally, detailed guidance and worked examples are given for analysing results to
give estimates of uncertainty in the measured performance.
60068-3-11 © IEC:2007 – 7 –
ENVIRONMENTAL TESTING –
Part 3-11: Supporting documentation and guidance –
Calculation of uncertainty of conditions in climatic test chambers
1 Scope
This part of IEC 60068 demonstrates how to estimate the uncertainty of steady-state
temperature and humidity conditions in temperature and humidity chambers. Since this is
inextricably linked to the methods of measurement, these are also described.
This standard is equally applicable to all environmental enclosures, including rooms or
laboratories. The methods used apply both to temperature chambers and combined
temperature and humidity chambers.
This standard is meant to help everyone using climatic test chambers. Those already familiar
with uncertainty of measurement will find it useful for guidance on typical sources of
uncertainty and how they should be quantified and combined. It is also intended to assist the
first-time or occasional user who has little or no knowledge of the subject.
To discuss uncertainty, it is important first to understand what is being measured or
characterized. The calibration or characterization of the performance of a chamber is
concerned with the humidity and temperature of the air in the chamber, as experienced by the
item under test, at a given set point. This should not be confused with characterizing or
calibrating the chamber sensor, which is a separate matter.
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.
IEC 60068-3-5: Environmental testing – Part 3-5: Supporting documentation and guidance –
Confirmation of the performance of temperature chambers
IEC 60068-3-6: Environmental testing – Part 3-6: Supporting documentation and guidance –
Confirmation of the performance of temperature/humidity chambers
ISO 3534-1:2006, Statistics – Vocabulary and symbols – Part 1: General statistical terms and
terms used in probability
ISO 3534-2:2006, Statistics – Vocabulary and symbols – Part 2:Applied statistics
International Vocabulary of basic and general standard terms in metrology. ISO, Geneva,
Switzerland 1993 (ISBN 92-67-10175-1) – VIM
– 8 – 60068-3-11 © IEC:2007
Guide to the expression of uncertainty in measurement. ISO, Geneva, Switzerland 1993.
(ISBN 92-67-10188-9) – GUM
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
calibration authority
laboratory or other organization that performs calibrations and is itself accredited by the
appropriate national accreditation body
3.2
climatic test chamber
enclosure
chamber or enclosed space where the internal temperature or temperature and humidity can
be controlled within specified limits
3.3
combined standard uncertainty
standard uncertainty of the result of a measurement when that result is obtained from the
values of a number of other quantities, equal to the positive square root of a sum of terms, the
terms being the variances or covariances of these other quantities weighted according to how
the measurement result varies with changes in these quantities
See also GUM.
3.4
correction
value added algebraically to the result of a measurement to compensate determinable
systematic error
See also VIM.
3.5
confidence level
value of probability associated with a confidence interval
NOTE The confidence level is the likelihood that the “true value” lies within the stated range of uncertainty usually
expressed as a percentage, e.g. 95 %.
See also ISO 3534-1.
3.6
coverage factor
numerical factor to multiply the combined standard uncertainty to obtain an expanded
uncertainty
NOTE A coverage factor of k=2 corresponds to a confidence level of approximately 95 % if normally distributed
and if the number of degrees of freedom is sufficiently large.
See also GUM.
60068-3-11 © IEC:2007 – 9 –
3.7
dew point
temperature at which the partial pressure of the water vapour is equal to the saturation vapour
pressure over water or ice
NOTE The temperature to which the air would have to cool (at constant pressure and constant water vapour
content) in order to reach saturation. A state of saturation exists when the air is holding the maximum amount of
water vapour possible at the existing temperature and pressure.
3.8
dispersion
spread of repeated measurements of a quantity
3.9
drift
change in the indication of a measuring system not related to a change in the quantity being
measured
See also VIM.
NOTE The drift since the last calibration can be estimated and a correction applied to measured values.
3.10
error
difference between result of a measurement and the true value
3.11
expanded uncertainty
quantity defining an interval about the result of a measurement that may be expected to
encompass a large fraction of the distribution of values that could reasonably be attributed to
the measurand
See also VIM.
3.12
fluctuation
change (from the mean) in the temperature or humidity after stabilization from time to time at
a point in space
NOTE It may be measured by standard deviation or maximum deviation.
3.13
gradient
maximum difference in mean value, after stabilization, at any moment in time between two
separate points in the working space
3.14
incident air
conditioned airstream which flows into the working space
3.15
partial vapour pressure
contribution of water vapour in a given volume of air at a constant pressure and temperature
of the atmosphere
3.16
reference instrument
previously calibrated instrument used to measure the conditions within the enclosure
– 10 – 60068-3-11 © IEC:2007
3.17
relative humidity
ratio of actual partial vapour pressure to the saturation vapour pressure at any given
temperature and pressure, expressed as a percentage (% RH)
3.18
repeatability
closeness of agreement between independent results obtained in the normal and correct
operation of the same method on identical test material, in a short space of time, and under
the same test conditions (such as the same operator, same apparatus, same laboratory)
3.19
resolution
smallest changes between indications of the chamber controller display that can be
meaningfully distinguished
3.20
saturation vapour pressure
when a given volume of air, at a constant temperature, has water vapour present and is
incapable of holding more water vapour it is said to be saturated
3.21
stabilization
achievement of the state of temperature/humidity in the chamber when all mean values in the
working space are constant and have maintained temperature/humidity within a given
tolerance
3.22
standard deviation
measure of the dispersion of a set of measurements
NOTE The standard deviation, s, is the best estimate of sigma (the population standard deviation).
See also GUM and/or VIM.
3.23
standard uncertainty
uncertainty of the result of a measurement expressed as a standard deviation
See also GUM.
3.24
tolerance
acceptance limit specified or chosen for a process or product
See also ISO 3534-2.
3.25
traceability
property of the result of a measurement or the value of a standard whereby it can be related
to stated references, usually national or international standards, through an unbroken chain of
comparisons, all having stated uncertainties
NOTE The unbroken chain of comparisons is called a traceability chain.
See also ISO 3534-1 and VIM.
60068-3-11 © IEC:2007 – 11 –
3.26
true value
value which characterizes a quantity, perfectly defined in the conditions which exist when that
quantity is considered
NOTE The true value of a quantity is a theoretical concept and, in general, cannot be known exactly but is
estimated by measurement.
See also ISO 3534-1.
3.27
uncertainty
parameter, associated with the result of a measurement, which characterizes the dispersion
of the values that could reasonably be attributed to it
3.28
uncertainty budget
list of sources of uncertainty compiled with a view to evaluating a combined standard
uncertainty associated with a measurement result
3.29
uncertainty contribution
input to an uncertainty budget
3.30
working space
part of the chamber in which the specified conditions can be maintained within the specified
tolerances
4 Concept of uncertainty
4.1 Uncertainty, error and “true value”
In every measurement – no matter how careful – there is always a margin of doubt about the
result. In simple terms, the uncertainty of a measurement is a quantification of the doubt
about the measurement result.
While discussing uncertainty we often also need to consider a related but separate concept,
“error”. A measurement “error” is the difference between the measured value and the “true
value” of the thing being measured.
The “true value” of any quantity is in principle unknowable. This leads to a problem since the
“error” is defined as the result of a measurement minus the “true value”. Sometimes this
difference can be estimated. Both terms are best avoided as much as possible and, when
necessary, should be used with care. Discussion of “error analysis”, which used to be
included in many scientific papers, should have been entitled “analysis of the probable limits
of error”, or more properly, ”analysis of uncertainty”. In older publications the term “error” was
widely used when ‘uncertainty’ would have been the correct term.
Uncertainty is not the same as error. If the conditions in a test chamber are measured with a
calibrated instrument and the result is 75 % RH when the chamber controller says 90 % RH,
that does not mean the uncertainty is 15 % RH. It is known that the relative humidity is 75 %
RH. One is aware that either the controller reading is wrong or the chamber is operating
incorrectly. It has an error estimated to be 15 % RH. The uncertainty is a characteristic of the
measurement that gave the answer 75 % RH. Could that be wrong and, if so, by how much?
– 12 – 60068-3-11 © IEC:2007
When considering “true value”, uncertainty, and error, one of the most important sources of
this type of information for a measuring instrument is its calibration certificate. It is vital to use
all of the information provided by the calibration certificate to ensure that the best estimate of
the test uncertainties are obtained.
4.2 Statements of uncertainty
4.2.1 General
When reporting the results of a measurement, three numbers are necessary for a
metrologically correct and complete statement of the result of each measurement point. For
example, the complete statement could be:
The “true value” is: 39,1 °C ± 0,3 K with 95 % confidence:
• 39,1 °C is the best estimate of the true value;
• ±0,3 K is the confidence interval;
• 95 % is the confidence level.
An explanation of these three components follows.
4.2.2 Best estimate of the true value of the measured quantity
Often this will simply be the reading on the calibrated reference instrument which, in the case
of a climatic test, could be the temperature measurement system and/or hygrometer reading,
or if the chamber has been calibrated it could be the chamber controller display. If the
calibration shows either for an instrument or for a chamber controller that an error exists
(which is not an uncertainty), this should be used to apply a correction. For example, if the
calibration of a thermometer shows that it reads 1 K high, 1 K should be subtracted from the
reading to obtain the best estimate of the true value.
4.2.3 Confidence interval
This is the range of measured values within which the “true value” lies with a given level of
confidence. In our example this interval is ±0,3 K.
4.2.4 Confidence level
The “confidence level” of a measurement is a number (e.g. 95 %) expressing the degree of
confidence in the result. This is the probability that the real “true value” lies in the given
range. Most sets of data are normally distributed and about 68 % of the values will fall within
plus or minus one standard deviation of the mean. About 95 % of the values can be expected
to fall within plus or minus 2 standard deviations (95 % confidence level). Put another way,
when many such measurements are performed not more than 1 in 20 will lie outside the
stated limits. Hence multiplying the standard deviation by 2 is an accepted way of
encompassing 95 % of the range of values. With a 95 % confidence level, we are 95 % sure
that the “true value” lies in the stated range.
It is conventional to work at the 95 % confidence level. Higher confidence levels can be used
but the confidence interval will increase.
4.2.5 Statement of uncertainty
In the above example the statement of uncertainty is that the temperature was 39,1 °C± 0,3 K
with 95 % confidence. 39,1 °C was the best estimate of the temperature but because of the
uncertainties there is a possibility of it being in the range 38,8 °C to 39,4 °C with a confidence
of 95 %.
60068-3-11 © IEC:2007 – 13 –
4.3 Combining uncertainties
Uncertainty contributions shall be expressed in similar terms before they are combined. They
shall be in the same units and at the same level of confidence. All contributions should be
converted into standard uncertainties (i.e. having a confidence level of plus or minus one
standard deviation). This is discussed further in Clauses 9 and 10.
5 Tolerance
When a test item is to be conditioned one of the first questions asked is, “Will the chamber
achieve and maintain the required conditions?” This is asked since the test specification will
often set a tolerance for the required condition e.g. ±2 °C and ±5 % RH. In deciding whether a
tolerance is met, the uncertainty in the measured chamber performance shall be taken into
account.
Tolerances are not the same as uncertainties. Tolerances are acceptance limits which are
chosen for a process or product. Most often the aim of knowing the uncertainty in a chamber’s
performance is to decide whether a tolerance is met. In deciding this, the deviation from the
required condition, together with the uncertainty, shall be considered. Using the values cited
in 4.2.5, it is certain to within 95 % that the true temperature is between 38,8 °C and 39,4 °C.
If the required condition is 40 °C ± 2 K, then the probability that the true temperature lies
within the tolerance is considerably better than 95 % because the entire confidence interval
lies within the range of the tolerance.
If the measured humidity is 81,7 % RH, and the confidence interval is ±3,6 % RH at a 95 %
confidence level, then it is certain to within 95 % that the true humidity is between 78,1 % RH
and 85,3 % RH. If the required condition is 85 ± 5 % RH, even though the measured condition
is within this range, the probability that the true humidity is within ±5 % RH of the set point is
significantly less at a 95 % confidence level, because the entire confidence interval does not
lie within the range of the tolerance. However, from the uncertainty, there are statistical
methods for making a good estimate of how likely this is.
6 Humidity and temperature measurement
When taking humidity measurements there are many ways of approaching the situation. It is
generally assumed that the water vapour content of the air is uniform throughout the chamber.
This is a reasonable assumption, and people who have performed measurements of humidity
at multiple points in a chamber can confirm that this is normally the case. However, this does
not mean that the relative humidity is uniform.
Dew point, being directly related to vapour pressure, can be assumed to be uniform across
the chamber and is not affected by temperature. It may be that during routine tests, humidity
measurement is only made in one place. However, at some point, either during the test or
when the chamber is operating under similar conditions, humidity measurements shall be
made in at least two places so that an uncertainty can be assigned to the assumption that the
vapour content of the air is uniform.
For most environmental tests, the required humidity is specified in terms of relative humidity.
The importance of relative humidity arises because the behaviour of most organic materials
depends on this parameter. Factors such as physical expansion of plastics and wood,
biological activity, electrical impedance and corrosion rates are examples of processes that
are affected by the relative humidity.
In a chamber the vapour pressure is often nearly uniform.
– 14 – 60068-3-11 © IEC:2007
Even when the air is thoroughly stirred there are often temperature differences from place to
place in chambers and, although the water vapour pressure is often nearly uniform, the
temperature differences cause differences in the relative humidity. A single humidity
measurement at only one location is often sufficient to tell us about the vapour pressure in the
rest of the chamber. The single measurement should be made at a central point or on the
incident air side of the object under test.
The measurement can be made with any hygrometer, but normally it will be one of three
types:
− a dew-point (dp) hygrometer (mirror condensation);
− a psychrometer (wet/dry); or
− a relative humidity probe.
Examples are shown in Annex A.
7 Methods for determining climatic test chamber uncertainties
There are three basic methods for determining conditions in a climatic chamber. These three
methods reflect the different requirements in different types of testing and there are good
reasons for each approach. These methods are illustrated in Figure 1.
60068-3-11 IEC:2007 – 15 –
©
1 2
Chamber status
Empty chamber Chamber typically Conditions measured
loaded at time of test
Humidity sensor
Multipoint Multipoint
Multipoint positioned
positioning
predefined positions positioned around the
around typical load
load
Single point. e.g. Single point. e.g. Multipoint
Single point. e.g. Multipoint around Multipoint
centrally located or the working space centrally located positioned around centrally located positioned around
Temperature sensor
adjacent to control on the incident air the load on the incident air the load
positioning
sensor side side
The suitability of the None
The effect of the load
load must be
Loading considerations must be considered
considered
Chamber repeatability Chamber repeatability Not applicable
Facility repeatability
Combine to show worst Combine to show
Combination
case condition averaged condition
techniques
Calibration equipment Uncertainty calculation
uncertainty
Uncertainty of
conditions
IEC 670/07
Figure 1 – Approaches to calibration method and uncertainty calculation
– 16 – 60068-3-11 © IEC:2007
7.1 Empty chamber
7.1.1 Advantages
Advantages are as follows:
a) The entire working space is calibrated.
b) Calibration need be carried out only once or twice a year.
c) Re-calibration is not required when the test sample is changed.
d) The suitability of the chamber can be assessed without subjecting the test sample to
conditioning.
e) Relatively low cost. Only one set of calibrated instruments required for many chambers.
7.1.2 Disadvantages
Disadvantages include:
a) The effect of the test sample is difficult to quantify, although it may be negligible for
samples that are very small compared with the chamber. It is very difficult to assign an
uncertainty to the effect of the load.
b) The effect of heat-dissipating test samples is very hard to quantify.
c) Drift, resolution, and repeatability of the chamber controller shall be assessed and their
contributions to the uncertainty calculations shall be included.
7.2 Typical load
Calibration of the chamber with a typical load is ideal where very similar tests are repeated.
7.2.1 Advantages
Advantages are as follows:
a) The affect of the load on the control of the chamber can be accurately assessed without
subjecting the test sample to an unknown stress.
b) The smallest suitable chamber that produces satisfactory conditions can be chosen
prior to test.
c) Careful positioning of the sensors can give detailed information about critical parts of
load.
d) Anomalies from dissipating loads can be quantified.
e) Relatively low cost. Only one set of calibrated instruments required for many chambers.
7.2.2 Disadvantages
Disadvantages include:
a) Re-calibration is required when the test sample is changed significantly.
b) Drift, resolution and repeatability of the chamber controller shall be assessed and their
contributions to the uncertainty calculations shall be included.
60068-3-11 © IEC:2007 – 17 –
7.3 Measurement of conditions in the chamber during the test
7.3.1 Advantages
Advantages are as follows:
a) This method gives the best estimate in the measured value of the conditions
experienced by the item under test. It is ideal when different kinds of loads and different
tests are being performed.
b) The effect of the load on the control of the chamber can be accurately assessed.
c) History of chamber calibration drift need not be assessed.
d) Careful positioning of the sensors can give detailed information about critical parts of
the load.
e) Anomalies from heat dissipating loads can be quantified.
f) This method can be economical because the chamber is not calibrated for conditions
that are not required.
7.3.2 Disadvantages
Disadvantages include:
a) Measurement equipment is required for every test.
b) Uncertainty calculations shall be made for every test.
c) Can be the most expensive method because measurement equipment is required all the
time.
7.4 Conditions to measure
If measurements are made at the time of the test, then an uncertainty can be calculated for
that condition. Alternatively, a calibration of the chamber could be performed for each
condition for which the chamber is to be used. However, in practice it is not always necessary
to perform a calibration at every possible condition.
If measurements are not made at the time of the test, the whole of the measurement
sequence and the analysis shall be repeated for a set of conditions that cover at least a range
of use. For evaluation an example is given in IEC 60068-3-6.
For temperature only (i.e. humidification OFF) this should include sufficient measurement
points to cover:
− the highest temperature;
− the lowest temperature;
− at least two temperatures with the cooling ON;
− at least two temperatures with the heating ON.
In addition to the temperature-only measurements above, measurements should be performed
for at least two humidity values, covering the range, for any of the above conditions where
humidity tests are to be performed.
It is necessary to perform so many measurements because each of the humidity and
temperature control systems can cause the chamber to have different gradients and
fluctuations. The temperature control is often much worse when the humidity system is on.
– 18 – 60068-3-11 © IEC:2007
If the chamber is only used at a few specific set points then only these need to be calibrated.
When the test is not performed at one of the calibrated levels, it is necessary to interpolate
between two calibrated levels. Interpolation should be used with caution and preferably only if
− the calibrated levels are reasonably close to the test level,
− the services used for each calibrated level (refrigeration, dehumidifiers, heaters, etc.) are
the same.
7.5 Measurements required
The measurements required are the same for all the methods; it is simply a matter of when
the measurements are made and how the results are analysed.
7.5.1 Temperature
For temperature measurement, an array of temperature sensors is used to measure the
temperature at points distributed around the chamber. There are other standards
IEC 60068-3-5 that give measurement methods but they do not address the consequences of
uncertainty and only refer to empty chambers.
For an empty chamber, eight sensors are normally used at the corners of the working space,
and a ninth in the centre. For large chambers more sensors may be required.
For a typical load, or an item under test, eight sensors, one at each corner of the object, are
usually used. For very small test items fewer sensors may be sufficient, but at least four
should be used. For large or unusually shaped objects, or where some particular point on the
test item is of special interest, extra sensors should be employed as appropriate.
For a heat dissipating test item, the measurement of the incident air temperature is usually
considered to be the condition of interest for the report but the other sensors should still be
used so that the local effects of the heat from the test item can be quantified.
7.5.2 Humidity
For humidity measurement, a hygrometer is positioned centrally on the incident air side of the
test item or in the centre of an empty chamber. This can be any kind of hygrometer, but is
most likely to be a relative humidity sensor, a psychrometer, or a condensation (chilled mirror)
hygrometer. The vapour pressure can be computed from the humidity and temperature
measurements. The vapour pressure is assumed to be the same everywhere in the chamber
and the relative humidity is computed from this vapour pressure and the temperature at each
of the temperature measurement sensors.
For each condition, a measurement of vapour pressure gradients shall be made so that an
uncertainty due to this variation can be calculated. This can be done using several
hygrometer probes of any type. However, relative humidity probes and psychrometers are
also sensitive to temperature so usually the estimate obtained using these instruments will be
larger than the true value.
60068-3-11 © IEC:2007 – 19 –
Another method is to sample from several points through tubing routed to a single hygrometer
and switched in alternation.
The vapour pressure gradient is normally small and need only be evaluated occasionally.
7.5.3 Recording procedure
To ensure that a valid assessment can be made,at least 5, and preferably 20 or more
recordings should be taken from each sensor at each set condition. The recordings should be
taken over a sufficient period of time so that several control fluctuations of the chamber can
be recorded. A period of 30 min is normally sufficient.
Recordings are taken from the array of sensors after the chamber has stabilized at each set
condition.
Measurements should be taken frequently from each sensor throughout the test period. Table
A.1 shows a typical data set, together with some of the analysis.
It is essential to ensure that the intervals between the measurements do not coincide with the
cycling interval of the chamber.
7.6 Sources of uncertainty
In any measurement, uncertainties arise from four basic sources.
7.6.1 Calibration uncertainties
Calibration uncertainties for the instruments used are listed on the calibration certificate.
These are normally at the 95 % confidence level. When interpreting uncertainties stated on a
calibration certificate, care shall be taken to consider all aspects of uncertainty mentioned,
including instrument resolution and short-term changes noted during calibration, as well as
the measurement uncertainty.
7.6.2 Instrument uncertainties
Instrument uncertainties include factors such as the resolution, repeatability and drift of the
instruments used. Repeated measurements can guard against gross errors and give improved
confidence in the estimated uncertainties.
7.6.3 Uncertainties arising at the time of the measurement
The dominant uncertainties at the time of the measurement usually concern the gradients and
fluctuations in the conditions. The measurement method shall identify these gradients and
fluctuations.
7.6.4 Uncertainty by radiation
Radiation effects can be large in some chambers. If the temperature sensors give
measurements which are unexpectedly large or vary from test to test or if there is any reason
to suspect that radiation could be a problem (e.g. if the temperature of any part of the
chamber which can be sensed by the test item is significantly different from the set
temperature), then an extra test should be performed with temperature sensors which have
different radiation colours (e.g. a shiny one next to a black one). For temperatures above
+100 °C, the radiation effect has
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