SIST EN 14626:2005
(Main)Ambient air quality - Standard method for the measurement of the concentration of carbon monoxide by nondispersive infrared spectroscopy
Ambient air quality - Standard method for the measurement of the concentration of carbon monoxide by nondispersive infrared spectroscopy
This document specifies a continuous measurement method for the determination of the concentration of carbon monoxide present in ambient air based on the non-dispersive infrared measuring principle (NDIR). This document describes the performance characteristics and sets the relevant minimum criteria required to select an appropriate non-dispersive infrared carbon monoxide analyser by means of type approval tests. It also includes the evaluation of the suitability of an anayser for use in a specific fixed site so as to meet the Directives data quality requirements and requirements during sampling, calibration and quality assurance.
The method is applicable to the determination of the mass concentration of carbon monoxide present in ambient air in the range from 0 mg/m3 to 100 mg/m3 carbon monoxide. This concentration range represents the certification range for the type approval test.
NOTE 1 0 mg/m3 to 100 mg/m3 of CO corresponds to 0 mmol/mol to 86 mmol/mol of CO.
The method covers the determination of ambient air concentrations of carbon monoxide in zones classified as rural areas, urban-background areas and traffic-orientated locations.
NOTE 2 Other ranges may be used for measurement systems applied at rural locations monitoring Ecosystems.
The results are expressed in mg/m3 (at 293 K and 101,3 kPa).
When the standard is used for other purposes than the EU directive, the range and uncertainty requirements need not apply.
Luftqualität - Messverfahren zur Bestimmung der Konzentration von Kohlenmonoxid mit nicht-dispersiver Infrarot-Photometrie
Le présent document spécifie une méthode de mesurage en continu pour la détermination de la concentration en monoxyde de carbone dans l’air ambiant, selon le principe de mesure par rayonnement infrarouge non dispersif (NDIR). Le présent document décrit les caractéristiques de performance et fixe les criteres minimaux requis pour sélectionner un analyseur de monoxyde de carbone par rayonnement infrarouge non dispersif a l’aide d’essais d’approbation de type. Il présente également l’évaluation de l’aptitude a l’emploi d’un analyseur sur un site fixe spécifique de maniere a répondre aux exigences relatives a la qualité des données définies dans les Directives ainsi qu’aux exigences relatives au prélevement, a l’étalonnage et a l’assurance qualité.
La méthode s’applique a la détermination de la concentration massique en monoxyde de carbone présent dans l’air ambiant dans une plage de concentration en CO comprise entre 0 mg/m3 et 100 mg/m3. Cette plage de concentration représente l’étendue de mesure certifiée pour l’essai d’approbation de type.
NOTE 1 La plage comprise entre 0 mg/m3 et 100 mg/m3 de CO correspond a la plage comprise entre 0 µmol/mol et 86 µmol/mol de CO.
La méthode s’applique a la détermination des concentrations en CO présent dans l’air ambiant dans les zones classées comme zones rurales, périurbaines et proches d’axes de circulation automobile.
NOTE 2 D’autres plages peuvent etre utilisées pour des systemes de mesure appliqués aux zones rurales contrôlant des écosystemes.
Les résultats sont exprimés en mg/m3 (a 293 K et 101,3 kPa).
Lorsque la norme est utilisée a d’autres fins que la Directive UE, les exigences relatives a plage et a l’incertitude ne s’appliquent pas.
Qualité de l'air ambiant - Méthode de mesurage pour la détermination de la concentration du monoxyde de carbone dans l'air ambiant par la méthode a rayonnement infrarouge non dispersif
Le présent document spécifie une méthode de mesurage en continu pour la détermination de la concentration en monoxyde de carbone dans l'air ambiant, selon le principe de mesure par rayonnement infrarouge non dispersif (NDIR). Le présent document décrit les caractéristiques de performance et fixe les criteres minimaux requis pour sélectionner un analyseur de monoxyde de carbone par rayonnement infrarouge non dispersif a l'aide d'essais d'approbation de type. Il présente également l'évaluation de l'aptitude a l'emploi d'un analyseur sur un site fixe spécifique de maniere a répondre aux exigences relatives a la qualité des données définies dans les Directives ainsi qu'aux exigences relatives au prélevement, a l'étalonnage et a l'assurance qualité.
La méthode s'applique a la détermination de la concentration massique en monoxyde de carbone présent dans l'air ambiant dans une plage de concentration en CO comprise entre 0 mg/m3 et 100 mg/m3. Cette plage de concentration représente l'étendue de mesure certifiée pour l'essai d'approbation de type.
NOTE 1 La plage comprise entre 0 mg/m3 et 100 mg/m3 de CO correspond a la plage comprise entre 0 µmol/mol et 86 µmol/mol de CO.
La méthode s'applique a la détermination des concentrations en CO présent dans l'air ambiant dans les zones classées comme zones rurales, périurbaines et proches d'axes de circulation automobile.
NOTE 2 D'autres étendues peuvent etre utilisées pour des systemes de mesure appliqués aux zones rurales contrôlant des écosystemes.
Les résultats sont exprimés en mg/m3 (a 293 K et 101,3 kPa).
Lorsque la norme est utilisée a d'autres fins que la Directive UE, les exigences relatives a plage et a l'incertitude ne s'appliquent pas.
Kakovost zunanjega zraka – Standardna metoda za določanje koncentracije ogljikovega monoksida z nedisperzivno infrardečo spektroskopijo
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
SIST EN 14626:2005
01-september-2005
.DNRYRVW]XQDQMHJD]UDND±6WDQGDUGQDPHWRGD]DGRORþDQMHNRQFHQWUDFLMH
RJOMLNRYHJDPRQRNVLGD]QHGLVSHU]LYQRLQIUDUGHþRVSHNWURVNRSLMR
Ambient air quality - Standard method for the measurement of the concentration of
carbon monoxide by nondispersive infrared spectroscopy
Luftqualität - Messverfahren zur Bestimmung der Konzentration von Kohlenmonoxid mit
nicht-dispersiver Infrarot-Photometrie
Qualité de l'air ambiant - Méthode de mesurage pour la détermination de la
concentration du monoxyde de carbone dans l'air ambiant par la méthode a
rayonnement infrarouge non dispersif
Ta slovenski standard je istoveten z: EN 14626:2005
ICS:
13.040.20 Kakovost okoljskega zraka Ambient atmospheres
SIST EN 14626:2005 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST EN 14626:2005
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SIST EN 14626:2005
EUROPEAN STANDARD
EN 14626
NORME EUROPÉENNE
EUROPÄISCHE NORM
March 2005
ICS 13.040.20
English version
Ambient air quality - Standard method for the measurement of
the concentration of carbon monoxide by nondispersive infrared
spectroscopy
Qualité de l'air ambiant - Méthode normalisée de mesurage Luftqualität - Messverfahren zur Bestimmung von
de la concentration en monoxyde de carbone par la Kohlenmonoxid in Luft mit dem NDIR-Verfahren
méthode à rayonnement infrarouge non-dispersif
This European Standard was approved by CEN on 10 December 2004.
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 Central Secretariat 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 Central Secretariat has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,
Slovenia, 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
© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 14626:2005: E
worldwide for CEN national Members.
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SIST EN 14626:2005
EN 14626:2005 (E)
Contents
Page
Foreword.4
1 Scope .5
2 Normative references .5
3 Terms and definitions .6
4 Symbols and abbreviated terms .10
5 Principle.13
5.1 General.13
5.2 Measuring principle.13
5.3 Type approval test .13
5.4 Field operation and quality control.14
6 Sampling equipment .14
6.1 General.14
6.2 Sampling location.14
6.3 Sample inlet and sampling line .14
6.4 Particulate filter.15
6.5 Control and regulation of sample flow rate .15
6.6 Sampling pump for the manifold.15
7 Analyser equipment .16
7.1 General.16
7.2 Interferences due to infrared absorbing gases .16
7.3 Details about analyser equipment .16
7.4 Pressure measurement .17
7.5 Flow rate indicator.17
7.6 Sampling pump for the analyser.17
8 Type approval of CO-NDIR analysers.17
8.1 General.17
8.2 Relevant performance characteristics and performance criteria .17
8.3 Design changes .19
8.4 Procedures for determination of the performance characteristics during the laboratory test .19
8.5 Determination of the performance characteristics during the field test.29
8.6 Expanded uncertainty calculation for type approval.32
9 Field operation and ongoing quality control .33
9.1 General.33
9.2 Suitability evaluation.33
9.3 Initial installation.34
9.4 Ongoing quality control .35
9.5 Calibration of the analyser.36
9.6 Checks .37
9.7 Maintenance .40
9.8 Data handling and data reports.41
10 Expression of results .41
11 Test reports and documentation.42
11.1 Type approval tests .42
11.2 Field operation .42
Annex A (normative) Test of lack of fit.43
Annex B (informative) Sampling equipment.45
2
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EN 14626:2005 (E)
Annex C (informative) Sampling on micro scale.47
Annex D (informative) Schematics of non-dispersive infrared spectrometer.48
Annex E (informative) Manifold testing equipment .50
Annex F (normative) Type approval .51
Annex G (normative) Calculation of uncertainty in field operation at the 8-hour mean limit value.73
Bibliography.86
3
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SIST EN 14626:2005
EN 14626:2005 (E)
Foreword
This document (EN 14626:2005) has been prepared by Technical Committee CEN/TC 264 “Air quality”, the
secretariat of which is held by DIN.
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 September 2005, and conflicting national standards shall be withdrawn at the
latest by September 2005.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Cyprus, Czech Republic, Denmark,
Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
4
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SIST EN 14626:2005
EN 14626:2005 (E)
1 Scope
This document specifies a continuous measurement method for the determination of the concentration of carbon
monoxide present in ambient air based on the non-dispersive infrared measuring principle (NDIR). This document
describes the performance characteristics and sets the relevant minimum criteria required to select an appropriate
non-dispersive infrared carbon monoxide analyser by means of type approval tests. It also includes the evaluation
of the suitability of an anayser for use in a specific fixed site so as to meet the Directives data quality requirements
and requirements during sampling, calibration and quality assurance.
The method is applicable to the determination of the mass concentration of carbon monoxide present in ambient air
3 3
in the range from 0 mg/m to 100 mg/m carbon monoxide. This concentration range represents the certification
range for the type approval test.
3 3
NOTE 1 0 mg/m to 100 mg/m of CO corresponds to 0 µmol/mol to 86 µmol/mol of CO.
The method covers the determination of ambient air concentrations of carbon monoxide in zones classified as rural
areas, urban-background areas and traffic-orientated locations.
NOTE 2 Other ranges may be used for measurement systems applied at rural locations monitoring Ecosystems.
3
The results are expressed in mg/m (at 293 K and 101,3 kPa).
When the standard is used for other purposes than the EU directive, the range and uncertainty requirements need
not apply.
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.
ENV 13005, Guide to the expression of uncertainty in measurement
EN ISO 14956:2002, Air quality — Evaluation of the suitability of a measurement procedure by comparison with a
required measurement uncertainty (ISO 14956:2002)
ISO 6143, Gas analysis — Comparison methods for determining and checking the composition of calibration gas
mixtures
ISO 6144, Gas analysis — Preparation of calibration gas mixtures — Static volumetric method
ISO 6145 (all parts), Gas analysis — Preparation of calibration gas mixtures using dynamic volumetric methods
5
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SIST EN 14626:2005
EN 14626:2005 (E)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
ambient air
)
1
outdoor air in the troposphere excluding workplace air
3.2
ambient temperature
temperature at the sample inlet outside the monitoring station (sample temperature, outdoor temperature)
3.3
availability of the analyser
fraction of the total time period for which usually valid measuring data of the ambient air concentration is available
from an analyser
3.4
certification range
concentration range for which the analyser is type approved
3.5
calibration
comparison of the analyser response to a known gas concentration with a known uncertainty
3.6
combined standard uncertainty
calculation result of combining the uncertainties determined from all performance characteristics specified in this
document according to the prescribed procedures given in this document
3.7
coverage factor
numerical factor used as a multiplier of the combined standard uncertainty in order to obtain an expanded
uncertainty
3.8
designated body
body which has been designated for a specific task (type approval tests and/or QA/QC activities in the field) by the
competent authority in the Member States
NOTE It is recommended that the designated body is accredited for the specific task according to EN ISO/IEC 17025.
3.9
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
NOTE for the purpose of this document the expanded uncertainty is the combined standard uncertainty multiplied by a
coverage factor k = 2 resulting in an interval with a level of confidence of 95 %
3.10
fall time
difference between the response time (fall) and the lag time (fall)
)
1
As stated in the relevant EU legislation.
6
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EN 14626:2005 (E)
3.11
independent measurement
individual measurement that is not influenced by a previous individual measurement by separating two individual
measurements by at least four response times
3.12
individual measurement
measurement averaged over a time period equal to the response time of the analyser
3.13
influence quantity
quantity that is not the measurand but that affects the result of the measurement (VIM 2.7), either an interferent
influence quantity (i.e. the concentration of a substance in the air under investigation that is not the measurand), or
an external influence quantity (i.e. a quantity that is not the measurand nor the concentration of a substance in the
air mass under investigation)
NOTE Examples are:
presence of interfering gases in the flue gas matrix (interferent influence quantity);
temperature of the surrounding air (external influence quantity);
atmospheric pressure (external influence quantity);
pressure of the gas sample (external influence quantity).
3.14
interference
response of the analyser to interferents
3.15
interferent
component of the air sample, excluding the measured constituent, that effects the output signal
3.16
lag time
time interval from the instant at which a step change of sample concentration occurs at the inlet of the analyser to
the instant at which the output reading reaches a level corresponding to 10 % of the stable output reading
3.17
lag time (fall)
lag time for a negative step change
3.18
lag time (rise)
lag time for a positive step change
3.19
limit value
level fixed on the basis of scientific knowledge, with the aim of avoiding, preventing or reducing harmful effects on
human health and/or the environment as a whole, to be attained within a given period and not to be exceeded once
attained
3.20
lack of fit
maximum deviation of the average of a series of measurements at the same concentration from the linear
regression line
3.21
long-term drift
difference between zero or span readings over a determined period of time (e.g. period of unattended operation)
7
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SIST EN 14626:2005
EN 14626:2005 (E)
3.22
monitoring station
enclosure located in the field in which a CO analyser has been installed in such a way that its performance and
operation comply with the prescribed requirements
3.23
parallel measurement
two measurements from different analysers, sampling from one and the same sampling manifold, starting at the
same time and ending at the same time
3.24
performance characteristic
one of the parameters assigned to equipment in order to define its performance
3.25
performance criterion
limiting quantitative numerical value assigned to a performance characteristic, to which conformance is tested
3.26
period of unattended operation
time period over which the drift is within the performance criterion for long-term drift
3.27
repeatability (of results of measurement)
closeness of the agreement between results of successive individual measurements of the same measurand
carried out under the same conditions of measurement [1]
NOTE These conditions are called laboratory repeatability conditions and include:
the same measurement procedure;
the same observer;
the same analyser, used under the same conditions;
at the same location;
repetition over a short period of time.
3.28
reproducibility under field conditions
closeness of the agreement between the results of simultaneous measurements with two analysers in ambient air
carried out under the same conditions of measurement
NOTE 1 These conditions are called laboratory repeatability conditions and include:
the same measurement procedure;
two identical analysers, used under the same conditions;
at the same monitoring station;
the period of unattended operation.
NOTE 2 In this document the reproducibility under field conditions is expressed as a value with a level of confidence of 95 %.
3.29
response time
time interval from the instant at which a step change of sample concentration occurs at the inlet of the analyser to
the instant at which the output reading reaches a level corresponding to 90 % of the stable output reading
3.30
response time (fall)
response time at a negative step change
NOTE Response time (fall) is the sum of the lag time (fall) and the fall time.
8
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SIST EN 14626:2005
EN 14626:2005 (E)
3.31
response time (rise)
response time at a positive step change
NOTE Response time (rise) is the sum of the lag time (rise) and the rise time.
3.32
rise time
difference between the response time (rise) and the lag time (rise)
3.33
sampled air
ambient air that has been sampled through the sampling inlet and sampling system
3.34
sampling inlet
entrance to the sampling system where ambient air is collected from the atmosphere
3.35
short-term drift
difference between zero or span readings at the beginning and end of a 12 h period
3.36
standard uncertainty
uncertainty of the result of a measurement expressed as a standard deviation
[ENV 13005]
3.37
surrounding temperature
temperature of the air directly surrounding the analyser (temperature inside the monitoring station or laboratory)
3.38
type approval
decision taken by a designated body that the pattern of an analyser conforms to the requirements as laid down in
this document
3.39
type approval test
examination of two or more analysers of the same pattern which are submitted by a manufacturer to a designated
body including the tests necessary for approval of the pattern
9
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SIST EN 14626:2005
EN 14626:2005 (E)
4 Symbols and abbreviated terms
For the purposes of this document, the following symbols and abbreviated terms apply
A availability of the analyser;
a
av average concentration of the measurand during the field test;
b sensitivity coefficient of the analyser to sample gas pressure change expressed as a percentage of the
gp
measured value, obtained during the laboratory type approval test;
b sensitivity coefficient of the analyser to sample gas temperature change;
gt
b sensitivity coefficient of the analyser to surrounding air temperature change;
st
b sensitivity coefficient of the analyser to electrical voltage change;
V
C CO concentration of the applied gas;
C average concentration of the measurements at sampling gas pressure P
P1 1;
C average concentration of the measurements at sampling gas pressure P
P2 2;
C concentration of the reference standard;
R
C concentration of site standard;
s
C average concentration of the measurements at span level at the beginning of the drift period;
s,1
C average concentration of the measurements at span level at the end of the drift period;
s,2
c test gas concentration;
t
C average concentration of the measurements at sample gas temperature T ;
T1 1
C average concentration of the measurements at sample gas temperature T ;
T2 2
C average concentration reading of the measurements at voltage V ;
V1 1
C average concentration reading of the measurements at voltage V ;
V2 2
C average concentration of the measurements at zero at the beginning of the drift period;
z,1
C average concentration of the measurements at zero at the end of the drift period;
z,2
av
average of at least four independent measurements during the constant concentration period (t );
c
C
const
av
average of at least four independent measurements during the variable concentration period (t );
v
C
var
average difference of parallel measurements;
d
f
d the ith difference in a parallel measurement;
f,i
D long-term drift at span concentration c
l,s t;
D long-term drift at zero;
l,z
10
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EN 14626:2005 (E)
D short-term drift at span level;
s,s
D short-term drift at zero;
s,z
D difference sample/calibration port;
sc
E sample system collection efficiency;
ss
F response factor in concentration units per voltage output of the analyser;
r
P sampling gas pressure P ;
1 1
P sampling gas pressure P ;
2 2
R mean analyser response to the test gas directly sampled by the analyser;
d
R mean analyser response to the test gas via the sample manifold;
m
s repeatability standard deviation at zero;
r,z
s repeatability standard deviation at concentration c ;
r,ct t
s reproducibility standard deviation under field conditions is the repeatability standard deviation;
r,f
s repeatability standard deviation;
l
T surrounding air temperature;
T sample gas temperature T ;
1 1
T sample gas temperature T ;
2 2
t relative difference between response time (rise) and response time fall;
d
t response time (fall);
f
T surrounding air temperature at the laboratory;
l
t two-sided Students t-factor at a confidence level of 0,05, with n-1 degrees of freedom;
n-1, 0,05
t response time (rise);
r
t time period of the field test minus the time for calibration, conditioning and maintenance;
t
t total time period with validated measuring data;
u
t whole number of t and t pairs;
V S02 zero
V minimum voltage V (V) specified by the manufacturer;
1 min
V maximum voltage V (V) specified by the manufacturer;
2 max
V voltage obtained when the reference standard is injected;
r
V voltage obtained when the site standard is injected;
s
V voltage obtained when zero gas is injected;
z
average of measurements;
x
11
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SIST EN 14626:2005
EN 14626:2005 (E)
x first average of the measurements at T just after calibration;
1 l
x second average of the measurements at T just before calibration;
2 l
X difference between the readings of the recent zero check and the most recent calibration;
z
x average of the measurements at concentration c ;
ct t
X averaging effect;
av
x average of the measurements using the calibration port;
c
X influence quantity of CO with concentration 500 µmol/mol;
CO2,z,ct 2
X influence quantity of H O with concentration 19 mmol/mol;
H2O,z,ct 2
x the ith measurement;
i
influence quantity of the interferent at concentration c ;
X
t
int,ct
X influence quantity of the interferent at zero;
int,z
X lack of fit (largest residual from the linear regression function);
l
X influence quantity of NO with concentration 1 µmol/mol;
NO,z,ct
X influence quantity of N O with concentration 50 nmol/mol;
N2O,z,ct 2
x
average of the measurements using the sample port;
s
X difference between the readings of two consecutive span checks;
s
x average of the measurements at T or T ;
T min max
X difference between the readings of two consecutive zero checks;
z
(x ) the ith measurement result of analyser 1;
1,f i
(x ) the ith measurement result of analyser 2 at the same time as the measurement of analyser 1;
2,f i
Z reading of the first zero check;
1
Z reading of the second zero check;
2
∆P measured pressure drop induced by the manifold pump;
m
∆R change in the analyser’s response due to the influence of the pressure drop induced by the manifold
a
pump, expressed as a percentage;
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SIST EN 14626:2005
EN 14626:2005 (E)
5 Principle
5.1 General
This document describes the method for the measurement of the concentration of carbon monoxide in ambient air
by means of the non-dispersive infrared spectrometry principle. The requirements, the specific components of the
non-dispersive infrared analyser and its sampling system are described. For the analyser a number of performance
characteristics with associated minimum performance criteria are given. The actual values of these performance
characteristics for a specific type of analyser have to be determined in a so-called type approval test for which
procedures have been described. The type approval test comprises a laboratory and a field test. The selection of a
suitable analyser for a specific measuring task in the field is based on the calculation of the expanded uncertainty
of the measuring method. In this expanded uncertainty calculation the actual values of the various performance
characteristics of a type approved analyser and the site-specific conditions at the monitoring station are taken into
account. The expanded uncertainty of the method shall meet the requirements of the (EU) legislation.
Requirements and recommendations for quality assurance and quality control are given for the measurements in
the field (see 9.4).
5.2 Measuring principle
Ambient CO concentration is measured with use of non-dispersive infrared methods. The attenuation of infrared
light passing through a sample cell is a measure of the concentration of CO in the cell, according to the Lambert-
Beer law. Not only CO but also most heteroatomic molecules will absorb infrared light, in particular water and CO
2
have broad bands that can interfere with the measurement of CO. Different technical solutions have been
developed to suppress cross-sensitivity, instability and drift in order to design continuous monitoring systems with
acceptable properties. For instance:
measuring IR absorption of a specific wavelength (4,7 µm for CO);
dual-cell monitors, using a reference c
...
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