Workplace exposure - Guide for the use of direct-reading instruments for aerosol monitoring - Part 2: Evaluation of airborne particle concentrations using Optical Particle Counters

This Technical Report describes the principle underlying evaluation of one or more health related aerosol fractions using an optical particle counter and details its limits and possibilities in the field of occupational hygiene.
The method complements conventional long-term aerosol particle sampling and offers possibilities of:
-   instantaneous (direct reading) measurement;
-   time-related monitoring;
-   investigation of space-related aerosol evolution (mapping);
-   assessment of particle size distribution.
The method enables e.g.:
-   detection and relative quantification of concentration peaks due to specific operations (bagging, sanding, etc.);
-   identification of most exposed workers with a view to more detailed studies of risks and prevention measures to be applied;
-   detection of dust emission sources and their relative magnitudes.
Basically, OPCs count airborne particles and are therefore suitable for measuring concentrations expressed in number of particles per unit volume of air. The applicability of the method is limited by the particle size and concentration ranges of OPC instruments, usually approximately 10-1 µm to 101 µm and 100 particles/cm3 to 103 particles/cm3, respectively.
Depending on specific conditions, the OPC method allows filter collection of an aerosol fraction, in the best case close to a health-related fraction (see EN 481), provided the OPC has the relevant sampling efficiency over its optical particle size range. If this is not the case, at least a sufficient aspiration efficiency is required to cover the size range of particles which can be detected and measured by the OPC optical system.
Converting count-based particle number concentrations into mass concentrations based on estimated particle size is indirect and therefore the accuracy of the conversion is limited by several simplifying assumptions:
-   identical optical parameters for both the calibration aerosol and the measured workplace aerosol;
-   (...)

Exposition am Arbeitsplatz - Leitfaden für die Anwendung direkt anzeigender Geräte zur Überwachung von Aerosolen - Teil 2: Ermittlung der Konzentration Luft getragener Partikel mit optischen Partikelzählern

Exposition au poste de travail - Guide d'utilisation des instruments à lecture directe pour la surveillance des aérosols - Partie 2 : Evaluation des concentrations de particules en suspension dans l'air à l'aide de compteurs optiques de particules

Le présent Rapport Technique du CEN décrit le principe de l'évaluation d'une ou plusieurs fractions d'aérosols liées à la santé à l'aide d'un compteur optique de particules et en précise les limites et les possibilités dans le cadre de l'hygiène du travail.
Complémentaire de l'échantillonnage classique de longue durée des particules d'aérosols, cette méthode offre des possibilités :
-   de mesurage quasi instantané (lecture directe) ;
-   de surveillance dans le temps ;
-   d'examen des variations spatiales des aérosols (cartographie) ;
-   d'évaluation de la distribution granulométrique.
A titre d'exemples, cette méthode permet :
-   la détection et la quantification relative des pics de concentration dus à des opérations spécifiques (ensachage, ponçage, etc.) ;
-   l'identification des travailleurs les plus exposés en vue d'études plus approfondies des risques et des mesures de prévention à appliquer ;
-   la détection des sources d'émission de poussières et leurs importances relatives.
Fondamentalement, les COP effectuent un comptage de particules en suspension dans l'air et sont donc adaptés au mesurage des concentrations exprimées en nombre de particules par unité de volume d'air. L'applicabilité de la méthode est limitée par les gammes de tailles de particules et de concentrations des appareils de COP qui, en général, sont respectivement de l'ordre de 10-1 µm à 101 µm et 100 particules/cm3 à 103 particules/cm3.
La méthode de COP permet, moyennant certaines conditions, de collecter sur un filtre une fraction de l'aérosol, proche dans le meilleur cas de la fraction liée aux problèmes de santé (voir EN 481), à condition que le COP ait une efficacité de captage pertinente sur sa gamme optique de taille de particules. Si tel n'est pas le cas, une efficacité d'aspiration suffisante est au moins requise pour couvrir la classe granulométrique des particules qui peuvent être détectées et mesurées par le système optique du COP.

Izpostavljenost na delovnem mestu - Vodilo za uporabo instrumentov z neposrednim odčitavanjem za monitoring aerosolov - 2. del: Vrednotenje koncentracije lebdečih delcev z uporabo optičnih števcev delcev

To tehnično poročilo opisuje načelo, ki je podlaga za vrednotenje enega ali več z zdravjem povezanih aerosolnih frakcij z uporabo optičnih števcev delcev ter podrobno opisuje omejitve in možnosti na področju higiene pri delu. Ta metoda dopolnjuje konvencionalno dolgoročno vzorčenje aerosolnih delcev in ponuja možnosti za: takojšnje merjenje (z neposrednim odčitavanjem); časovni monitoring; raziskovanje prostorske sprostitve aerosolov (preslikava); ocenjevanje razporeditve velikosti delcev. Metoda na primer omogoča: zaznavanje in relativno količinsko določanje največjih vrednosti koncentracije zaradi določenih delovanj (polnjenja v vreče, brušenje itd.); prepoznavanje najbolj izpostavljenih delavcev za izvedbo bolj podrobnih študij o tveganjih in preventivnih ukrepih; zaznavanje virov emisij prahu in njihovih relativnih razsežnosti. V bistvu OPC štejejo lebdeče delce in so potemtakem primerni za merjenje koncentracij, izraženih v številu delcev na enoto volumna zraka. Uporaba metode je omejena z velikostjo delca in razponi koncentracije  inštrumentov OPC, ki so običajno od 10-1 μm do 101 μm  in od 100 delcev/cm3 do 103 delcev/cm3. Odvisno od posebnih pogojev, metoda OPC omogoča zbiranje aerosolne frakcije s filtrom, v najboljšem primeru blizu z zdravjem povezani frakciji (glej EN 481), pod pogojem, da je OPC ustrezno učinkovit pri vzorčenju optičnega razpona velikosti delcev. Če temu ni tako, se zahteva vsaj zadostna učinkovitost pri izsesavanju, ki zajema razpon velikosti delcev, ki ga OPC optični sistem lahko zazna in meri. Pretvorba koncentracij števila delcev, osnovanih na štetju, v masno koncentracijo, osnovano na ocenjeni velikosti delcev, je neposredna; potemtakem je natančnost pretvorbe omejena z več naslednjimi poenostavitvenimi predpostavkami: identični optični parametri za aerosol pri kalibraciji in aerosol, merjen na delovnem mestu; vsi prešteti delci aerosola na delovnem mestu so okrogli z geometričnim premerom, enakim določenemu optičnemu premeru in z identično gostoto; učinkovitost OPC pri izsesavanju in prenosu je poznana ali ocenjena glede na inženirske modele. Potemtakem je potrebna potrditev ocenjene masne koncentracije iz OPC porazdelitve velikosti delcev z konvencionalno metodo vzorčenja. Ocenjene masne koncentracije glede na podatke OPC so zgolj pokazatelj in se jih ne more uporabiti za neposredno primerjavo z pravno uveljavljenimi mejami poklicne izpostavljenosti.

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SLOVENSKI STANDARD
SIST-TP CEN/TR 16013-2:2010
01-november-2010
,]SRVWDYOMHQRVWQDGHORYQHPPHVWX9RGLOR]DXSRUDERLQVWUXPHQWRY]
QHSRVUHGQLPRGþLWDYDQMHP]DPRQLWRULQJDHURVRORYGHO9UHGQRWHQMH
NRQFHQWUDFLMHOHEGHþLKGHOFHY]XSRUDERRSWLþQLKãWHYFHYGHOFHY
Workplace exposure - Guide for the use of direct-reading instruments for aerosol

monitoring - Part 2: Evaluating airborne particle concentrations using Optical Particle

Counters

Exposition am Arbeitsplatz - Leitfaden für die Anwendung direkt anzeigender Geräte zur

Überwachung von Aerosolen - Teil 2: Ermittlung der Konzentration Luft getragener
Partikel mit optischen Partikelzählern

Exposition au poste de travail - Guide d'utilisation des instruments à lecture directe pour

la surveillance des aérosols - Partie 2 : Evaluation des concentrations de particules en

suspension dans l'air à l'aide de compteurs optiques de particules
Ta slovenski standard je istoveten z: CEN/TR 16013-2:2010
ICS:
13.040.30 Kakovost zraka na delovnem Workplace atmospheres
mestu
SIST-TP CEN/TR 16013-2:2010 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST-TP CEN/TR 16013-2:2010
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SIST-TP CEN/TR 16013-2:2010
TECHNICAL REPORT
CEN/TR 16013-2
RAPPORT TECHNIQUE
TECHNISCHER BERICHT
May 2010
ICS 13.040.30
English Version
Workplace exposure - Guide for the use of direct-reading
instruments for aerosol monitoring - Part 2: Evaluation of
airborne particle concentrations using Optical Particle Counters

Exposition au poste de travail - Guide d'utilisation des Exposition am Arbeitsplatz - Leitfaden für die Anwendung

instruments à lecture directe pour la surveillance des direkt anzeigender Geräte zur Überwachung von Aerosolen

aérosols - Partie 2 : Evaluation des concentrations de - Teil 2: Ermittlung der Konzentration Luft getragener

particules en suspension dans l'air à l'aide de compteurs Partikel mit optischen Partikelzählern

optiques de particules

This Technical Report was approved by CEN on 13 March 2010. It has been drawn up by the Technical Committee CEN/TC 137.

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.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: Avenue Marnix 17, B-1000 Brussels

© 2010 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 16013-2:2010: E

worldwide for CEN national Members.
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Contents Page

Foreword ..............................................................................................................................................................4

Introduction .........................................................................................................................................................5

1 Scope ......................................................................................................................................................6

2 Principles of the method .......................................................................................................................7

2.1 Light scattering ......................................................................................................................................7

2.2 Working principle...................................................................................................................................7

3 OPC performance characteristics ........................................................................................................8

4 Number and mass concentrations .......................................................................................................8

5 Mass concentrations of thoracic and respirable aerosol fractions ..................................................9

6 OPC use ............................................................................................................................................... 10

6.1 General ................................................................................................................................................. 10

6.2 Airflow adjustment.............................................................................................................................. 11

6.3 Calibration of particle count response ............................................................................................. 11

6.4 Calibration of particle diameter response ........................................................................................ 11

6.5 Mass concentration response ........................................................................................................... 11

7 Fundamental and practical limitations ............................................................................................. 12

7.1 Refraction index and particle density ............................................................................................... 12

7.2 Forward scattering instruments ........................................................................................................ 12

7.3 Limitation in particle size ................................................................................................................... 12

7.4 Coincidence error and concentration limitation .............................................................................. 12

7.5 Aerosols from several sources ......................................................................................................... 12

8 Instrumentation characteristics ........................................................................................................ 13

8.1 Aspiration system ............................................................................................................................... 13

8.2 Integrated collection filter .................................................................................................................. 13

8.3 Sampling head .................................................................................................................................... 13

8.4 Optical cell ........................................................................................................................................... 13

8.5 Electronics ........................................................................................................................................... 13

8.6 Case of laser instruments .................................................................................................................. 13

9 Aerosol measurement by OPC .......................................................................................................... 13

9.1 Operating procedure .......................................................................................................................... 13

9.2 Cartography of workplace ................................................................................................................. 14

9.3 Working shift monitoring ................................................................................................................... 14

9.4 Sampling record.................................................................................................................................. 14

9.5 Cleaning and maintenance ................................................................................................................ 14

Annex A (informative) Evaluation of an OPC as an instrument for thoracic and respirable mass

concentrations .................................................................................................................................... 15

A.1 Introduction to workplace evaluation ............................................................................................... 15

A.2 Procedure for field comparison of the OPC with the reference sampler ...................................... 15

A.2.1 General ................................................................................................................................................. 15

A.2.2 Comparison of a static OPC with a static reference sampler ........................................................ 16

A.2.3 Comparison of mass concentrations for the respirable or thoracic aerosol fractions

calculated from OPC data with a reference sampler ....................................................................... 16

A.3 Calculation methods .......................................................................................................................... 16

A.3.1 General ................................................................................................................................................. 16

A.3.2 Estimation of the correction coefficient ........................................................................................... 16

A.3.3 Exclusion of outliers .......................................................................................................................... 17

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A.3.4 Residual uncertainty after transformation by the correction function .......................................... 17

A.3.5 Equivalence .......................................................................................................................................... 17

A.4 Periodic validation ............................................................................................................................... 17

A.5 Documentation .................................................................................................................................... 18

A.5.1 General ................................................................................................................................................. 18

A.5.2 Description of the OPC and the reference sampler ......................................................................... 18

A.5.3 Critical review of sampling process .................................................................................................. 18

A.5.4 Circumstances of field comparison .................................................................................................. 18

A.5.5 Details of experimental design .......................................................................................................... 18

A.5.6 Data analysis ........................................................................................................................................ 18

A.5.7 Equivalence .......................................................................................................................................... 19

A.6 Nomenclature ....................................................................................................................................... 19

Annex B (informative) Example for the determination of the correction coefficient for an OPC ............. 20

Bibliography ...................................................................................................................................................... 23

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Foreword

This document (CEN/TR 16013-2:2010) has been prepared by Technical Committee CEN/TC 137

“Assessment of workplace exposure to chemical and biological agents”, the secretariat of which is held by

DIN.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent

rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.

CEN/TR 16013, Workplace exposure ― Guide for the use of direct-reading instruments for aerosol

monitoring, consists of the following parts:
 Part 1: Choice of monitor for specific applications

 Part 2: Evaluation of airborne particle concentrations using Optical Particle Counters

 Part 3: Evaluation of airborne particle concentrations using photometers (in preparation)

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Introduction

Optical Particle Counters (OPC) count airborne particles and are therefore suitable for measuring

concentrations expressed in number of particles per unit volume of air. Counting-based measurement of mass

concentration and particle size estimation is indirect: a number of assumptions and approximations are made

to access the information sought. Nevertheless, optical particle counters can be used to evaluate the

efficiency of preventive actions and to monitor the spatial distribution and/or the temporal evolution of an

aerosol. In occupational hygiene, no standard recommends workers' exposure assessment using optical

particle counters. These instruments should instead be considered as permitting a complementary approach

to the conventional filter-based gravimetric method. A confirmation of OPC mass concentration by a

conventional sampling method with a calibrated instrument is recommended when comparing concentration

measurements with legal limit values.
An OPC method allows assessment of working place aerosol conditions including:

 almost instantaneous evaluation of particle concentration and size distribution;

 estimating concentration variations and mean concentration of aerosol particles during a working shift

period;

 sampling to constitute a particle sample for further analysis (when equipped with terminal filter).

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CEN/TR 16013-2:2010 (E)
1 Scope

This Technical Report describes the principle underlying evaluation of one or more health related aerosol

fractions using an optical particle counter and details its limits and possibilities in the field of occupational

hygiene.

The method complements conventional long-term aerosol particle sampling and offers possibilities of:

 instantaneous (direct reading) measurement;
 time-related monitoring;
 investigation of space-related aerosol evolution (mapping);
 assessment of particle size distribution.
The method enables e.g.:

 detection and relative quantification of concentration peaks due to specific operations (bagging, sanding,

etc.);

 identification of most exposed workers with a view to more detailed studies of risks and prevention

measures to be applied;
 detection of dust emission sources and their relative magnitudes.

Basically, OPCs count airborne particles and are therefore suitable for measuring concentrations expressed in

number of particles per unit volume of air. The applicability of the method is limited by the particle size and

-1 1 0 3

concentration ranges of OPC instruments, usually approximately 10 µm to 10 µm and 10 particles/cm to

3 3
10 particles/cm , respectively.

Depending on specific conditions, the OPC method allows filter collection of an aerosol fraction, in the best

case close to a health-related fraction (see EN 481), provided the OPC has the relevant sampling efficiency

over its optical particle size range. If this is not the case, at least a sufficient aspiration efficiency is required to

cover the size range of particles which can be detected and measured by the OPC optical system.

Converting count-based particle number concentrations into mass concentrations based on estimated particle

size is indirect and therefore the accuracy of the conversion is limited by several simplifying assumptions:

 identical optical parameters for both the calibration aerosol and the measured workplace aerosol;

 all counted particles of the workplace aerosol are spherical with a geometric diameter equal to the

determined optical diameter and with identical density;

 the aspiration and transmission efficiencies of the OPC are known or estimated from engineering models.

Therefore confirmation of the estimated mass concentrations from OPC particle size distributions by a

conventional sampling method is necessary (see [3]). The estimated mass concentrations from OPC data are

only indicative and cannot be used for a direct comparison with a legally enforced occupational exposure limit.

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2 Principles of the method
2.1 Light scattering

An aerosol particle scatters light energy through the effects of reflection, refraction absorption, and diffraction.

The amount of energy scattered can be calculated by applying Mie's theory (see [8]), which can be

summarised by the following simplified equation for a non-polarised monochromatic incident light beam and a

spherical particle:
1 λ
I = I i()α,n,θ + i()α,n,θ (1)
0 2 2 1 2 
8π r
where

I is the intensity of light scattered at angle θ, per unit cross-sectional area, in watts per square

metre;

I is the intensity of the incident beam, per unit cross-sectional area, in watts per square metre;

α is the particle size parameter, where α = π × D λ and D is the spherical particle diameter, in

micrometres;
n is the particle complex refraction index;
λ is the wavelength of incident light, in micrometres;

r is the distance from the centre of the scattering particle to the point where the intensity, I, is

measured, in micrometres;
θ is the scattering angle;
i , i are Mie intensity functions.
1 2

The particle diameter D can be deduced from Equation (1) by measuring the intensity of light scattered, when

the particle optical parameters and the incident light beam characteristics are known.

2.2 Working principle

OPCs are closed optical cell instruments featuring an aerosol aspiration system. They are characterised by

their very low optical measuring volume (of the order of 1 mm ) and by a flow rate often of the order of 1 l/min

(see [9]). This allows particles to be drawn individually into the sensing zone and recording of the light

scattered by each particle. Discrete pulses are counted and their size measured.

The aerosol to be investigated is aspirated through the instrument sampling probe by a constant flow pump.

Particles pass one by one into the optical cell, where each particle is illuminated by a focused light beam of

specified characteristics and scatters this light according to its properties (complex refraction index, size,

shape).

Particles move perpendicularly to the plane formed by the focused light incident beam and the scattered light

reception beam. Optical parts are swept by filtered air to prevent any particle deposition inside the optical cell.

The scattered light is focused onto a photo detector and recorded as a pulse. From the pulse size, the particle

size is inferred assuming spherical particles. A quantity of signals during predefined integration time can be

converted into mass concentration, usually after calibration using the investigated aerosol.

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3 OPC performance characteristics

OPC performance characteristics vary according to the aerosol particle sampling efficiency of the sampling

head, the type of light used (monochromatic or polychromatic), its intensity (incandescent or laser lamp), the

cell optical arrangement (choice of scattering axis, width of reception solid angle), the sensitivity of the

photosensitive component (photodiode, photomultiplier) and the electronic discriminating power (pulse

frequency and pulse size measurement).

The limited flow rate, often of the order of 1 l/min, restricts the chances of attaining aerodynamic conditions

favourable to good aspiration orifice efficiency and ensuring full transmission of particles to the optical cell.

OPC flow rate system characteristics (aspiration orifice and tube geometries, air velocities and flow rates) are

such that particle losses are mainly inertial and therefore greater for larger particles (especially those larger

than 10 µm).

Maximum concentration that can be measured by an OPC is limited to a few thousand particles per cubic

centimetre to avoid coincidence error by passing several particles simultaneously through the optical sensing

volume.
4 Number and mass concentrations

OPC counting for a time t, in minutes, gives the number N of particles, counted and classified by size in

different channels.

Knowing the airflow Q aspirated by the OPC, it is simple to calculate the particle number concentration in

terms of number of particles per unit volume of air C :
C = 0, 001 (2)
Q ×t
where

C is the particle number concentration in terms of number of particles per unit volume of air, in

1/cm ;
N is the number of particles counted;
Q is the airflow aspirated by the OPC, in litres per minute;
t is the time, in minutes.

Mass concentration C is expressed in particle mass per unit volume of air. Based on the assumption that

particles are spherical and identify the particle geometrical diameter as its optical diameter, the mass of a

particle classified in channel i, m can be calculated from the equation:
−12
m = π D ρ (3)
i i
where
m is the mass of a particle classified in channel i, in milligrams;

D is the mean diameter between channel i left-hand and right-hand thresholds, in micrometres, as

selected by the manufacturer or specified by the user;
ρ is the particle density, in kilograms per cubic metre.
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The mass of all particles counted by the OPC is:
m = m N (4)
∑ i i
where
m is the mass of all particles counted by the OPC, in milligrams;
m N is the mass of particles classified in channel i.
i i
The mass concentration of the sampled aerosol C is given by the equation:
C =1000 (5)
Q × t
where

C is the mass concentration expressed in particle mass per unit volume of air, in milligrams per cubic

metre.

NOTE Light scattering parameters are not those characterising aerosol mass concentration. The light scattering

response is dependent on the index of refraction of particles, and not on their density. On the other hand, particle mass

concentration depends on particle density, and not on the index of refraction. There is no known physical link between

these two properties of matter. This problem should be minimised by an appropriate calibration, but it assumes a fair

stability of particle composition and size distribution on the site, even when aerosol concentration changes, see [4].

5 Mass concentrations of thoracic and respirable aerosol fractions

The dichotomous model of aerosol fractioning as it progresses though the respiratory tract enables us to

calculate the thoracic fraction concentration C or the respirable fraction concentration C from the total

T R

airborne particle concentration C using the penetration probabilities given in EN 481.

For the thoracic fraction:
C = C P (D ) P (D ) F (D ) dD (6)
T m I ae T ae m ae ae
For the respirable fraction:
C = C P (D ) P (D ) F (D ) dD (7)
R m I ae R ae m ae ae
where
C is the thoracic fraction concentration, in milligrams per cubic metre;
C is the respirable fraction concentration, in milligrams per cubic metre;

P (D ) is the inhalable sampling convention (expressed as a fraction instead of a percentage), as

I ae
defined in EN 481;

P (D ) is the thoracic sampling convention (expressed as a fraction instead of a percentage), as

T ae
defined in EN 481;
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P (D ) is the respirable sampling convention (expressed as a fraction instead of a percentage), as

R ae
defined in EN 481;

F (D ) is the mass particle size distribution function per unit particle diameter, in 1/µm;

m ae
D is the particle aerodynamic diameter, in micrometres.

For the aerosol sampled by the OPC the corresponding thoracic and respirable fraction concentrations the

integrals are converted into summations over all channels by the appropriate weight.

For the thoracic fraction concentration:
P (D )
I ae,i
C =1000 m (D )N P (D ) (8)
T i i i T ae,i
Q ×t A(D )
ae,i
and for the respirable fraction concentration:
P (D )
I ae,i
C =1000 m (D )N P (D ) (9)
R i i i R ae,i
Q ×t A(D )
ae,i
where

A(D ) is the aspiration efficiency of the OPC as a function of aerodynamic diameter.

If the aspiration efficiency of the OPC is unknown, the aspiration efficiency of the OPC entry nozzle in calm

and moving air, can for circular tubular nozzles be estimated by the models presented by Brockmann (see [2])

and for horizontal omni-directional slot entries by the models presented by Witschger et al., Roger et al. and

Görner et al., see [12], [10] and [6].

Based on the hypothesis that the particles are spherical and their optical and geometrical diameters are

identical, the particle aerodynamic diameter, D can be deduced from the geometrical diameter, D , using the

ae,i i
equation:
D = D (10)
ae,i i
where
D is the particle aerodynamic diameter, in micrometres;
ae,I
D is the geometrical diameter, in micrometres;
3 3
ρ is 10 kg/m ;
ρ is the density of particles sampled by the OPC, in kilograms per cubic metre.
6 OPC use
6.1 General

An OPC should meet the general requirements for the determination of airborne agent concentrations at the

working place given in EN 482. Several tests are required to achieve this objective.

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6.2 Airflow adjustment

Instrument airflow adjustment should be performed under conditions of normal OPC usage using a calibrated

flow meter.
6.3 Calibration of particle count response

This kind of calibration is usually performed by manufacturer. It should be checked that every particle entered

optical cell is counted and reciprocally that there are no counts due to electronic noise.

6.4 Calibration of particle diameter response

Channel thresholds can be calibrated using spherical, individually dispersed particles (e.g. certified

polystyrene particles) in suspension in clean air with a diameter close to the value corresponding to the

threshold of the considered channel. In principle, this calibration should be performed for all counter channels.

However, a single channel may be calibrated and the counter response curve calculated for the relevant

range. The latter method requires knowledge of the counter construction parameters and the light scattering

calculation algorithm according to Mie's theory, see [8] and [1].
6.5 Mass concentration response

The particle density ρ of the investigated aerosol (see Equation (3)) should be introduced to convert the OPC

particle number counts into a mass response. But this density is often unknown or cannot be known for

heterogeneous aerosols. The mass response should then be adjusted using a correction factor k obtained by

comparing the calculated mass concentration C from the OPC data with the reference mass concentration C

m r
according to:
k = (11)
where
k is the correction factor;
C is the reference mass concentration, in milligrams per cubic metre;
C is the calculated mass concentration, in milligrams per cubic metre.

Some OPCs allow local calibration using the investigated aerosol. This involves collecting on an instrument-

incorporated filter the aerosol particles that have crossed the optical cell. The real mass of the aerosol, for

which the instrument has counted the particles, is thereby determined by weighing and C is deduced from the

collected particle mass and sampled air volume.

In other cases, the reference concentration C is obtained by resorting to a proven method applied in parallel

(sampling point as near as possible, identical sampling time).

The correction factor, k, is only valid for the identical and stable aerosol. Recalibration should be performed in

each new case (change of measuring location, measured aerosol type or composition).

---------------------- Page: 13 ----------------------
SIST-TP CEN/TR 16013-2:2010
CEN/TR 16013-2:2010 (E)
7 Fundamental and practical limitations
7.1 Refraction index and particle density

Calculation of OPC response in mass concentration terms depends particularly on the refraction index n, and

the particle density ρ. Parameters n and ρ are not linked to each other. Therefore the response may vary

depending on refraction index and particle density, even if the particle number and size per unit volume

remains the same (see [3] and [4]). Mass concentration calibration is therefore essential for each alteration of

investigated aerosol.
7.2 Forward scattering instruments

OPC response in relation to particle size depends, amongst other factors, on the scattered light detection

angle. In the case of forward scattering (where the investigated scattered beam is close to the incident light

axis), there is no unequivo
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