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Watson, J.G.; Chow, J.C.; Tropp, R.J.; Wang, X.L.; Kohl, S.D.; Chen, L.-W.A. (2013). Standards and traceability 1
for air quality measurements: Flow rates and gaseous pollutants. Mapan-Journal of Metrology Society of India, 2
28(4): accepted. 3
4
5
6
Pre-Print 7
Standards and Traceability for Air Quality Measurements: Flow Rates and Gaseous 8
Pollutants 9
10
11
12
13
14
John G. Watson
1,2
15
Judith C. Chow
1,2
16
Richard J. Tropp
1
17
Xiaoliang Wang
1
18
Steven D. Kohl
1
19
L.W. Antony Chen
1
20
21
22
1
Division of Atmospheric Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, 23
Nevada, USA 24
2
State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, 25
Chinese Academy of Sciences, 10 Funghi South Road, Xi’an High Tech Zone, Xi’an, Shaanxi, 26
China 27
28
29
30
31
32
33
34
35
36
37
38
39
1
Abstract 40
Accurate and precise flow rate and gas concentration measurement standards are needed 41
for comparable air quality measurements. Transfer standards are most often used for calibration, 42
performance testing, and auditing of field monitors. These must be traceable to primary 43
standards that are in turn derived from fundamental units of length, mass, temperature, and time. 44
Flow rates and volumes are measured by devices based on positive displacement, pressure 45
differences, and temperature increases or decreases. Stable gas concentrations are prepared in 46
non-reactive pressurized containers by gravimetric and dilution methods. Reactive gases are 47
generated from photochemistry and permeation devices. These standards are used to determine 48
the accuracy, precision, and validity of air quality measurements, and these attributes should be 49
reported with the measurement values. 50
Introduction 51
Air quality measurements are acquired for multiple purposes using a wide variety of 52
physical and chemical methods. All of these methods require some sort of standardization that 53
allows the values acquired to be compared with each other and with regulatory limits. As more 54
air quality monitors are installed worldwide, and as the number of measured observables 55
increases, there is a need to summarize and review the standardization process and available 56
resources. Various standard-setting organizations, such as the International Standards 57
Organization (ISO, 2013) and American Society of Testing and Materials (ASTM, 2013), have 58
issued procedures for creating and using measurement standards. Unfortunately, these 59
procedures are available only at high cost, and several have not been updated to reflect current 60
technology. Fortunately, summaries and reviews of the topic have been published, along with 61
more specific literature on individual standards, and the most useful of these are identified here. 62
The following discussion defines common terms related to air quality standards, identifies their 63
uses, and describes options available for flow rate/volume and gas concentration measurements. 64
Suspended particle (PM) measurement standards will be addressed in a forthcoming publication. 65
Definitions 66
“Standards” and “standardization” refer to several different things in air quality science 67
and are defined as follows: 68
Air quality regulatory standards: Ambient air quality standards (AAQS) have been
69
established in many countries as a means to protect public health and welfare (Bachmann, 70
2
2007; Cao et al., 2013; Chow et al., 2007; Vahlsing and Smith, 2012; WHO, 2006; World 71
Bank, 2007). AAQS define a measured indicator (usually the criteria contaminants of carbon 72
monoxide [CO]; nitrogen dioxide [NO
2
]; sulfur dioxide [SO
2
]; ozone [O
3
]; and PM mass and 73
lead [Pb]), an averaging time [e.g., 5 minutes; one-hour; 24-hour; or one year], a statistical 74
form [e.g., annual averages; maximum value; or upper 98
th
percentile], and concentration 75
limits for each averaging time. Emission standards are also set for large industrial sources 76
and engine exhaust. 77
Air quality measurement standardization: Procedures and methods are defined for 78
determining compliance with regulatory standards. This standardization can be: 1) 79
performance-based, in which specifications are given for the measured quantities and 80
tolerable deviations from the desired values, or 2) design-based, in which the dimensions and 81
types of measurement equipment are defined (Chow, 1995; Watson et al., 1995). The U.S. 82
EPA’s (2012) Federal Reference Methods (FRMs) represent a combination of performance 83
and design standardization. Figure 1 shows examples of performance- and design-based 84
standards for PM
2.5
(mass of particles with aerodynamic diameter < 2.5 µm) FRMs. The 85
disadvantage of design-based methods are that newer, and possibly more accurate, 86
technology must demonstrate equivalence with methods that may have been found to be less 87
applicable to modern pollution situations. An example of this is standardization of the 88
Method 5 stack test method (U.S. EPA, 2008) which draws effluent through a hot filter to 89
measure PM, then through impinger solutions that are intended to capture condensable gases, 90
but in reality collect soluble SO
2
and organic compounds (England et al., 2000). Dilution 91
sampling systems (Chang et al., 2004; Watson et al., 2012), similar to those used for engine 92
exhaust, provide more realistic PM emissions. 93
Air quality measurement standards: These are methods or materials of well-known 94
properties used for measurement calibration, auditing, and performance testing, and are the 95
subject of this paper. These are classified as: 96
- Primary Standard: Mixtures, substances or devices with properties or values derived
97
from fundamental physical parameters of length, mass, temperature, and time as related 98
to primary standards established by national and international metrology institutes. 99
Primary standards are prepared and maintained under well-controlled environmental 100
conditions. 101
3
- Transfer (Secondary) Standard: Mixtures, substances or devices with values based upon 102
comparison with primary standards and traceable to them. Transfer standards may also 103
be primary standards, but they are often less costly and available in larger quantities than 104
primary standards. As several transfer standards are used in field applications without 105
environmental control, and are often less costly to produce, transfer standards may be less 106
accurate and precise than primary standards. 107
- Reference Materials: Substances representing different environmental matrices that have 108
been analyzed by multiple laboratories and have had values assigned based on inter-109
laboratory agreement. These are sometimes used as primary standards, but they are more 110
appropriate as audit standards. They are often used to evaluate the effect of interferences 111
that are encountered in environmental measurements. 112
- Calibration Standards: Transfer standards at various concentration levels used to relate 113
the output of a measurement method to a concentration level. 114
- Performance Test Standards: Transfer standards at various concentration levels used to 115
evaluate instrument calibration during normal operation. These are often the same as the 116
calibration standards. They are often used in “zero/span” tests. 117
- Audit Standards: Transfer standards with traceability independent of those used for 118
calibration and performance testing. Performance audits are performed periodically to 119
evaluate the accuracy of a method calibration and are performed by someone other than 120
the person who normally operates the instrument. 121
Flow Rate/Volume Measurement Standards 122
The product of volumetric flow rate (F
v
) and sampling time (t) equals the volume (V) 123
passing through the measurement device: 124
V=F
v
t (1) 125
Since t can be accurately measured, quantification of V and F
v
are interchangeable. Many 126
modern air quality measurement devices contain some form of flow measurement and timing 127
device. Accurate flow rates are essential for all air quality monitoring devices and for preparing 128
their measurement standards. Collocated comparisons of monitoring devices often show near-129
perfect correlations between the reported concentrations, but slopes differing from unity by 130
±10% or more. The first suspect in these cases is differences in the sample volume or flow rate. 131
4
Given the importance of flow rate and volume values, there is a surprisingly small 132
literature describing and comparing the different measurement and standardization options. 133
Baker and Pouchot (1983a; 1983b) provide the most concise, accurate, and accessible summary 134
of flow and volume measurement standards while Bean (Bean, 1971) contains a more 135
comprehensive treatment. These methods can be classified into the following categories: 136
Positive displacement systems have precise internal volumes that can be filled and emptied
137
as a function of time. Since these volumes are relatable to the primary meter standard in 138
Paris, most of these are suitable as primary standards. 139
Head devices restrict the flow with an obstruction, thereby changing the flow speed and 140
reducing the pressure. The pressure reduction can be related to the flow rate as determined 141
by a primary standard. 142
Rotational systems consist of a turbine or fan that spins at a higher rate when flow rate
143
increases. These are calibrated to primary standards. 144
Gravitational resistance variable area devices allow the flow to raise a weight within an 145
annulus that varies with height. These are calibrated to primary standards. 146
Thermal devices measure the temperature lost from a heated wire with a constant electrical 147
current as the gas passes over it or the increase in gas temperature as it passes over a surface 148
maintained at a constant temperature. These quantify mass as opposed to volumetric flow 149
rates and are calibrated to primary standards. 150
Table 1 describes some of the primary and transfer standards for flow rate/volume 151
commonly used for air quality purposes, and Figure 2 illustrates some of their appearances. 152
Most gas flow measurement devices are calibrated for air to standard conditions, which 153
are usually 1 atm of pressure, but with standard reference temperatures of 0 °C, 20 °C, or 25 °C. 154
Flow rates must be adjusted from the ambient conditions to the standardized conditions reported 155
with the measurement standard. It is good practice to record these conditions on the device 156
itself, as they are often specified only in the accompanying calibration certificate or operating 157
manual. After the real-world ambient flow rate is determined, it may be necessary to adjust this 158
to a new standardized condition for reporting purposes. The U.S. EPA (2013) FRM defines 159
PM
10
levels for standard sample volumes at 1 atm pressure and 25 °C, while other criteria 160
pollutant concentrations (including the PM
2.5
and Pb fractions of PM
10
) are reported at ambient 161
5
temperatures and pressures. Head and gravity devices also depend on the upstream and 162
downstream pressures; when one of these differs from 1 atm, adjustments must be made and 163
these do not necessarily follow the ideal gas law. For example, a rotameter reading scales as the 164
square root of the ratio of gas densities (which depend on temperature, pressure, and molecular 165
weight of the gas mixture), rather than as the direct ratios indicated by the ideal gas equation. A 166
rotameter used as an in-line flow indicator should be re-calibrated against an appropriate transfer 167
standard at the inlet to the measurement instrument. 168
Mass flow (F
m
) needs to be converted to volumetric flow (F
v
) for most air quality 169
applications: 170
F
v
= (RT/MP)F
m
(2) 171
which requires knowledge of the absolute temperature (T) in the flow meter, the molecular 172
weight of the gas (M), the gas pressure at the inlet of the flow meter (P), and the ideal gas 173
constant (R) in units consistent with the other measurements. Note that the original calibration 174
of the mass flow meter is probably related to standard air composition (21% O
2
and 78% N
2
), so 175
that adjustments are needed when measuring other gases in the preparation of gas measurement 176
standards as described below. Other gases in air constituting <1% (e.g., CO
2
) are usually 177
neglected as they add small additional uncertainty. 178
Gas Concentration Measurement Standards 179
Many gases are considered important to air quality, both for their direct effects on the 180
environment and for their participation in atmospheric chemical reactions that create secondary 181
gases and particles with adverse effects. Much effort has been expended in creation of gas 182
measurement standards for CO, NO
2
, and SO
2
, and O
3
, as these are regulated as criteria 183
pollutants. Certain volatile organic compounds (VOCs) are O
3
and PM precursors, and some of 184
the VOCs are classified as Hazardous Air Pollutants (HAPs), so appropriate standards have been 185
created. Standards for chemical end-products, such as peroxyacetyl nitrate (PAN) and nitric acid 186
(HNO
3
) are also useful for better understanding atmospheric transformation processes. 187
When the gases are non-reactive, small amounts can be mixed and stored in pressurized 188
containers with the balance being taken up by a non-reactive gas. Depending on the reactivity of 189
the pollutant (minor) gas, the makeup gas may consist of clean air (nitrogen and oxygen mixture 190
scrubbed of other contaminants), pure nitrogen, or a noble gas such as helium or argon. The 191
container walls may also react with the gas (Kebbekus and Scornavacca, 1977). Common 192
6
materials considered to be minimally reactive are passivated stainless steel (polished to a mirror-193
like finish), stainless steel, glass, aluminum, Teflon, Tedlar (polyvinyl fluoride), and Mylar. 194
Most commercially-prepared gases are distributed in stainless steel cylinders or passivated 195
spheres and have lifetimes of a year or two. While glass containers may be used to create 196
mixtures in a standards laboratory, they are impractical for field use owing to their fragility and 197
potential danger when under high pressure. Teflon and Tedlar can be used for short-term storage, 198
but gases will diffuse through these materials over long time periods. This permeability is 199
advantageous for permeation tubes, as described below. 200
Barratt (1981) and Namiesnik (1984) provide good descriptions of methods for preparing 201
standard gas mixtures, along with several other reviews (ISO, 2003a; Moss, 2001; Naganowska-202
Nowak et al., 2005). ISO (2001a; 2001b; 2002; 2003a; 2003b; 2003c; 2005a; 2007; 2008; 203
2009a; 2009b; 2009c) has a series of procedures for standard gas preparation. The more 204
common methods are summarized in Table 2, and Figure 3 shows examples of some of the 205
apparatus used to prepare primary standards, illustrating the substantial investment needed for 206
this work. Most stable gas standards are obtained from a specialty gas supplier with a 207
traceability certificate, as illustrated in Figure 4. This original certificate should be placed in a 208
quality assurance (QA) file and a copy should accompany the cylinder wherever it is used. The 209
information should include the nominal concentration, the balance gas, the traceability trail, the 210
date of creation, and the maximum lifetime of the mixture. Reputable suppliers will allow the 211
cylinders to age for a few weeks, often rotating them to assure a uniform mixture, and test them 212
against their primary standards to assure the accuracy of the concentrations. This is why the 213
specified and actual concentrations may differ. 214
Field dilution systems (Saltzman and Wartburg, Jr., 1965; Thomas and Amtower, 1966; 215
Wright and Murdoch, 1994) are used to obtain a range of concentrations for calibration and 216
auditing purposes. These systems incorporate activated charcoal, particle filters, and Drierite to 217
remove water vapor to create clean air as the dilution gas (Turner et al., 1981). While these are 218
adequate for removing criteria pollutants and most VOCs, they may not be sufficient to remove 219
all types of gases (e.g., halocarbons). Concentrations in the range of 20 to 1000 ppb are often 220
required for these purposes. As modern air quality monitors have become more sensitive (~1 221
ppb detection limits), and as these lower levels are relevant to atmospheric chemistry, there is 222
increasing interest in accurate calibration at the lowest concentrations (Chow and Watson, 2008).
223
7
Adjustments for standard gas flow measurements must be made for the gas composition since its 224
molecular weight may differ from that used to calibrate the dilution system’s flow meters. 225
Gases (e.g., CO, nitrogen oxide [NO], and SO
2
), halocarbons, and a reasonable number of 226
VOCs can retain their concentrations in pressurized containers with an appropriate carrier gas for 227
several years. Reactive gases must be generated on-site, often using stable gas standards as 228
reactants. Most oxides of nitrogen (NO
x
) analyzers are calibrated with NO, as they convert NO
2
229
to NO before detection. It is still necessary to generate known amounts of NO
2
, however, to 230
evaluate the conversion efficiency. This is done with gas-phase titration (Bertram et al., 2005), 231
in which an ultraviolet (UV) lamp generates O
3
that reacts with NO to create NO
2
. The UV-232
generated O
3
can also be used as a calibration gas when it is related to a highly maintained 233
primary UV absorption standard (Early et al., 1998a; 1998b). Many modern O
3
monitors contain 234
a small UV O
3
generator used for daily zero/span performance tests. PAN must be generated on-235
site from liquid evaporation or a portable smog chamber (Ciccioli et al., 1992; Edney et al., 236
1979; Grosjean et al., 1984; Holdren and Spicer, 1984; Krognes et al., 1996; Lonneman et al., 237
1982). 238
Permeation tubes (ASTM, 2005; Brito and Zahn, 2011; Gameson et al., 2012; ISO, 2002; 239
Kanda et al., 2005; Kim et al., 2012; Knopf, 2001; Maria et al., 2002; Mitchell et al., 1992a; 240
Mitchell et al., 1992b; Neuman et al., 2003; O'Keeffe and Ortman, 1966; Pitombo and Cardoso, 241
1990; Scaringelli et al., 1967; Scaringelli et al., 1970; Singh et al., 1977; Spinhirne and Koziel, 242
2003; Susaya et al., 2011; Susaya et al., 2012; Thorenz et al., 2012; Torres et al., 1981; Tumbiolo 243
et al., 2005; Washenfelder et al., 2003; Zabiegala et al., 2006) offer the widest range of gas 244
compositions and are especially applicable to reactive gases that can be compressed to a liquid 245
under pressure. Permeation tubes are based on Fick’s law of diffusion (Fick, 1855) in which 246
gases move from areas of higher to lower concentrations. The relevant gas is compressed into a 247
container consisting of or capped by a permeable membrane. Common membrane materials are 248
Teflon, silicone rubber, polypropylene, polyester, Tedlar, and Nylon, selected based on their 249
durability and affinity for the gas being generated. The emission rate is determined by the mass 250
loss over time, as determined by periodic weighings of the tube with a microbalance calibrated 251
against traceable mass standards. In this sense, permeation tubes can be considered to be 252
primary standards. The tubes must be maintained at a constant temperature in a continuous flow 253
of a clean carrier gas, as the diffusion depends on both the temperature and the concentration 254
8
gradient of the pollutant gas across the membrane. Many reactive pollutant standard 255
concentrations, particularly HNO
3
, ammonia (NH
3
), hydrogen sulfide (H
2
S), and several VOCs, 256
can be obtained from permeation tubes that would not remain stable for long periods in a 257
container. 258
Using Standards to Evaluate the Measurement Process 259
An air quality measurement consists of four attributes (Watson et al., 2001), each of 260
which requires measurement standards for its quantification: 1) the value of the measurement 261
(C
m
), which is most commonly reported and used for determining compliance with AAQS; 2) the 262
precision (σ
m
), which is the standard deviation of repeated measurements of a known 263
concentration (C
i
), where i refers to one of the many responses to a performance standard; 3) the 264
accuracy (A), which is the relative difference between C
m
in response to sampling of an audit 265
standard (C
t
); and 4) validity, the extent to which specified measurement procedures were 266
followed, including maintenance or recalibration when A and C
i
exceed pre-set tolerances. A and 267
σ
m
are calculated as: 268
t
tm
C
CC
A
)(100
(%)
(3) 269
and 270
)1(
])([
2
n
AvgCC
iii
m
i=1 to I (total number of performance tests) (4) 271
Validity is expressed in terms of pre-defined data flags that indicate the deviations from 272
the standard operating procedures (SOPs). These flags do not necessarily invalidate the C
m
, but 273
they assist the interpretation of the information when used for data analysis and modeling. 274
Table 3 summarizes calibration, performance test, and audit activities and the standards 275
used for each one. These tests provide the input data for applying Equations 3 and 4 and for 276
assessing the validity of air quality measurements. 277
Summary 278
Accurate and precise measurement standards for flow rate/volume and gas concentrations 279
are essential components of any air quality monitoring program. Several methods are available 280
for generating these standards and tracing them to fundamental primary standards. These 281
methods are not necessarily equivalent, and compensation must be made for differences between 282
9
the gas mixtures and the operating environments. The molecular weight, pressure, and 283
temperature are important variables for both flow rates and gas concentrations. 284
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11
ISO (2003c). ISO 6145-6:2003: Gas analysis -- Preparation of calibration gas mixtures using dynamic volumetric 391
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592
593
594
595
596
597
598
599
600
a)
b)
Figure 1.
E
2.5 µm) F
sampling
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Bottom p
a
quality sta
n
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xamples of
p
ederal Refere
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ffectiveness
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dards (AAQ
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Particle Penetration Fraction
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erformance a
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ce Methods (
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urve (Watso
n
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pecification f
o
S
).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
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Particle
Penetration
Fraction
n
d design stan
d
FRMs; (U.S.
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n
et al., 1983;
o
r a PM
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Wedding an
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(mass of par
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.S.EPA, 199
7
d
Carney, 198
3
et used to me
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4
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) for a PM
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: performance
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size-selectiv
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n
ce with ambi
m
eter <
-based
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inlet.
ent air
16
601
a)
b)
c)
d)
Figure 2. Examples of primary and transfer standards for flow rate/volume. Primary standards: a) Bell Prover; and 602
b) Bubble Meter. Transfer standards: c) Rotameters (left) and Orifice Calibrator (right). Magnahelic above 603
rotameters measures pressure drop across the orifice; and d) Roots Meter. 604
605
17
a)
b)
Figure 3. Examples of gas preparation facilities for gas concentrations standards for: a) a volumetric mixing 606
chamber with clean gas dilution system (Indian Central Pollution Control Board; Delhi); and b) a precise weighing 607
system for quantifying gases compressed into a cylinder. The empty cylinder on the left has the same dimensions as 608
the primary standard cylinder on the right and accounts for air displacement buoyancy effects (Indian National 609
Physics Laboratory, New Delhi). 610
611
612
613
614
615
Figure 4.
E
E
xample of a
g
g
as cylinder ce
rtification to e
18
s
tablish tracea
b
b
ility to a pri
m
m
ary standard.
19
Table 1. Primary and transfer standards for flow rate/volume commonly used in air quality measurements. 616
Flow
Measurement
Device
(References) Operating Principles Comments
Bell Prover (Benkova
et al., 2011; Choi et
al., 2010; Pavlovic et
al., 2009; Ruegg and
Ruegg, 1990; Stasic
et al., 2007)
Positive displacement. The open end of a
large cylindrical “bell” is immersed in a low-
volatility oil and counterbalanced by a weight.
Gas flow is introduced through a tube that
extends above the oil surface, and the change
in elevation of the cylinder above the oil is
measured as a function of time. Pressure and
temperature are monitored within the bell for
adjustment to standard conditions.
Primary standard, originally used by
natural gas suppliers to calibrate dry test
meters. Was commonly used to calibrate
high-volume sampler transfer standards,
but is not practical for the lower flow
rates of modern air quality monitors.
When water is used instead of oil as a
sealant, a correction is needed for
saturated water vapor in the trapped gas.
Piston Meter (Su et
al., 2011)
Positive displacement. A plug with a mercury
seal is tightly-fitted into a precisely-
dimensioned glass tube. The piston rises as
gas is introduced into the tube, and its change
in elevation as a function of time quantifies
the flow rate.
Primary standard, in which adjustments
are needed to correct for increased
pressure owing to resistance of the plug.
Bubble Meter (Clark
et al., 2011; Lashkari
and Kruczek, 2008;
Scott, 1992; Waaben
et al., 1978)
Positive displacement. Gas flows through a
soap-saturated water solution, creating a
bubble that clings to the sides of a vertical
cylinder. The bubble rises and indicates the
volume passing through the system as a
function of time.
Primary standard, in which the bubble
provides minimal resistance to the air
flow, but adjustments are needed to
account for the water vapor added to the
gas stream after passing through the soap-
solution. Modern implementations pulse
the gas through the cylinder and count the
number of pulses per unit time.
Roots Meter
(Cornelius, 1997)
Positive displacement. Two rotating lobes
move in response to flow being drawn through
the meter. The lobes seal to create a constant
volume that moves the flow to the outlet. The
number of revolutions, along with the gas
volume displaced, gives an accurate
measurement of the total gas volume.
Transfer standard, because an empirical
discharge coefficient depending on the
volume between the lobes. The discharge
coefficient remains constant for a given
meter, so it is often used as a primary
standard.
Wet Test Meter
(Smith and Eiseman,
1939)
Positive displacement. A compartmentalized
inner cylinder with slits cut into it is located in
an outer drum half filled with water. The
measured gas is pumped into one of the
submerged compartments on the inner drum,
displacing the water and turning the drum.
The number of rotations is related to the flow.
Transfer standard, owing to the need to
determine a discharge coefficient based
on the drum geometry and resistance to
rotation. Since the discharge constant
remains the same after standardization to
a primary source, this is often used as a
primary laboratory standard.
Dry Gas Meter (Fritz,
2009)
Positive displacement. Internal bellows
inflate and deflate as the measured gas passes
through the meter.
Transfer standard, commonly used in
buildings to measure natural gas
consumption. Smaller units are also used
in some air quality samplers to measure
integrated sample volumes. Several
filling and emptying cycles are needed as
the volume is not constant within a cycle
and measurement does not necessarily
stop at the beginning portion of the cycle.
Orifice Meter (Chahal
and Hunter, 1976)
Head device. One or more orifices of known
area are placed at the inlet to a sampling
system and the pressure drop is measured
across the orifice. These can also be located
Transfer standard; a simple, inexpensive
and accurate flow device. A water
anemometer can be used to measure
pressure drop, though Magnehelics are
20
in a sample flow line, most often in the form
of a venturi with pressure drop measured at
the narrowest point. The pressure drop is
related to the flow measured by a primary
standard via a calibration curve.
more practical. The orifice is commonly
used to set the mass flow controller for
the high-volume sampler used to measure
Total Suspended Particulate (TSP) for
lead quantification.
Critical
Orifice/Throat
(Bogema and
Monkmeyer, 1960;
Brenchley, 1972;
Carter et al., 2011;
Chen et al., 2007;
DeNardi and Sacco,
1978; Li and Johnson,
2011; Lodge, Jr. et
al., 1966; Povey and
Beard, 2008; Urone
and Ross, 1979a;
Wang and Zhang,
1999; Wedding et al.,
1987; Zimmerman
and Reist, 1984)
Head device. When the ratio of downstream
to upstream pressure for flow through an
orifice drops below 0.53 (i.e., for air, slightly
different for other gases), the velocity
stabilizes at the speed of sound and does not
increase. Flat plates with a small hole and
hypodermic needles have been used as critical
orifices. The critical throat is a variation that
reduces the 0.53 downstream/upstream ratio,
thereby requiring less energy for the flow
mover.
Transfer standard, although theoretically a
primary standard, it is treated as a transfer
standard calibrated against a primary
standard to account for imprecise
dimensions of the orifice. These are often
used behind a substrate that collects gases
and/or particles, but a high capacity pump
is needed to retain the desired pressure
difference.
Laminar Flow Meter
(Bean, 1971; Feng et
al., 2011)
Head device. It consists of a tube through
which the gas flows under laminar conditions.
The pressure drop across the tube is directly
proportional to the volumetric flow rate and
gas viscosity.
Transfer standard; a simple, inexpensive
and accurate flow device. It has lower
pressure drop and particle losses than
orifice meters and is therefore suitable for
in-line flow measurement. The pressure
drop can be measured by a Magnahelic
gauge, and need to be calibrated against a
primary standard.
Rotameter (ASTM,
2004; Caplan, 1985;
Mironov and
Freidgei, 1972; Motit
and Nistor, 1988;
Urone and Ross,
1979b; Veillon and
Park, 1970;
Wojtkowiak and
Popiel, 1996)
Gravity/variable area device. A float is
located in a tapered tube that increases its area
with vertical distance from the flow entry
point. The flow raises the float in the tube
until the upward aerodynamic force equals the
weight of the float. Floats of different sizes
and densities can be used in the same tube to
cover a wide range of flow rates.
Transfer standard, calibrated against a
primary standard. The scale is non-linear
unless the tapered tube follows a complex
pattern that is difficult to manufacture.
Most rotameters have slightly angled
bores in the shape of an inverted and
truncated cone. For a spherical float,
readings are taken at the middle of the
ball. Non-spherical floats should have the
reading position well-marked.
Turbine Meter (Shu,
2000; Wolf and
Carpenter, 1982)
Rotational device. A propeller is located in a
tube through which the flow is directed. The
number of rotations per unit time is related to
the flow rate.
Transfer standard, in which a discharge
coefficient is determined by calibration
against a primary standard.
Mass Flow Meter
(Huijsing et al., 1988;
Povey and Beard,
2008; Tison, 1996;
Wedding, 1985)
Thermal cooling. A hot-wire anemometer is
located in a sampling duct and a constant
current is run through the wire. The
temperature of the wire is measured with a
thermistor, and the reduction in temperature is
related to the gas flowing across it.
Transfer standard, calibrated against a
primary standard. These are most
commonly used in modern air quality
monitors as they can be located in-line
and converted to volumetric flow by the
data acquisition software. Temperature
and pressure of the gas are monitored
along with the temperature change of the
hot wire.
617
618
21
619
Table 2. Methods for creating standard gas concentrations. 620
Method
(References)
Principles Comments
Gravimetric (ISO,
2001a; Milton et al.,
2011; Tohjima et al.,
2005)
The difference in weight of two identical
containers, one empty and one containing the gas
mixture, is translated to the gas concentration in the
filled cylinder.
For a two gas mixture compressed to
a known pressure, the specific
gravity can be solved to determine
the number of moles of two gases
with known molecular weights.
Modern microbalances with ~1 µg
precision can determine
concentrations with ~100 ppm
precision.
Gas stream mixing
(Scherberger et al.,
1958; Thomas and
Amtower, 1966)
Precise flows of the pollutant and carrier gas are
metered into a mixing chamber prior to
pressurization into a container or presentation to a
monitoring system.
Precise temperature and pressure
control are essential. Calibration of
the flow meters must be adjusted to
the molecular weight of the gas, as
well as temperature and pressure for
mass flow meters.
Bolus injection
(Jardine et al., 2010)
A known volume of pure pollutant, usually
measured with a syringe, is injected into a mixing
chamber of known volume with internal stirring.
The bolus can also be injected into a zero-air
stream; this is appropriate for an integrating
monitor, but not for a continuous monitor.
Bolus injection is useful for
synchronizing the time of several
instruments on a single manifold and
for determining the response
functions for a detector.
Exponential dilution
(Bruner et al., 1973;
Greenhouse and
Andrawes, 1990;
Nozoye, 1978; Ritter
and Adams, 1976;
Sedlak and Blurton,
1976)
A mixing chamber of known volume is filled with a
pure gas (pollutant), or a known concentration in a
carrier gas, then clean carrier gas is introduced.
The concentration in the outflow decreases
exponentially with time.
A rotor or fan rapidly mixes the gas
in the chamber.
Evaporation (ISO,
2005b; Kida et al.,
2011; Konieczka et
al., 1991; Thorenz et
al., 2012)
A liquid with known vapor pressure is located in a
vessel with a known head space. The metered
carrier gas sweeps through the saturated head
space. Concentration in the head space is
determined from the thermodynamic properties of
the liquid.
Temperature and pressure control in
the head space must be precise. The
sweep rate must be much less than
the evaporation rate of the liquid.
Temperature may be elevated above
ambient to increase evaporation.
621
622
22
Table 3. Primary and transfer standards with calibration, performance test, and audit frequencies for flow rates and gas concentrations at an air quality 623
monitoring site, modified from Watson et al. (2000). 624
Observable
(Method)
Percent
Tolerance
Primary
Standard
Calibration
Standard
Calibration
Frequency
Performance
Test
Standard
Performance
Test
Frequency
Performance
Audit
Standard
Performance
Audit
Frequency
Flow Rates
TSP mass
(high-volume sampler)
±5% Bell Prover
(>1,000
L/min)
Calibrated orifice/
Roots meter
Quarterly Calibrated
orifice
Monthly Calibrated
orifice
Yearly
PM
2.5
mass
(2 single-channel
FRM
a
samplers)
±5% NIST
b
-
certified
bubblemeter
(1-25 L/min)
Mass flowmeter/
bubblemeter
Quarterly Calibrated
bubblemeter
(Gillibrator)
Monthly Mass flow
meter
Yearly
Gas Concentrations
NO/NO
x
(chemiluminescence)
±10% NIST-
traceable NO
mixture
Certified NO mixture
and dynamic dilution
Quarterly or
when out of
specification
Span with
certified NO
and zero with
scrubbed air
Daily Certified NO
mixture with
dynamic
dilution
Yearly
O
3
(UV absorption) ±10% ARB
c
Primary UV
Photometer
Dasibi 1003H UV
photometer
Quarterly or
when out of
specification
Span with
internal O
3
generator and
zero with
scrubbed air
Daily Dasibi 1008
with
temperature
and pressure
adjustments
Yearly
CO (infrared
absorption)
±10% NIST-
traceable CO
mixture
Certified CO mixture
and dynamic dilution
Quarterly or
when out of
specification
Span with
certified CO
and zero with
scrubbed air
Daily Certified CO
mixture with
dynamic
dilution
Yearly
Non-methane
hycrocarbon (NMHC)
(flame ionization)
±10% NIST-
traceable
VOC mixture
Certified VOC and
dynamic gas dilution
Quarterly or
when out of
specification
Span with
certified VOC
and zero with
scrubbed air
Daily Certified
VOC mixture
with dynamic
dilution
Yearly
625
a
U.S. EPA Federal Reference Method 626
b
National Institute of Science and Technology, Gaithersburg, MD, USA 627
c
California Air Resources Board, Sacramento, CA, USA 628
629
