Observations on the effects of air flow on smoking
yields as applied to linear and rotary smoking machines.
By
I. Tindall, Cerulean, Rockingham drive, Linford Wood East, Milton Keynes,
United Kingdom1
T.Mason, Cerulean, Rockingham drive, Linford Wood East, Milton Keynes,
United Kingdom
It has been long established that air flow is one of the key factors that effect
smoking yields in routine smoking machines. This is acknowledged in ISO3308
by a requirement to measure and set air flow as part of the smoking criteria.
However it is becoming clear that the magnitude alone of the air flow is not
sufficient to obtain consistent results between machine types [1].
Current commercially available machine types have small but measurable
differences in mean yields. This paper explores these differences with particular
reference to air flow and poses some questions as to how these might be
minimised through attention being paid to design of air flow.
Introduction
Yields from commercially available routine analytical smoking machines have
been long studied with a view to ensuring that inter laboratory reproducibility is
well understood and considered when comparisons are being made between
cigarettes. This has become a more acute issue as countries and organisations
produce legislation on maximum yield levels and differences in measured yields
can become a decider in the acceptability of a product for sale [2].
Studies such as those conduced by ACS and CORESTA have led to
harmonisation efforts by the principle manufacturers of smoking machines to
ensure that the machine designs are not the source of measured variation in
yields. This was a complex task successfully managed by CORESTA and
resulted in significant reduction in offsets between machines.
Despite these efforts there are still some small but measurable differences in
yields when the machine type is considered. This could in principle lead to
discrepancies in yields as measured by regulating authorities and manufacturers
and be the source of dispute.
1
Author to whom correspondence should be addressed
1|Page
Difference in CO yields Linear - Rotary smoking machines
1.00
Higher linear yield
0.80
1mg
3mg
0.60
5mg
10mg
15mg
0.40
Difference(mg/cigt.)
0.20
0.00
9th
10th
11th
12th
-0.20
-0.40
-0.60
-0.80
-1.00
-1.20
-1.40
Higher Rotary yield
-1.60
-1.80
ACS meeting
Figure 1 : Comparison of CO yield for linear and rotary machines for nominal 1 to 15mg
yield cigarettes.
The figure 1 shows how the CO yields differ between rotary and linear machines
for a series of ACS studies, the degree of difference depending upon the nominal
yield of the cigarette, although the percentage bias is relatively constant. There is
a clear bias for higher yields from the rotary style machines. This difference has
been minimized by air flow modification kits that have been retrofitted to rotary
smoking machines.
There are also noted differences in NFPDM between linear and rotary types
(figure 2). Here ventilated lower tar cigarettes tend to have slightly higher yields
on rotary machines whilst with fuller flavour cigarettes the linear machines give
higher yields. This effect can also be seen when considering intensive smoking
of ventilated cigarettes when more tar is produced.
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Difference in NFPDM Linear - rotary smoking machines
1.00
0.80
Higher linear yield
1mg
0.60
3mg
5mg
10mg
15mg
Difference(mg/cigt.)
0.40
0.20
0.00
4th
5th
6th
7th
8th
9th
10th
11th
12th
-0.20
-0.40
-0.60
Higher Rotary yield
-0.80
-1.00
ACS study
Figure 2 comparison of NFPDM yields between rotary and linear smoking machines for a
series of Asia Collaborative Studies.
This is a result that has also been observed for CORESTA monitor cigarettes
such as the CM4 smoked under ISO conditions, a study conducted by the
CORESTA RAC group showed figures for CM4’s smoked on different types of
machines that underline the findings of the ACS studies (table 1).
Table 1 : Table of typical mean yields from CM4 cigarettes by machine type - source
CORESTA collaborative study 2004 [3]
Puff
count
9.27
8.95
9.06
TPM
Linear *
17.75
Rotary**
17.54
All types
17.62
*Cerulean SM400 and SM450
Water
Nicotine
NFPDM
CO
1.85
2.5
2.26
1.316
1.288
1.298
14.6
13.75
14.06
12.69
13.54
13.23
** kc-Borgwaldt RM20 and RM200
The reasons for these discrepancies are not well understood as the smoking
process is complex and destructive in nature. A greater understanding of the
nature and cause of offsets between machine types is fundamental to eliminating
these differences and producing truly harmonised smoking machines.
Some work has been conducted on discovering the cause for the slight
discrepancies between different types of linear smoking machines [1]. It has
been established that the direction of air flow as well as the magnitude contribute
to the observed yield of CO and NFPDM when monitor cigarettes are smoked to
the ISO regime.
3|Page
This may provide some guidance as to the cause of discrepancies between
common linear and rotary machines. Additional factors such as the lighting
regime, path between smoked cigarette and captured pad and losses of volatile
components from the pad could contribute to discrepancies and these should
also be investigated.
Experimental
In order to understand the causes for these apparent differences in yields a
rotary machine was designed and built. This machine was designed such that the
air flow at the smoking port position was along the axis of the cigarette and that
the distance from cigarette to the extraction hood was closely matched to a linear
smoking machine. In addition the smoking ports were designed so that a
Cambridge filter pad could be located immediately behind the smoking article so
as to mimic a linear smoking machine as well as having an arrangement where
by the smoke was collected in a single pad as is common for other rotary
machines. To eliminate any potential human errors the machine was fitted with a
coil lighter that automatically moved to the set position for lighting with a defined
pre-light period, time of lighting and voltage for the coil (controlled lighting
temperature). For mechanical reasons termination was performed by an optical
sensor rather than the cotton that would be used on a linear machine.
The whole design was such that it proved relatively simple to dismantle for
cleaning and for inspection of any trapping points where smoke condensate
could lie within the smoking process. Analysis of CO was by collection in gas
bags and verification using a Cerulean COA analyser, the same model as
supplied on many linear smoking machines.
The design intent was to make a linear smoking machine “in the round” but with
only a single puff engine. The minor modifications were intended to allow a series
of experiments that could provide indications as to the cause of the discrepancy
between linear and rotary configurations.
The air flow at the smoking port was intended to mimic that of the linear machine,
with uniform flow along the length of the smoking article and as little “funnelling”
of flow as was possible given constraints of sidestream vapours. To do this the
approach of having a large extraction aperture remote from the cigarette was
chosen, the schematic (figure 3) showing the extraction and air flow scheme.
4|Page
Linear
SM450
arrangemen
t
Air flow
Experimental Rotary
arrangement
Air flow
Figure 3 Schematic of air flow patters for linear and rotary smoking machines.
Baseline comparisons for yields were provided in the main by collaborative
studies and, where there was not data available in the public domain, from
comparisons made between SM400 and SM450 linear smoking machines. These
tests were carried out by an independent laboratory as part of the development
and qualification process of the SM450 linear smoking machine.
5|Page
Figure 4: Montage showing elements of rotary smoking machine, clockwise from top left
automatic lighting element, smoke pad holder for 20 cigarettes, termination sensor and
44mm pad holders for individual results
Various samples were smoked as part of the test performed and a range of
nominal tar values and smoking regimes were used. These are shown together
with the nominal and expected tar and CO yields in table 2. Expected yields were
normally an average of test values except in the case of the CM4 where the
expected yields have been sub divided by machine type.
Brand
CM4
Linear
(CORESTA
Rotary
Monitor)
All types
IM17
2R4F (University of
Kentucky)
1R5F (University of
Kentucky)
Nominal
NFPDM
14mg
14mg
14mg
13mg
9mg
Expected
NFPDM
14.52mg
13.82mg
14.17mg
13.4mg
9.2mg
CO Expected
2mg
1.785mg
2.83mg
12.47mg
13.44mg
12.95mg
15.79mg
12.3mg
Table 2 : Table of typical mean yields from monitor cigarettes
6|Page
Results
Results presented here have been obtained from a small data set of three
machines and a statistical significance beyond this small data set should not be
inferred.
A
C
H
2
C
C
1
D
D
A = 44mm filter pad
B = 92 mm filter pad
C = Angle bracket and pipe
work
C1 = filter pad holder
C2 = pipe work
D = smoking ring
E = Region behind 92mm pad
F = 92mm pad holder
G = puff engine
H = IM17 cigarette
1
F
B
E
G
Figure 5 : A schematic of capture arrangement with alternate capture pad position
Initial investigations concerning the proximity of the capture pad with respect to
the smoked article were conducted by varying the position of the Cambridge filter
within the smoking environment. It is possible that differences in NFPDM are a
result of condensate forming on the path to the capture pad. Using the
experimental apparatus shown schematically in figure 5 comparison tests were
conducted where the pad was positioned at point A and at point B. The
comparison in yields is shown in table 3.
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Mean value
“linear” pad Point
A
8.25
1.136
15.19
18.82
Puff count
Nicotine mg/cig
NFDPM mg/cig
TPM mg/ cig
Mean value
“Rotary” pad Point
B
8.247
1.18
14.74
18.56
Mean value long
path “Rotary” pad
11.96
Table 3: Table of comparison yields for different capture pad locations
It can be seen that puff count is the same in the two experiments and tolerable
agreement in yields are achieved for other principle components.
Having determined that the path length has no appreciable effect upon yields
smoking experiments were conducted on a range of brands using the rotary
smoking machine with essentially horizontal air flow as described above.
Table 4 shows the obtained results from this small study group using ISO
smoking machine conditions and compared with expected norms. Target yields
are normally an average of test values for different machine types except in the
case of the CM4 where the results have been sub divided by machine type.
IM17
Nominal Tar /
mg
Range
NFDPM/mg
Range CO/mg
Target
NFDPM/mg
Achieved
NFPDM/mg
Target CO/mg
Achieved
CO/mg
2R4F
1R5F
13
9
2
13.4-15.3
8.6-9.2
1.66-1.79
15-15.8
13.40
11.4-12.3
8.69
2.72-2.98
1.66
13.18
8.66
1.71
15.00
15.20
11.40
11.36
2.72
2.63
CM4 [3]
Linear Rotary All types
14
13.82-14.52
12.47-13.44
14.52 13.82 14.17
14.05
12.47
13.44 12.95
12.55
Table 4: Table of comparison yields for standard monitor cigarettes and rotary machine
with horizontal air flow.
8|Page
Discussion
The mechanical constraints of rotary machines have forced the capture pad to be
sited at a location further from the cigarettes than in linear machines. Retention
on the Cambridge filter pad could be a potential cause of discrepancy both in
specific analytes and in overall tar weight. The Cambridge filter pad system (a
glass fibre filter stabilised by an organic binder) traps the particles present in the
cigarette smoke and is 99.9% efficient for particles greater than 0.1microns in
diameter. This efficiency is known to be a function of the nature and quantity of
material being trapped, the flow through the filter, the temperature and moisture
content of the filter [4].
The initial experiments that used different locations for the Cambridge pad
showed a slight discrepancy in NFPDM that was lower than has been observed
in collaborative studies (3% loss as opposed to 6% discrepancy). The CO yields
are within 1% or each other and the Nicotine variation is within the expected
statistical variation for such an experiment. It can be concluded that the slightly
longer path length is not a principle cause for any observed yield variation.
Extending the path length to 0.5m does reduce TPM yield through condensation
on the tube walls, so there should be pains taken to minimise path length to
prevent condensation, losses and carry over.
If the results obtained for smoking standards are expressed graphically, where
the difference in yield between standards and this rotary machine by brand types
is plotted, the pattern of difference can be compared with the same comparison
made in the ACS studies (linear – Rotary yields). Figure 6 shows the differences
for NFPDM and figure 7 for CO.
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Difference in Yield NFPDM linear - rotary smoking machines
1
Higher linear yield
0.753
0.8
0.6
0.47
0.4
0.07
-0.4
14mg CM4
mean
-0.216
13mg IM17
15mg
10mg
-0.159
5mg
-0.2
3mg
0
-0.23
14mg CM4
Rotary - CR20
0.03
14mg CM4
linear - CR20
-0.045
9mg 2R4F
0.054
2mg 1R5F
0.004
1mg
mg per cig
0.22
0.2
-0.6
-0.8
Higher Rotary yield
-1
Nominal yield for brands
Figure 6: Comparison of yield difference smoked under ISO conditions for NFPDM
between Linear and Rotary type machines for the 12th ACS study and the standard yield to
special rotary machine yields.
Difference in yields CO linear- rotary smoking machines
1
0.89
0.8
Higher linear yield
0.6
0.4
0.2
0.095
0.04
-0.2
-0.08
14mg CM4
Rotary - CR20
14mg CM4
linear - CR20
14mg CM4
mean
13mg IM17
-0.7
9mg 2R4F
-0.709
-0.342
2mg 1R5F
15mg
-0.295
10mg
-0.4
5mg
-0.2
3mg
0
1mg
mg CO / cig
0.4
-0.476
-0.6
-0.8
Higher Rotary yield
-1
Nominal Tar Yield
Figure 7: Comparison of yield difference smoked under ISO conditions for CO between
Linear and Rotary type machines for the 12th ACS study and the standard yield to special
rotary machine yields.
On the left of the graphs in figures 6 and 7 is the results of the 12th ACS study. A
negative result – bars below the x axis – indicate that the rotary machines give a
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higher result than linear machines whilst a bar above the x-axis indicates that
linear smoking machine give higher yields than rotary machines. In each case
there are definite trends dependant on machine types.
In contrast the right hand side of the two graphs shows a comparison between
the expected yields for monitors and those achieved by the rotary machine with
linear flow characteristics. In each case the trends present in the ACS data have
been minimised or eliminated. Of particular note is the correspondence of data
for CO from this modified rotary machine with linear smoking machines.
In figure 6 there is still a small trend in tar yields when compared with the
expected nominal values (lower yield than expected). This is a considerably
reduced trend than seen for the ACS results and does no show the negative
tendency at lower yield.
There appears to be a greater correlation between those results obtained on
linear machines with the results obtained on this rotary machine when compared
with other standard rotary smoking machines. It appears that the replication of air
flow of a linear system on a rotary platform has minimised the differences in
yields.
It has been shown [1] that air flow direction can have an influence on yields for
linear machines. In an experiment conducted on an ASM500 smoking machine
the direction of air flow was altered from essentially at right angles to the
cigarette being smoked to a flow that was more parallel to the cigarette
orientation, a direction that is common to the SM400, SM450 and KC20X. In
these studies there was a marked change in some of the monitored yields
indicating that the vector of air flow is important and not just the magnitude of air
flow during smoking.
The ISO3308 [5] standard defines air flow in a smoking machine in terms of a
point remote from the cigarette and does not specify the direction of air flow. Only
the magnitude of flow measured with an Omni directional probe is defined. The
direction of flow is not specified nor is the flow volume. Flow at points adjacent to
the measured point, or along the cigarette, are not defined. Consequently there
may be small but subtle differences in the air flow environment experienced by
the cigarette during smoking which may manifest its self in differences in yields
between machine types. It is probable that such discrepancies will be brand
dependant and could be amplified by certain constructions.
Inherent in the design of a rotary machine the vector of air flow is not constant
through the smoking process. During the interpuff period the cigarettes are
moved which may influence the free burn rate and hence the overall puff count
and yield. This difference in air flow vector could underpin some of the
discrepancies in yields between machine types. However when the air flow in a
rotary smoking machine is essentially parallel to the cigarette during puffing this
movement does not significantly contribute to a discrepancy in yield.
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Conclusions
The small data set used in this study and the single smoking regime limits the
conclusions that can be drawn.
The premise that the rotary and linear arrangements for smoking machines give
inherently different yields seems unfounded. Using a machine with air flow
direction essentially replicating linear smoking has provided some indications as
to the underlying causes for discrepancies in yields under the ISO smoking
regime.
The inherent movement of rotary schemes does not significantly affect measured
yields. Neither does the position of the capture system provided that good design
is employed to ensure that the generated condensate is transported completely
to the pad and that there are no residues left between pad and cigarette.
When rotary machines are constructed with the same general air pattern (vector)
as linear machines then in this small comparison study yields are obtained that
are consistent between the general machine types.
In particular the CO yield is sensitive to this change in flow vector.
The current study only compares ISO condition smoking and further
understanding of the effect of flow vectors on yields could be gained if other
regimes such as FTC, Canadian intense and Massachusetts are employed.
Further experimental work needs to be conducted on the effect of mass flow and
air vector uniformity if a defined set of machine parameters is to be created that
ensure that different machine styles always produce consistent and comparable
results.
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References
[1] Tindall.I, Mason.T; “The effect of flow vectors on the yields from “unusual
design cigarettes” when smoked in an ISO compliant manner”; Proceedings of
the 60Th Tobacco Science Research Conference, Montreal 2006
[2] EU Directive 2001/37/EC “Manufacture, presentation and sale of tobacco
products”
[3] CORESTA study for the estimation of the repeatability and reproducibility of
the measurement of nicotine free dry particulate matter, nicotine and CO in
smoke using the ISO smoking method. September 2004.
[4] Grob K., “Gas chromatography of cigarette smoke. Part III. Separation of the
overlap region of gas and particulate phase by capillary columns”, J. Gas Chrom.
3 (1965) 52-56
[5] ISO 3308:2000 “Routine analytical cigarette-smoking machines – definitions
and standard conditions”
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