A test piece for determining dense end offset repeatability
and reproducibility using the microwave method
Authors: Gary Pitt, Cerulean, Rockingham Drive, Linford Wood East, Milton Keynes, MK14 6LY
Ian Tindall, Cerulean, Rockingham Drive, Linford Wood East, Milton Keynes, MK14 6LY
The use of microwave devices for the measurement of mean density and dense end offset
has become commonplace in the analysis of cigarette production. However a practical
problem remains in relating the measurements obtained to the readings from such devices.
Density profile measurements are generally “noisy” and lacking in definition. The user relies
upon the computer algorithms to provide a reading of dense end offset that may not be
evident from visual inspection alone.
The dense end offset can be defined as the distance from the cigarette end where the
profile first equals the density mean. In practice the density varies in such a way as to instil
a degree of uncertainty in the measurement apparatus. It is often difficult to distinguish the
effect of measurement error on the given dense end figure.
The paper describes the development of a simple test piece that allows repeatable
measurement of the density profile using the microwave method. The test piece is based
upon creating a rod made of segments of distinctly different densities but with a known
mean density. In this way the dense end offset can be accurately predicted and measured
for confirmation. Detail of the construction is provided.
Future use of a test piece to provide a verification method for moisture content as measured
by the microwave method is discussed.
Introduction
The physical and structural aspects of cigarette construction may be an important factor in
the combustion process and ultimately the “yields” of cigarettes. Different methods have
been employed to examine the density of cigarettes such as neutron Radiography [1] and
x-radiography [2]. Today density measurements can be simply performed using the
microwave method [3,4] which allows rapid estimation of mean densities of cigarettes in a
non-destructive manner. This can give average density or density profiles depending on
use. Calibration and confidence in the maintenance of this calibration remain problems to
overcome with this method.
The microwave resonance technique is a non-direct measurement method, therefore a
calibration must be carried out. Normally this consists of measuring a representative sample
of cigarettes and then comparing the allocated microwave results to a chosen reference
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method. In the case of moisture measurement this may be an approved oven method. In
the case of density this would be an average density measurement constructed from the
dimensions of the cigarette and the measured weight of tobacco.
In the case of density measurements, cigarettes do not show a uniform distribution of
tobacco throughout the rod and where the microwave measurement is made influences the
outcome of the experiment unless an average measurement is made. Moreover cigarettes
are subject to environmental conditions and have a tendency to change properties with
time.
A method for verifying density measurement of the microwave resonator system and setting
a “benchmark” to be used for recalibration and drift checks is of value when using this
technology.
Where two cigarettes are profiled for density one is the “back” and one the “front” as
determined by the direction of the lap. The dense area is determined by looking for the
point at which the actual density curve intersects the mean density line. This intersection
point is given a distance from the input end of the cigarette and this portion of the cigarette
is known as the “dense end” (see figure1).
Figure 1: Dense end of cigarette
The dense end offset is the front dense area minus the back dense area divided by two.
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Experimental methods
The microwave resonator technique is fast, highly selective, is independent of density and
weight and non destructive. This last property allows the use of “standards” for verification
of the function of the instrument.
The microwave technique is based on the dipole behaviour of water molecules. A harmonic
electromagnetic resonator field is built up by a microwave generator. The water within the
cigarette rod interacts with the field and the changes in the field are recognised by a
detector that measures the shift in resonance frequency and the change in magnitude. By
measuring these two parameters the moisture and density can be measured separately.
Larger molecular groups or ions do not follow the field changes and so do not interfere with
the measurements and so gives high selectivity to water.
Two types of equipment were used for these tests, a Cerulean QTM9PX microwave
moisture and density device fitted with TEWS microwave system (MW3100) cell that
provides average moisture and density results and a Cerulean C2 on line measurement
system also fitted with TEWS microwave measurement system (MW3050) (figure 2).
Figure 2: Measurement cell
Experiments were performed in a temperature and humidity controlled environment (22°±
1° C, 60% +/- 5%RH). Samples were conditioned in this environment to ensure that
equilibrium in moisture was obtained between tests.
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Samples of cigarettes (Benson and Hedges Gold) were used for comparative tests. Filter
rods prepared by Filtrona Jarrow were used for base tests. Filter rod 1 (White wrapped
acetate section material code ABR003888 measured density 135mg/cm 3) and filter rod 2
(black carbon paper section material code PBR003468 measured density 450mg/cm 3) were
measured separately and then combined in a composite sample using RYO papers, see
Figure 3: For dimensions
.
ABR003888
PBR003468
17.5mm
17.5mm
ABR003888
PBR003468
17.5mm
17.5mm
70mm
Figure 3: Composite filter rod
A final composite check rod was manufactured from derived specifications by Filtrona
Jarrow see table 1.
Active Paper Dual Rod Filter (APD008645)
Full Rod
Length
Configuration
Circ
Base
(WA)
Base
(MD)
Wrapped acetate
Length
Material -Tow
Tip Constituent Pressure
Drop
Paper
Length
Material - Paper
Tip Constituent Pressure
Drop
108
3 UP filter rod
24.5
18
8.0Y28,000
mm
(18WA + 36CP + 18WA +
36CP)
mm
mm
Working to a finished weight
18
280mm Carbon Paper
mm
Working to a finished weight
Table 1: Machine made filter check rod specification
In each case measurements were carried out by passing the filter rod through the
microwave cell and recording the raw density and moisture measurements as derived by
the TEWS device.
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Results Discussion
Testing causes some small changes in the measured moisture content of rods. This is a
combination of microwave cell and the process of transporting rods within the instrument.
The data shown in table 2 and figure 4 shows the mean moistures obtained for both filter
rods and tobacco columns obtained by subsequent microwave measurements (10
measurements separated by 3 minutes, then allowed to stabilise for 12 hours in the
controlled environment). It is clear from the data that a calibration error can be obtained if
rods are repeatedly used without equilibrating with the environment first. This is more
pronounced for tobacco rods. To avoid error all measurements made required a
“stabilisation” period in a controlled environment before repeat measurements were made.
Run
1
2
3
4
5
6
7
8
9
10
12 hour
recovery
Cigarette
Batch 1
10.253
10.59
10.097
10.127
9.886
10.002
9.828
9.823
9.656
9.808
10.275
Cigarette
batch 2
9.824
9.889
9.839
9.801
9.892
9.592
9.72
9.647
9.574
9.525
9.876
Filter
batch
8.73
8.406
8.276
8.412
8.218
8.391
8.378
8.532
8.514
8.386
8.575
Table 2: drying out effect of microwave measurement as observed in measurements 30
minutes apart
Mean moisture
11
Overnight recovery
10.5
R2 = 0.7329
units %
10
R2 = 0.7313
9.5
cigarette batch 1
cigarette batch 2
9
filter batch
8.5
8
1
2
3
4
5
6
7
8
9
10
12 hours
Time / batch
Figure 4: Graph showing effect on moisture of microwave cell
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The stabilisation period was chosen as 12 hours after considering the time taken for a
tobacco rod to recover from 48hrs of zero humidity when placed in a 60% RH 22°C
environment, figure 5.
Mean moisture with time from samples held 48hours in Zero Humidity
10
9
8
mean moisture %
7
6
5
4
3
2
1
0
07:00
08:12
09:24
10:36
11:48
13:00
14:12
15:24
16:36
Time
Figure 5: Time constant for cigarette moisture recovery
The composite check rod was then passed through the TEWS cell and the profile of density
measured using a 1 mm step resolution. The transit time for a 70mm long rod was 2 sec. A
profile density is shown in figures 6 and 7 – direction of travel reversed. The “interface”
between the two dense areas is clearly shown. Using a 20%:80% measurement for the
resolution, it can be calculated that the system can resolve dense areas of 3.7mm under
these conditions.
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Figure 6 : Profile of fabricated dense end sample, densest element first
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Figure 7: Profile of fabricated dense end sample, least dense element first
In practice the rods produced in this way are fragile and not easily produced. A test batch of
rods was produced on a filter maker with different densities and these were again run
through the C2 machine for comparison. The machine made rods were made from
segments of nominal densities 135mg/mm3 and 450mg/mm3. The elements were of
nominal length 18mm and 36mm. The resolution as measured by the 20:80 method was
comparable to the fabricated rods and provides a good method of establishing the “dense
end” region.
The rods were not visibly damaged by transit through the machine and consequently show
the potential for forming a check rod.
Conclusions
The method of using segmented filter rods of controlled known density allows the
construction of a check rod that can be used for verifying the density measurement of a
microwave method system such as the TEWS unit.
Tobacco rods do not readily perform this function due to tobacco loss and drying out effects
whilst filter rods provide a more stable option. The drying effects of the methods necessitate
the stabilisation of the rod moisture between measurements and so a set of well defined rod
samples provide the best method of gaining confidence in the calibration of the microwave
cell. The long term stability of the check rod sample means that this method can be
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employed as means of establishing drift and also of production line confidence building and
notifying non-technical users that calibration may be required.
Acknowledgements
The authors would like to thank Cerulean and Molins PLC for the support offered
throughout the preparation for this paper.
The authors would also like to thank Filtrona Filters for their help and advice in the
preparation of samples used in this study.
References
[1] Brenizer, J.S., Sulcoski, M.F. “Observations of Density Variations in Tobacco Rods by
Neutron radiography” Beitrage zur Tabakforschung International 14 (1987) 21-28
[2] Goldring , L.S., Fiore,J.V., Gadiziala, A.C. “X-ray Study of Cigarette Structure” Beitrage
zur Tabakforschung 7 (1973) 121 -137
[3] Herremann,R.,“New technique for moisture and density measurement in tobacco and
cigarettes” Tobacco Journal International 2 (1996) 29
[4] Herrmann.R, “A microwave moisture measurement system based on the resonator
method” Schulungsunterlagen Technische Akademie Esslingen Weiterbildungszentrum,
Lehrgang Nr. 18976/41.454 (Feuchtemessung Stoffen)
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