WO2011086527A1

WO2011086527A1 - Apparatus and method for automatic positioning cigarette paper and measuring its permeability

Info

Publication number: WO2011086527A1
Application number: PCT/IB2011/050181
Authority: WO
Inventors: Alessandro Piombini; Mauro Lucchesi; Marco Bianucci
Original assignee: Extrasolution S.R.L.
Priority date: 2010-01-14
Filing date: 2011-01-14
Publication date: 2011-07-21
Also published as: ITPI20100004A1; IT1397555B1

Abstract

The gas permeability of cigarette paper provided with portions or cross bands having, thanks to specific treatments such as impregnation with chemicals, painting, gluing films, or the like, specific chemical-physical properties, has to be measured. A machine for measuring the gas permeability of such portions or cross bands is provided with an apparatus for detecting such portions of cross bands and automatically placing them with a great accuracy in a permeability measuring station of the machine. The apparatus inspects the dielectric properties of the paper thanks to peculiar capacitive means and is able of a great sensitiveness and accuracy at placing the portions or bands in the permeability measuring station.

Description

[Title established by the ISA under Rule 37.2] APPARATUS AND METHOD FOR AUTOMATIC POSITIONING CIGARETTE PAPER AND MEASURING ITS PERMEABILITY

Field of the Invention

The present invention concerns a method, and a machine, for automatically measuring the gas permeability of cigarette paper.

In particular, the invention concerns a method, and a machine, for measuring the gas permeability of areas of cigarette paper in the form of a web, such areas having specific permeability characteristics, in which said areas are detected and automatically positioned in a measuring station of the machine

Description of the Prior Art

In the field of cigarettes production is more and more used cigarette paper comprising evenly distributed stripes or cross bands having specific chemical-physical properties. For instance, they are known papers comprising bands provided with an amount of small holes, as well as bands covered or impregnated with substances able to provide them with specific filtering or auto-extinguishing features or the like, and a number of patents have been filed which describe cigarette paper defined according to the position, the size and the chemical-physical properties of such stripes or bands.

In the cigarette paper production process they are used many kinds of treating machines both for manufacturing and for inspecting the produced paper. Among the operations which are commonly performed there are cutting, rewinding, perforating, performing chemical treatments, applying chemicals, gluing films or layers of paper sheets, measuring of thickness, width, or other chemical properties. Machines and methods for manufacturing and inspecting cigarette paper provided with cross bands are described for instance in US patents n. 4,845,374; n. 5,966,218; n. 6,020,969; n. 6,198,537 in which optical devices are used for detecting and inspecting, in a running paper web, cross bands having specific properties with regards to not treated areas of the paper web.

In the last period it is becoming widespread cigarette paper providing cross bands whose main feature is to have a specific permeability to specific gases, which is different from the gas permeability of different areas of the cigarette paper, for instance for giving the cigarette auto-extinguishing properties in specific sections of the cigarette. At this aim it is becoming more and more important the possibility of inspecting the gas permeability of the above cross bands in a rapid, precise and reliable way. They are known gas permeability measuring machines in which a web of cigarette paper is fed in a planar arrangement by two winder rollers towards a measuring station made of two measuring chambers where the paper is fed therebetween so that it keep the chambers separated; when a cross band to be inspected is at the measuring station the paper is stopped, the chambers pinch the paper, and two sample gas are supplied under pressure into the chambers so that by measuring the changes of pressure the permeability value of the paper's cross band can be calculated. An apparatus of the above type is described in US patent application n. 2005/087202. In this kind of machines the band to be inspected is manually positioned in the measuring station even if in the application it is mentioned that the positioning operation can be performed by means of optical detection means, or even by non-optical means such as ultrasound or near infrared spectroscopic means. The above mentioned positioning methods and apparatus are very expensive and unreliable. In particular, optical systems are not reliable when the treated cross bands have optical properties similar to the optical properties of not treated areas, which is very common in cigarette paper.

In the paper industry there also exist capacitive inspection devices but they are used for inspecting the thickness and/or the humidity of a running web of a paper, such as, for instance in Canadian patents n. 1277707; n. 1278355; or n. 1184268 or in US patent n. 4,610, 530.

Due to the above it is felt the need of exploiting a method and a machine for detecting and correctly positioning cigarette paper provided with cross bands having specific chemical-physical properties in a gas permeability measuring station of the machine for measuring the gas permeability of such cross bands, and, more in general, in machines for treating web material in which the cross bands have to be properly positioned within the machine

Summary of the Invention

It is object of the present invention to propose a rapid, automatic and reliable method for detecting specific areas in cigarette paper and measuring the gas permeability of such areas.

It is further object of the present invention to propose a machine for measuring the permeability of cigarette paper comprising means for detecting the areas to be measured and for automatically and properly positioning them in a measuring station of the machine.

The above objects are obtained by a method for measuring the permeability to a gas of cigarette paper provided with portions having specific chemical-physical properties, in which the method comprises steps of: feeding said cigarette paper through a detecting station in which occurs a step of surveying parameters of said cigarette paper that characterize the response of a surveying electric circuitry comprising at least a capacitor, between the conductors thereof being comprised said cigarette paper operating as the dielectric; verifying the presence of said portions performed by checking that the values of the response of said surveying electric circuitry to said parameters are comprised in a predefined range to which is attributed the presence of said portions between the conductors of said capacitor; adjusting the feeding movement of said cigarette paper according to the results of said verifying step for positioning said portions in a measuring station; measuring the gas permeability of said portions performed in said measuring station.

The detection of the above portions takes place thanks to the different electric or dielectric properties which they have with regard to the rest of the cigarette paper, and which are given them by the specific treatment made for modifying some characteristics of the paper such as the gas permeability. Such detection method is reliable and cost effective since the surveying circuitry contains only simple electric components. When the portions to be measured have been detected they can be properly and automatically positioned in the measuring station of the machine with savings in time and human work.

Advantageously the surveying circuitry is made so that it converts variations of the dielectric properties of the paper running through the capacitor in variations of the frequency of the current oscillating in the surveying circuitry. In this way the step of detecting parameters comprises measuring the frequency with which the electric current oscillates in the surveying circuitry comprising said at least one capacitor. Downstream, still in the surveying circuitry, the above frequency variations are converted into voltage variations which are easy to measure in specific section of the circuitry, so that the step of verifying the presence of said portions or cross bands between the conductors of the capacitor consists of verifying that the voltage is comprised within a specific range of values.

In particular, the surveying circuitry is designed so that said at least one capacitor is housed in a self-oscillating circuit so that the presence or lack of the above portions or bands between the conductors of the capacitor involves a capacitive variation and then a frequency variation of the self-oscillating circuit. The above frequency variations are then transformed in voltage variations so that the presence or lack of portions or bands in said capacitor can be checked by verifying the range of values in which is comprised the measured voltage.

In a specific embodiment the cigarette paper is in the form of a web provided with cross bands having specific chemical-physical properties which are able to decrease the permeability of the bands themselves to one or more specific gas.

The above objects are also obtained by a machine for measuring the gas permeability of cigarette paper provided with portions having specific chemical-physical properties comprising: a measuring stations comprising means for feeding a test gas to which the permeability of said portions of cigarette paper has to be measured and, in particular, comprising two measure chambers where said portions are placed between so that they separate said two measure chambers, in which the permeability to said gas test of said portion is measured by measuring the changes of concentration of said gas test which permeates from a measure chamber to the other measure chamber; a detection station in which is placed a detection apparatus comprising at least a capacitor housed in a surveying electric circuitry, said detection station being placed upstream said measuring station in a running path of said cigarette paper, so that said cigarette paper runs between the conductors of said at least a capacitor and it acts as the dielectric of said capacitor; feeding means for feeding said cigarette paper along said running path; and at least a processing unit for receiving inputs from said surveying electric circuitry and consequently controlling said feeding means for positioning said portion of cigarette paper into said measuring station.

The apparatus allow to automatic place specific portions of the sheet or web of paper in measuring stations of machines for measuring the permeability of cigarette paper. The surveying circuitry is very simple and reliable even when the detection performed through optical means would be very difficult, thanks to the fact that are used the dielectric properties of the portions of paper which have been subjected to specific treatments.

Advantageously the surveying circuitry also comprises at least a self-oscillating circuit comprising said at least a capacitor, and the self-oscillating circuit is comprised in a superheterodyne circuit. Thanks to the above circuits, which are composed of very cheap components, little variations of the dielectric properties of some portions of the paper can be easily detected in the form of frequency variations of the surveying circuitry.

Still advantageously, the surveying circuitry comprises at least a PLL (phase-locked loop) circuit connected so that it receives as an input the output signal of said superheterodyne circuit properly filtered, and it is used in a frequency to voltage converter configuration, so that the frequency variations at its input are converted into voltage variations and this last voltage values can be received and processed by the processing unit. Thanks to a very cheap component such as a PLL the processing unit is able to process voltage signals which are much simpler to manage with regard to frequency signals.

Preferably, the feed motor of the feed means is a stepper motor. With this kind of motor it is possible to adjust the movement of the paper very accurately in particular if a movement of the paper equal to the length of a portion to be detected corresponds to several steps of the motor, preferably at least 3 or 4 steps.

Still advantageously, in case the portions are periodic bands, the surveying circui comprises at least two capacitors in parallel spaced in the paper feed direction with a pitch which is equal to the pitch between the bands to be detected. In this way the difference between the dielectric properties of the bands and the dielectric properties of the rest of the paper is increased in proportion with the number of capacitors, so making easier the detection of the bands.

Advantageously, a first conductor of a capacitor is comprised in a first surveying plate integral to the structure of the machine, and a second conductor of the same capacitor is comprised in a second surveying plate that can be magnetically connected to the first surveying plate. The second plate, which needs electric connections for connecting to the rest of the surveying circuitry which is housed under the first plate, do not need to be wired or firmly fastened by screw means to the first plate, so that it can be easily removed by pulling it upwards.

Brief Description of the Drawings

These and more features and advantages of the present invention will be more easily comprehensible thanks to the following description of a preferred embodiment of the invention, given as a non limiting example, with reference to the accompanying drawings in which:

figure 1 shows a schematic side view of a machine for measuring the permeability of cigarette paper according to the present invention;
figure 2 shows a top view of the machine of fig. 1;
figure 3 shows a flow chart of a detecting apparatus for detecting portions or bands having specific chemical-physical properties, in cigarette paper, comprised in a machine according to the invention;
figure 4 shows a chart of the voltage received by the processing unit of the apparatus according to the flow chart of fig. 3, as a function of the movement, between the conductors of the capacitors of the apparatus, of cigarette paper having periodic cross bands provided with specific chemical-physical properties.

Detailed Description of Preferred Embodiments

With reference to figs. 1 and 2 it is labelled as a whole with M a machine for measuring the permeability of cigarette paper in which, by means of rollers, R, a web of cigarette paper, N, is fed in a planar configuration from a bobbin, B, of cigarette paper, to a measuring station, P, of a measuring head, T, of the machine.

The web of cigarette paper is provided with treated portions or cross bands, C, which in the present embodiment are cross bands having specific chemical-physical properties different form the rest of the web of cigarette paper N. In particular, for instance, the cross bands C have a gas permeability lower than the gas permeability of the rest of the web of paper for providing the cigarettes with sections having self-extinguishing properties. The specific permeability of the cross bands C can be obtained in many ways. For instance, it can be obtained when the paper is still in the form of a pulp, or it can be obtained by pouring or spraying fluids on the surface of the paper, by adsorption processes, by gluing films of specific materials, or still other ways.

The measuring station P is composed of two measure chambers where the cigarette paper is fed therebetween in a planar configuration so that the cigarette paper separates the chambers; when a cross band to be measured is properly placed in the measuring station the movement of the paper is stopped, the chambers clamp the paper, and a test gas, which is CO₂ or CO, and a carrier gas, usually nitrogen, are supplied in the chambers and the pressure in the chambers is kept substantially the same, so that, by measuring the variations of concentration of the test gas in the carrier gas it can be obtained the permeability of the paper to that test gas.

In particular, the permeability measuring station is composed of two metallic chambers with at least one of said chambers is movable for performing the above clamping action, and the sample of paper to be measured is interposed between the chambers so that when it is clamped it form a thin wall separating the chambers. The test gas and the carrier gas then flow in the upper and in the lower chamber, respectively. The carrier gas carries the test gas permeated from the upper chamber into the lower chamber towards a specific sensor which is able to detect it so allowing to calculate the permeability of the sample of paper.

In addiction, it is useful to underline that, in the above described measuring station, the shape and dimensions of the two chambers, together with the shape, size and layout of the holes for the passage of the gas, are decisive parameters for a proper working of the machine. In this view, when defining the specifications of the machine, the shape and dimensions of that components are calculated and simulated so that the test gas propagation rate in the two chambers in the direction perpendicular to the paper is much lower than the propagation rate of the carrier gas or the test gas in the direction parallel to the paper. The above feature allow to obtain that the concentration of the test gas in the inlet chamber (the upper chamber), is exactly the nominal value, while the concentration of the test gas in the outlet chamber (the lower chamber) is substantially zero (that is the test gas is completely carried away by the carrier gas), which is a mandatory condition in order to obtain a proper accuracy of the calculated permeability value.

In fact the value of permeability of the cross bands of the cigarette paper must be accurately measured in order to assure a proper working of the self-extinguishing sections that cause the cigarette to die out when the combustion reach the self-extinguishing section and the smoker is not inhaling from the filter of the cigarette. In this kind of cigarettes, the cross bands C usually have all the same length, few millimetres, and they are evenly spaced, so that they have to be accurately placed in the measuring station P of the measuring machine M.

A detecting apparatus, labelled as a whole with 100, is placed in a detecting station which comes between the bobbin B and the measuring station P and it detects the cross bands C and consequently controls the movement of the web of paper N in order to properly place the detected cross bands C in the measuring station P.

The apparatus 100 comprises a first surveying plate, 10, whose distance from the measuring head T can be accurately set, and a second surveying plate 20, magnetically coupled to the first surveying plate 10. Reference means and electric contact means are provided between the first 10 and the second 20 surveying plate for properly defining the mutual position of the surveying plates and for allowing the electric connection between the electric circuitry of the first surveying plate and the electric circuitry of the second surveying plate. The surveying plates coupled together form a passing through hollow space, having a calibrated height, through which passes the web of paper N. The surveying plated house a surveying circuitry, 30.

The surveying circuitry 30 of the detecting apparatus 100 is connected to a processing unit, 40, which in turns controls a stepper motor, 50 , which rotates one of the feeding rollers R for moving the web N along a feeding path towards the measuring station P.

The surveying plate 10 slides in the direction of the movement of the paper in order to adjust its distance from the measuring head T. At this aim, a supporting frame, 60, comprises a base, 61, where are mounted both the measuring head T and two supporting members, 62, 63, for supporting longitudinal rail members, 64, along which slides the surveying plate 10. The position of the surveying plate is adjusted thanks to a micrometer screw, 65, manually rotated through to a knob, 66, placed on the outer side of the supporting member 62.

The surveying circuitry 30 comprises two capacitors, 31, connected in parallel and spaced in the direction of movement of the paper with a pitch which is the pitch between the cross bands C. In particular, in the upper surface of the first plate 10 are housed two first conductors, 31b, of said capacitors, which are cross oriented with regard the direction of movement of the paper, and corresponding second conductors, 31a, are housed in the bottom surface of the second surveying plate, 20. The gap between the

conductors

31a and 31b is the height of the hollow space formed by the surveying

plates

10 and 20, and it is a little greater than the thickness of the web of paper N which acts as the dielectric of the capacitors.

In fig. 3 is shown a flow chart describing the way of working of the surveying circuitry and the processing unit which controls the feed motor. The capacitor 31 (or, according to the embodiment of figs. 1 and 2, the capacitors 31 in parallel) is part of an harmonic self-oscillating circuit, 32, of an LC type, which, for a proper gain, has to oscillates at high frequencies of the magnitude of tens of MHz (about 20 MHz in the present embodiment). The oscillator 32 is part of a superheterodine circuit, 33, whose further main components are a local quartz oscillator, 34, working at a frequency near the frequency of the LC self-oscillator (in the present embodiment 20 MHz), and a superheterodine mixer, 35, which receive as an input the signal of the

oscillators

32 and 34 and gives as an output two signals having frequencies which are the sum and the difference of the frequencies of the input signals.

The low frequency signal is then separated from the high frequency signal by a low pass filter and it is supplied to a PLL circuit (phase-locked loop), 37, which is calibrated for operating, in a proper range of frequencies which is much lower than the frequencies of the

oscillators

32 and 34, as a frequency to voltage converter. In fact, the voltage of the input filter of the phase ring of the PLL is sent to the processing unit 40 that, processing the received voltage values, determines if a cross band C or a different section of the web of paper N is passing between the

conductors

31a and 31b of the capacitors 31.

The processing unit 40, having determined the position of the cross bands C passed through the surveying circuitry 30 is able to control the stepper motor 50 for stopping the web paper when a cross band C is in the measuring station P. Once the band C is properly placed in the measuring station the control system of the machine enables the operation of permeability measurement.

The method of the present invention for detecting cross bands having specific chemical-physical properties in a web of paper and for automatically positioning it in a measuring station of the machine for measuring the permeability to specific gas, is particularly advantageous when it is used in a machine as above described.

The web of paper N provided with the portions or cross bands C which have been treated for giving them specific chemical-physical properties, is the dielectric of the capacitors 31. When, during the feeding movement of the web N, a cross band C (or any kind of portion of paper having different electric or dielectric properties) passes through the conductors 31, the value of the dielectric constant of the capacitors 31 changes. The above change causes a modification of the electric parameters of the surveying circuitry 30, and this last modification is used for detecting the presence of the cross bands C between the conductors of the capacitors 31. In particular, the modification of the dielectric constant causes a variation in the frequency of the current in the LC self-oscillating circuit 32in which the capacitor is definitely the external part of a capacity-voltage transducer able to detect capacitive changes less than 0,1%. It has been verified that in cigarettes with self-extinguishing bands the frequency variation in the self-oscillator 32 is about 50 KHz. Such a frequency variation, very low compared to the nominal frequency of the oscillator 32 that is about 20 MHz, would be very difficult to investigate, with reliable and cheap means, without the presence of the superheterodine circuit that houses the self-oscillator 32. The signal outputted by the superheterodine mixer, properly filtered by the low pass filter 36 has a frequency which is the difference between the frequency of the self-oscillator 32 and frequency of the local oscillator 34, that is, in the present embodiment, a frequency in a range from 50 to 500 KHz. In this way it is much easier to detect the variation of about 50 KHz since it corresponds to a variation from 10% to 100%. By using the PLL circuit 37, which, compared to different frequency to voltage converters has a wider frequency range and it is cheaper, the frequency variation is converted into a voltage variation which are then transmitted to the processing unit 40.

The processing unit 40 receives, moment after moment, the voltage values transmitted by the surveying circuitry 30, it processes them with a proper sample rate and it consequently controls, by the feeding motor 50, the feeding movement of the web of paper. With reference to fig. 4, once a first band C has passed through the capacitor 31, the processing unit 40 has stored a minimum voltage value and a maximum voltage value and it defines a voltage range I₀ and a voltage range I₁ around such minimum and maximum values, respectively. While the processing units keeps moving the web of paper, it check the presence of the cross bands C between the conductors of the capacitors 31 by comparing the voltage values received from the surveying circuitry 30 with the previously defined ranges I₀ and I₁. The width of the conductors of the capacitors 31 is lower than the length of a band C of an amount which is at least one step of the stepper motor 50 so that the maximum voltage value can be revealed for at least two steps of the stepper motor 50.

When the presence of a cross band C between the conductors of the capacitors 31 has been detected and verified, thanks to the fact that the distance from the detecting station and the measuring station is known, the processing unit is able to compute the number of steps of the stepper motor 50 needed to stop the band C exactly in correspondence with the measuring station P, and it consequently controls the stepper motor for correctly feeding the web of paper N towards the measuring station.

At every passage of a band C between the conductors of the capacitors 31, the processing unit 40 store the maximum voltage value and properly adjust the range around the maximum values defining, each time, a new range I₂, I₃ and so on, that will be the reference range for the detection of the following band C. In particular, in fig. 4 it is shown a chart of the voltage transmitted by the surveying circuitry 30 to the processing unit 40 during the feeding movement of the web paper N: the segments of constant maximum voltage represent the situation in which a band C completely covers the area of the capacitors, while the raising and descending ramps represent moments in which between the conductors of the capacitors 31 there are in part the bands C and in part the rest of the paper having a different dielectric constant.

As it would be cleat, the above described method allow a very accurate placement of the cross bands C in the measuring station P of the machine and it can be performed by the above described apparatus which is made with very simple, cheap and reliable components.

The features and advantages of the method and the machine above described remains obviously safe also in case that changes or modifications are carried out to the above described embodiment of the present invention.

As concerns the method of the invention the skilled person is obviously able to understand that the way of processing the electric parameters of the surveying circuitry 30 could be different. For instance, the surveying circuitry 30 could be of a different type so that of a different type would also be the signals transmitted to the processing unit 40, for instance signals with variation of the frequency. The processing unit 40 could be programmed with an automatic set-up function to be made when it starts working, for defining the reference ranges, as well as it could perform the calculation of the feeding commands to transmit to the motor 50 in a different way, for instance if it is not a stepper motor. The extent of the voltage ranges I could be either manually or automatically defined, for instance as a function of the difference between the maximum voltage value and the minimum voltage value, and, in the same way, other parameters could be manually or automatically defined.

Also to the above described machine according to the present invention various changes or modifications can be carried out, both as concerns its mechanical structure and in its electric and electronic components, as a function of the specific application. The surveying circuitry 30 is placed under the first plate 10 in order to have the lowest cable's length and consequently the lowest parasitic capacitances in parallel with the capacitors 31 which would limit the accuracy of the detecting apparatus 100. Nevertheless, except the capacitors 31, the other components of the surveying circuitry 30 could be housed outside the machine, for instance near the processing unit 40 or somewhere else. In the described embodiment two capacitors 31 are used in parallel in order to increase the sensitiveness of the surveying circuitry, and, at this aim, when it is possible and needed, a greater number of capacitors in parallel could be used arranged with a pitch which is equal to the pitch between the cross bands C to be revealed. Obviously, it is possible and useful when the cross bands C are evenly distributed all with a same pitch. Differently, when there is a different arrangement of the portions C there has to be used a single capacitor 31. The second surveying plate 20 is magnetically secured to the first surveying plate 10 in order to connect the different sections of the surveying circuitry. This feature is useful for allowing a fast and simple removal of the upper surveying plate 20, for inspection or for replacing the conductors of the capacitors 31. At this aim, an advantageous arrangement of the surveying circuitry 30 consists of providing a single conductor 31a in the first plate 10 even when in the second plate 20 are housed a greater number of conductors 31b. Obviously, in this case the area of the conductor 31a has to cover at least the overall plant dimensions of the upper conductors 31b. In fact, if the web of paper N is changed, and a change of the pitch and size of the cross bands C to be revealed, it has to be replaced only the cheap and simple printed circuit board comprising the conductors 31b in the upper plate 20 with a new one. Since the first plate 10 is mounted on longitudinal rails 64 the distance of the detecting apparatus 100 from the measuring station P can be finely adjusted, even if a more simple structure could be designed in which the detecting apparatus 100 is at a fixed distance from the measuring station P.

The surveying circuitry 30 is designed for transmitting to the processing unit 40 signals which are easy to process and it comprises simple and cheap components, but it could comprise further or different electric and/or electronic components suitable for transmitting to the processing units signals and electric parameters which change at the variation of the dielectric means which is between the conductors of the capacitors 31.

These and more changes or modifications could be carried out to the method and machine of the present invention still remaining within the ambit of protection defined by the following claims

Claims

Method for measuring the permeability to a gas of cigarette paper provided with portions (C) having specific chemical-physical properties, characterized in that it comprises steps of:

feeding said cigarette (N) paper through a detecting station (100) in which occurs a step of surveying parameters of said cigarette paper (N) that characterize the response of a surveying electric circuitry (30) comprising at least a capacitor (31), between the conductors thereof being comprised said cigarette paper (N) operating as the dielectric;

verifying the presence of said portions (C) in said detecting station (!00) performed by checking that the values of the response of said surveying electric circuitry (30) to said parameters are comprised in a predefined range of values to which is attributed the presence of said portions (C) between the conductors of said capacitor (31);

adjusting the feeding movement of said cigarette paper (N) according to the results of said verifying step for positioning said portions (C) in a permeability measuring station (P);

measuring the gas permeability of said portions (C) performed in said measuring station (P).
Method according to claim 1 characterized in that said step of surveying parameters comprises surveying the frequency changes of the electric current in said surveying circuitry (30).
Method according to claim 1 or 2 characterized in that said step of surveying parameters comprises detecting the voltage between specific points of said surveying circuitry (30) and said step of verifying the presence of said portions (C) is carried out by checking that said voltage is comprised within a predefined range.
Method according to the previous claim characterized in that said step of surveying parameters is obtained by housing said capacitor (31) in a self-oscillating circuit (32) so that said portions (C) going through the conductors of said at least one capacitor (31) corresponds to frequency changes in said self-oscillating circuit (32).
Method according to the previous claim characterized in that said frequency changes are properly transformed into voltage changes and the values of said voltage are compared with at least a range of voltage values to which is attributed the presence of said portions (C) between the conductors of said capacitor (31).
Machine (M) for measuring the gas permeability of cigarette paper provided with portions (C) having specific chemical-physical properties characterized in that it comprises:

- a measuring station (P) comprising means for feeding a test gas to which the permeability of said portions (C) of cigarette paper has to be measured;

- a detection station in which is placed a detection apparatus (100) comprising at least a capacitor (31) housed in a surveying electric circuitry (30), said detection station being placed upstream said measuring station (P) in a running path of said cigarette paper, so that said cigarette paper runs between the conductors (31a, 31b) of said at least a capacitor (31) and it acts as the dielectric of said capacitor (31);

- feeding means (R, 50) for feeding said cigarette paper along said running path; and

- at least a processing unit (40) for receiving inputs from said surveying electric circuitry (30) and consequently controlling said feeding means (R, 50) for positioning said portions (C) of cigarette paper into said measuring station (P).
Machine (M) for measuring the gas permeability according to the previous claim characterized in that said measuring station (P) comprises two measure chambers where said portions (C) are placed between so that they separate said two measure chambers, in which the permeability to said test gas of said portions (C) is measured by measuring the changes of concentration of said test gas which permeates from a measure chamber to the other measure chamber.
Machine (M) according to claim 6 or 7 characterized in that said surveying circuitry (30) comprises at least a self-oscillating circuit (32) comprising said at least a capacitor (31).
Machine (M) according to the previous claim characterized in that said self-oscillating circuit (32) is comprised in a superheterodine circuit (33).
Machine (M) according to the previous claim characterized in that said detecting apparatus 100 comprises at least a PLL circuit (37) connected so that it receives as an input the output signal of said superheterodine circuit (33) properly filtered, and it is used in a frequency to voltage converter configuration, the values of said voltage being received and processed by said processing unit (40).
Machine (M) according to claim 6 or followings characterized in that said feeding means (R, 50) comprises at least a stepper motor (50).
Machine (M) according to claim 6 or followings characterized in that it comprises a plurality of capacitors (31) in parallel spaced in the feed direction of said cigarette paper (N).
Machine (M) according to claim 6 or followings characterized in that a first conductor (31a) of said at least a capacitor (31) is comprised in a first surveying plate (10) integral to the frame of said machine (M), a second conductor (31b) of said at least a capacitor (31) being comprised in a second surveying plate (20) that can be magnetically connected to said first surveying plate (10).