Permeability monitoring of sheet materials

A device or method for monitoring the gas-permeability of a sheet material is disclosed. The device comprises a first gas-flow chamber having an inlet, which, during operation of the device is closed by the sheet material extending across the inlet. A second flow chamber has an outlet by which a gaseous medium can be directed towards the sheet material with a leakage path whereby a significantly greater proportion, suitably 90%, of the gas-flow in the second flow chamber is permitted to leak to the atmosphere. This leakage path is provided by spacing the gas outlet of the second chamber from the position in which the sheet material extends across the gas inlet of the first chamber. A measurement means, preferably a pressure transducer is connected across a low-impedance laminar-flow device.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention concerns improvements relating to the monitoring or 
determination of the permeability of sheet materials, particularly but not 
exclusively webs of paper used in the tobacco industry. 
2. Brief Description of the Prior Art 
In United Kingdom Patent Specification No. 1,239,408, it has been proposed 
that, in the manufacturing of cigarettes, the permeability of a web of 
cigarette paper should be monitored continuously for the purpose of 
identifying any portions of the web which are unsatisfactory. A device 
disclosed in that specification for monitoring paper web comprises a 
suction chamber disposed in contact with one side of the web and a second 
chamber disposed in contact with the other side thereof. A bleed hole 
communicates with the second chamber, whereby ambient air may flow into 
the second chamber when a partial vacuum is established in the suction 
chamber. Pressure indicating means is operative to measure the pressure in 
the second chamber, the pressure therein varying in accordance with 
variations in the permeability of the web which is conveyed between the 
two chambers. The use of the partial vacuum is disadvantageous in that 
paper dust and other debris are likely to be sucked into and cause 
malfunctioning of the device. The monitoring device of the aforesaid 
Specification also has the disadvantage that both chambers have to be 
maintained in contact with respective sides of the web in order to produce 
necessary hermetic seals therewith. 
To our knowledge, there is no device currently available which adequately 
meets the practical requirements for continuous on-line monitoring of 
paper webs of the tobacco industry, such webs including, for example, webs 
of tipping, filter-plug wrapping and cigarette wrapping material. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a monitoring device 
which does meet the above-mentioned requirements. 
The invention provides a device for monitoring the gas permeability of a 
sheet material comprising a first flow chamber having an inlet which, 
during operation of the device, is closed by the sheet material extending 
across said inlet, gas-measurement means operable to measure gas flow or 
gas pressure in said first flow chamber, and a second flow chamber having 
an outlet whereby a gaseous medium, suitably air, may be directed towards 
said sheet material, there being provided a gas-leakage path whereby a 
significantly greater proportion of the gas flow in said second flow 
chamber is permitted to leak to atmosphere than that proportion which 
passes through said sheet material into said first flow chamber. 
The gas leakage path is preferably provided by the simple expedient of 
spacing the outlet of the second flow chamber from the position occupied 
by sheet material when the sheet material extends across the inlet of the 
first flow chamber to close the inlet. 
The present invention further provides a method of monitoring the 
permeability of sheet material, wherein the sheet material is placed 
across an inlet of a first flow chamber to close said inlet, a gas flow is 
directed towards said sheet material from a second flow chamber, there 
being provided a gas-leakage path from said second flow chamber, whereby a 
significantly greater proportion of the gas flow in said second flow 
chamber is permitted to leak to atmosphere than that proportion which 
passes through said sheet material into said first flow chamber, and the 
gas flow or gas pressure in said second flow chamber is measured. 
When paper webs of the tobacco industry are to be monitored, the pressure 
in the first flow chamber should be very low, for example it should be 
maintained at a value of not more than about 5 mm W.G. Conveniently the 
pressure in the first flow chamber is maintained at a value in the region 
of 1 mm W.G., in which case the pressure in the second flow chamber can be 
expected to be about 8 mm W.G. Suitably the proportion of the flow of gas, 
air for example, which passes through the leakage path is at least 90% of 
the gas flwo entering the second flow chamber.

DETAILED DESCRIPTION OF THE INVENTION 
The permeability monitoring device of FIG. 1 comprises a first flow chamber 
1 of frusto-conical form and a second flow chamber 2 of similar 
frusto-conical form. The larger-diameter open ends of the chambers 1 and 2 
are in opposed relationship, but are somewhat spaced apart from each 
other. The spacing may be of the order of 0.4 mm. Mounted above and below 
the chambers 1 and 2 are guide rollers 3 and 4 about which, in use of the 
device, a paper web 5, tipping paper for example is trained. The rollers 3 
and 4 are so disposed in relation to the inlet end of the chamber 1 that a 
run of the web 5 is held securely across the inlet of the chamber, thus 
closing the inlet. The edges of the inlet may be smoothly rounded, as 
shown, to enhance the closure and facilitate free movement of the web. The 
web 5 extends from a supply bobbin (not shown) and is caused to be 
conveyed along the path indicated by arrow 5' in FIG. 1 by drive means 
(not shown). 
The flow chamber 2 communicates via a duct 6 with a source 7 of air under 
pressure, from which air is supplied to chamber 2 at a constant flow rate. 
The flow chamber 1 communicates at its downstream end with a 
multi-capillary laminar-flow element 8. The element 8 comprises four 
capillary tubes, of which two appear in FIG. 1. For the purpose of 
measuring the pressure drop across the element 8, a pressure transducer 9 
is connected thereacross. The element 8 possesses a linear 
pressure-drop/flow characteristic and thue measurement of the pressure 
drop by the transducer 9 provides also a measure of the air flow through 
the flow chamber 1. 
In operation of the device, air at a constant low pressure, 8 mm W.G. for 
example, is supplied to the flow chamber 2 from the source 7, 
Approximately 95% of the air supplied to the chamber 2 passes therefrom 
through the annular gap 2' between the opposed ends of the chamber 1, 
covered by the web 5, and the chamber 2, which gap provides a leakage 
path. The remainder of the air flow passes through the paper web 5 into 
the flow chamber 1, the flow rate in the latter being measured by means of 
the laminar flow element 8 and pressure transducer 9. 
If the paper web 5 is tipping paper, then preferably the web is fed through 
the permeability monitoring device with the side of the web which is 
intended to be at the outer side of the cigarette tippings disposed 
towards the flow chamber 2. In this way it is ensured that the air flow 
through the web 5 induced by the monitoring device is in the same 
direction as the air flow through the tippings when the cigarettes are 
smoked. 
It will be appreciated that the permeability monitoring device shown 
diagrammatically in FIG. 1 possesses the merit of avoiding the use of 
dust-attracting partial vacuums and of being self-cleaning. Also rubbing 
contact is avoided at that side of the tipping web which is to be at the 
outside of the cigarette tippings. 
It is advantageous to maintain a very low air pressure regime in the 
monitoring device, as higher pressures would result in uneconomical waste 
of air from the leakage path. Furthermore, as the web 5 is moving through 
the monitoring device at a significant speed, for example 140 m per minute 
with tipping paper, there will be a tendency for the web to entrain air 
from within the chamber 1 and, if the pressure in the chamber 1 was too 
high, for the web 5 to lift away from the mouth of the chamber 1 in the 
region at which the web 5 passes from the monitoring device. This could 
produce a leakage path from the chamber 1, resulting in readings from the 
pressure transducer 9 which were no longer accurately indicative of the 
permeability of the web 5. 
Since the pressure in the flow chamber 1 is of a low value, it is necessary 
that the means employed to measure the air flow in the chamber 1 should be 
of adequately low impedance. Flow-measuring means of low impedance other 
than a laminar flow device, a thermal-effect flow-measuring device for 
example, could be utilized. 
In FIG. 2 there is shown an inlet-end view of a form of first flow chamber 
which is suitable for use when the web being monitored, for instance a 
tipping paper web, has two longitudinal lines of ventilation perforations. 
Such a web 10 is indicated by broken lines, the two lines of perforations 
being indicated by chain lines 11. As may be seen from FIG. 2, the 
modified form of flow chamber comprises a central partition 12. By virtue 
of the presence of the partition 12, there are in effect two separate flow 
chambers 1' and 1". With this arrangement, two flow-measuring devices, 
(each similar to the device 8, 9), one in association with each of the 
flow chambers 1', 1". Thus, if there occurs a malfunction of on-line 
perforation devices producing the lines 11 of perforations, this may be 
detected and a feedback signal to the malfunctioning perforation device 
provided.