SENSOR SYSTEM

Field of the Invention

The present invention relates to a sensor system for measuring the concentration of a gaseous sterilization agent in a sterilization process utilized in production of packages. The present invention also relates to a device for sterilization of packages, comprising such sensor system.

Technical Background

One device for sterilization of containers 8 is shown in Fig. 1 , (from SE0203692-9). The device 1 typically consists of a housing divided into zones. In a first zone, the heating zone 2, hot air is introduced for heating the packages. In a second zone, the sterilization zone 3, gaseous hydrogen-peroxide is introduced for sterilizing the heated containers 8. The hydrogen-peroxide is injected through a nozzle 13. In the third zone, the venting zone 4, hot sterile air is introduced for venting the containers 8, and in the final zone, the filling zone 5, the packages 8 are filled with sterile content. The containers 8 are conveyed through the device 1 arranged on a conveyor belt 10. In Fig. 1 the containers stand on their closed top end 11 with their open bottom end 12 facing upwards. The gaseous hydrogen-peroxide is generally supplied in liquid form to a gasifier (not shown), in which it is mixed with a heated stream of sterilized air. This stream of sterilizing agent and air is injected into the separate containers 8 in the sterilization zone 3. The degree of sterilization is determined by such factors as what concentration of hydrogen-peroxide the individual packages 8 are exposed to, exposure time, temperature etc. Too low concentrations of hydrogen-peroxide will result in an insufficient degree of sterilization, and too high concentrations can lead to traces of hydrogen-peroxide in the filled packages 8, which is not acceptable. Further, too high concentrations are economically unfavourable. In an alternative, related sterilization method the sterilizing agent is allowed to condense on the surfaces of the packages, and is thereafter evaporated.

Another device for sterilization of packages is shown in Fig. 2. In this device 21 a web 24 of package material is directed, by means of directing rollers 25, 27,

through a bath 29 containing a liquid hydrogen-peroxide solution. After leaving the bath 29 the web passes between a two squeegee rollers 31 , distributing the hydrogen- peroxide evenly over the web 24. The web 24 then enters a heating chamber 37 through an entrance opening 39, is directed through the heating chamber 37 by means of directing rollers 33, 35, and exits the chamber 37 same through an exit opening 41. The heating chamber 37 comprises a heating arrangement, schematically outlined by the dash-line box 43, for effecting evaporation of the hydrogen-peroxide on the web. The rate of vaporization obviously depends on, e.g., heating temperature and ambient concentration of hydrogen peroxide. If the concentration of hydrogen peroxide is too low in the heating chamber, the hydrogen peroxide will vaporize too quickly resulting in a risk of insufficient sterilization. On the other hand, if the concentration is too high the risk of unallowable residues of hydrogen peroxide remaining on the web as it leaves the heating chamber increases. Also shown in Fig. 2 is the main catalyst 45, which effects decomposition of hydrogen-peroxide prior to discharge into the ambient air. It is apparent that in any of the above cases an accurate measurement of the sterilizing-agent concentration is crucial. If the concentration measurement method is crude, it results in that too much sterilizing agent is used, since a certain minimum level of sterilization needs to be ensured. This in turn leads to longer vaporization times, which slows down the production rate. Traditionally, concentration measurements are performed in one of two ways:

• Indirectly, e.g., by means of measuring the mass flow or volume flow of liquid sterilization agent combined with the mass flow or volume flow of heated air, as well as parameters such as temperature and pressure. The advantage of the indirect method is that it generally is cost efficient and accurate, at least over longer periods of time. Examples of drawbacks are that it is quite slow in terms of response to changes, since it measures mean consumption of sterilization agent and air over time, which results in the use of non-optimized amounts of sterilization agent. Further, key information for efficient process control is related to information concerning each individual package. Such information would ensure safe handling, traceability and minimum wastage. This key information is not obtainable with said indirect method.

• Directly, e.g., by arranging concentration sensors in specific locations. Some existing concentration sensors used thus far have proven to be accurate, but are generally, e.g., in the case of IR-sensors, too fragile, too bulky and overly expensive for the intended use.

Further, the environment where the sensor needs to be located is harsh, since hydrogen peroxide is corrosive, so the first criterion is that the sensor material can withstand this harsh environment. The second criterion is that the sensor material should not degrade the unstable hydrogen peroxide. A sensor generally interacts with a small fraction of the total volume of sterilizing agent, which is why the second criterion is not as strict as the first.

By sterilization is meant removal or killing of microorganisms to a certain, application-specific extent. Depending on the application different levels of sterilization can be chosen to comply with the desired shelf life of the packaged product.

As used herein, sterilization treatment is a generic term for operations performed in connection to sterilization of an object, e.g., one of the processes described above, gassing for exposure of the sterilization agent, followed by evaporation and or venting for removal of sterilization agent after exposure. Summary of the Invention

An object of the present invention is to provide a sensor system which, when used sterilization treatment, eliminates or at least alleviates drawbacks present in prior art. Another object of the present invention is to provide a sterilization device, which allows for improved process control by using the inventive sensor system. These purposes are achieved with a system in accordance with claim 1, and a device using said system. Preferred embodiments are disclosed in the dependent claims.

The invention thus relates to a sensor system for measurement of a concentration of a sterilizing agent in a sterilizing device utilized in production of packages. The system comprises a sensor, adapted for arrangement in a volume of sterilization gas for measurement of concentration of the same, an electronic processing circuit, for evaluation of an output from the sensor, and an I/O-unit for communication with process-control systems. The inventive sensor system further comprises a high voltage power supply for operation of the sensor, a first electrode, which together with a second electrode arranged at a distance from the first forms a corona gap. The system utilizes measurement of electrical properties of this gap, when a voltage is applied over the gap and a corona current is flowing between the electrodes, to determine the concentration of sterilizing agent in the gap.

The inventive sensor system provides an on-line concentration measurement, which is simple, affordable and reliable. The simplicity vouches for a rugged sensor

system. The affordable cost makes it possible to arrange, if considered necessary, several sensors in a single sterilization device without affecting the total cost of the sterilization device significantly. The reliability and accuracy of the measurements ensures the possibility of minimizing the consumption of sterilization agent while securing a predetermined degree of sterilization. The sensor system enables monitoring of sterilization degree for individual containers. Containers deviating in some way (related to their exposure of sterilization agent) can be individually marked and discarded or followed-up.

In one or more embodiments the system may comprise a temperature sensor arranged in the same volume of sterilization gas. The use of a temperature sensor enables correlation between sensor output and temperature in the ambient gas. Since the output is somewhat temperature dependent the use of a temperature sensor is necessary in order to accurately deduce absolute concentrations of the sterilizing agent.

The first electrode may also have a lower electric potential than the second electrode. This relation is referred to as a "negative potential". The use of a negative potential for the corona gap has proven to result in an improved discrimination between water and sterilizing agent, which both enables an improved measurement of sterilization agent and the possibility of distinguishing between water and sterilization agent. In one or more embodiments the system may be adapted to ramp the voltage applied over the corona gap and to evaluate the resulting electric properties of the corona gap as a function of voltage for deduction of condensation and/or the status of the sensor. Condensation of sterilization agent is a key parameter in many sterilization processes. Using the inventive sensor system for this measurement makes it possible to determine the occurrence of condensation (e.g. the actual dew point) with high accuracy and at a high rate. Ramping of the voltage also makes it possible to check the status of the corona gap, and thereby to automatically send status information to the control system. In this way service of the sensor system may be tailor-made to an individual application, reducing service costs. In one or more embodiment the first electrode has the form of a sharp tip and the second electrode provides a protective structure, preferably in the form of a generally U-shaped loop or a dome. Using surrounding structure protecting the electrode as an earth point reduces the number of components in the sensor. Also, less material around the electrode increases the flow of gas around the electrode, which improves the accuracy as well as not affecting the stream of sterilization gas

significantly. For basically the same reason the use of a U-shaped loop also decreases the build-up of chemical stabilizers, present in the sterilization agent, onto the surrounding structure. The second electrode could also have the form of an L (compare with the electrode of an ordinary spark plug. In one or more embodiments the first electrode, the second electrode, and, when present, the temperature sensor may be led through a common isolator, preferably constructed from a ceramic material. The use of a common isolator enables production of a straightforward and rugged sensor. The use of a ceramic material is appropriate from several aspects. For instance, ceramic is a good electric isolator, is durable, and complies with high hygienic standards. Other potential materials include glass, polymers etc.

In one embodiment of the invention a device for sterilization treatment of packages is provided, said device comprising an inventive sensor system. The device for sterilization treatment of packages may further comprise means to correlate a certain package, a certain group of packages, or a portion of a package-material web to the concentration of sterilization agent applied to that particular package, group of packages or portion of package-material web. This follow-up feature implements a beneficial feature of the inventive sensor system. It makes the sterilization process more time efficient, more cost efficient and more reliable. Examples of follow-ups include, but is not limited to, physical marking of a container or group of container, logging a concentration measurement to a predetermined container identification, and so forth. Generally speaking, a sterilization device using the inventive system may benefit from all the advantages related to the inventive sensor system.

Brief Description of the Drawings

Fig. 1 is a perspective, partly cut away, view showing a first container- sterilizing device.

Fig. 2 is sectional side view showing a second container-sterilizing device.

The devices of Figs. 1 and 2 have been presented earlier. Fig. 3 is an explanatory sketch concerning the principle of corona discharge.

Fig. 4 is a circuit diagram illustrating the principle of corona current measurement.

Fig. 5 is a graph showing experimental results.

Fig. 6 is a graph of corona current as a function of corona potential, with and without condensation on the corona electrode.

Fig.7 is a perspective, sectional view of a concentration sensor used in one embodiment of the invention, arranged in a duct. Fig. 8 is a perspective view of a sensor system according to a second embodiment of the present invention.

Fig. 9 is a side view of the sensor system of Fig. 8.

Detailed Description of Preferred Embodiments Before focussing of the present sensor system, a brief discussion of some theoretical aspects of corona-current measurements follows, referring to Fig. 3.

Fig. 3 is an explanatory sketch illustrating the principle of corona discharge. A first electrode 101 is arranged at a distance from a second electrode 102 connected to an earth potential, together forming a corona gap. A positive, in this example, potential of several kV is applied to the electrode, resulting in an electrical field, illustrated by field lines 110. This will result in that free electrons 111 start to move in the applied electrical field, which results in an electrical current or corona current. As the electrons approach the electrode 101 they will accelerate to such extent that collisions with neutral molecules 112 will create more free electrons, and so forth. This avalanche effect will amplify the corona current.

The driving force of the above process is the relative difference in electric potential between the electrodes 101, 102, and the use of an earth potential for the second electrode 102 in the example(s) above and below should not be construed in a limiting sense. The term "positive potential" refers to the above situation, where the first electrode 101 has a higher electrical potential than the second electrode 102, though they both could be negative, positive or one of each, within the definition.

If ions, particularly electronegative ions 113, are present in the corona gap they will absorb colliding electrons 111, thus cutting of a number of avalanche branches and cause a markedly, measurable, decrease in the corona current. A voltage applied over the corona gap results in a corona current through the corona gap, which makes it possible to attribute the gap, a resistance, conductance and other electric properties. An experimental setup is explained in the following, referring to Fig. 4.

The electrode, a pointed metal tip 101, is surrounded by an open metal cage 102. The cage 102 is designed to allow gas to pass freely through it, and also to protect the pointed tip 101 from mechanical damage. The tip 101 is electrically insulated from its surroundings, including the metal cage 102, preferably by a non-porous ceramic insulator 103. The cage 102 will generally be connected to earth potential via electrical wiring 200. The assembly is located in a volume of gas 104 (such as humid air), for which it is desired to know the hydrogen-peroxide vapour concentration.

A high DC voltage (typically 3-8 kV), generated by a stabilised supply 202, is applied to the tip 101 via electrical wiring 201 and a resistor 105. The voltage is sufficient to cause a steady corona discharge 106 to occur at the free end of the metal tip 101. The magnitude of the corona discharge 106 is deduced by measuring the electrical current flowing through it, which in turn may be calculated from the voltage drop 107 occurring across the resistor 105. The magnitude of the current, and hence the voltage drop 107, is observed to decrease in approximately inverse proportion to the concentration of the hydrogen peroxide. There are several techniques for measuring the electrical properties (corresponding to corona current, corona conductance etc) of components (such as a corona gap) in high- voltage circuits, none of which will be discussed in detail here.

Relating to the theoretical discussion above, hydrogen peroxide gas is electronegative, which means that the molecules have a tendency of capturing free electrons. When introduced in the corona gap the hydrogen peroxide molecules will act as electron scavengers, capturing the free electrons creating the current through the corona gap. Since the molecules have such a large mass compared to the electrons they will effectively reduce the current. This reduction in current (or increase in resistance) is measurable and proportional to the concentration of hydrogen peroxide in the gap.

During the breakdown of hydrogen peroxide hydroxyl radicals are formed, and they are even stronger electron scavengers. Consequently, both of these species will effectively immobilize the electrons and thus reduce the corona current.

In practical applications related to sterilization of packages, hydrogen peroxide mixed with water and stabilisers are generally used as a hydrogen peroxide source. This means that there will be water molecules present in the corona gap as well. During the investigations related to the present invention, this presence was compensated for in the evaluation of the signal from the sensor.

Fig. 5 is a graph showing an actual variation in hydrogen-peroxide concentration over time as a white line superimposed overthe concentration evaluated

from measuring the corona current, which is drawn as a full line. It is evident from the graph that the excellent correlation between corona resistance (or current) and hydrogen-peroxide concentration makes it possible to use corona current as a measure of hydrogen-peroxide concentration. The lines surrounding the measurement curves correspond to intervals +/- 10 % relative to the actual variation.

During sterilization with a gaseous sterilizing agent, the dew point is of interest. In some cases it is desirable to avoid condensation of sterilizing agent, and in other cases still, condensation is a desired result. In any case, a reliable measure of the occurrence of condensation is desirable. Fig. 6 illustrates schematically how the presence of condensation can be established with the present sensor system by a correlation between corona voltage/potential and corona current. In the graph of Fig. 6, two curves are shown. The first curve (full line), illustrates the appearance of the curve without condensation. During ramping of the corona potential from zero and upwards the corona current will be zero until a certain threshold potential is reached. This corresponds to the potential where significant electron-generating avalanches start to occur. After this threshold the corona current increases proportional to the corona potential. The second curve (dashed line), illustrates the appearance of the curve when condensation is present on the electrode. This will result in leakage currents, unambiguously shown in the resulting graph as a clear increase in (false) corona current below the threshold potential. The ramping of the potential is readily performed.

Another example of the versatility of the sensor system is that its performance can be monitored continuously or at fixed intervals. This is possible by, e.g., monitor the threshold potential on a day-to-day basis. As the electrode wears down, the threshold potential will change, making it possible to schedule service or replacements without dismounting the sensor. Another way of monitoring the performance is to ramp the voltage, as in the dew point measurement or from a negative to a positive potential.

The concentration of sterilizing agent is likely to vary slowly or not at all during a sterilization process. A sudden abrupt peak in the corona current indicates that a physical object has hit the concentration sensor. Since physical objects are undesirable in a sterilization process, this information can be important for the process control. One embodiment of the sensor 300 utilized in the sensor system will be explained in the following referring to Fig. 7, where the sensor 300 is arranged in a duct 310, in which the hydrogen-peroxide concentration is to be measured.

The sensor 300 comprises a first electrode 301, in the form of a sharp metal tip, and a second electrode 302 in the form of a U-shaped loop 303. This loop 303 also

protects the first electrode 301 during handling and use. The first electrode 301 and the U-shaped loop 302 are positioned in an isolator 303, made of an electrically insulating material. When used for the intended purpose in a sterilizing device the isolator 303 has to be made of a material fulfilling existing standards, e.g., concerning hygiene related to food handling. Examples of classes of materials where approved candidates can be found are ceramics, glass, and polymers. In the illustrated example the isolator 303 comprise a ceramic material. The ceramic isolator 303 is in turn arranged in a sleeve 311, preferably made of stainless steel. The sensor may also comprise a temperature sensor 312, in the form of a thermocouple. An accurate measure of temperature is needed for an accurate measure of concentration. An in-situ measurement is therefore preferred. The sleeve 311 is fitted into an opening of the duct 310 and a sealing member (not shown) may be arranged in a peripheral groove 313 so as to seal the opening when a flange of the sleeve 311 is pressed towards the duct 310 by means of a fastening collar 314, which is screwed to the duct 310. There are a limited number of materials that are theoretically adapted for the active parts of the sensor (generally the electrodes 301, 302). The materials should not be degraded by the hydrogen-peroxide and not degrade the hydrogen peroxide. Examples include stainless steel, and aluminium, though several others are possible. In practice, a suitable sensor response to the measured substance may be more important than the second, or even than the first, criterion. Also, the electrode may comprise several materials, such as a core of one material coated with another material.

In practical application the sensor system is arranged in sterilization device. In the case of a device 1 in accordance to Fig. 1, where sterilization agent is sprayed into individual containers, the sensor is generally located in a duct leading the sterilization agent to the spray orifice 13. In the case of a device 21 in accordance to Fig. 2, with a sterilization bath 29 and a heating chamber 37, the sensor is generally located in a representative position in the heating chamber 37. During concentration measurements the sensor 300 is powered by a high- voltage supply with variable output. The signal from the sensor is processed in a microprocessor, where the relevant electrical properties of the corona gap are deduced. The microprocessor is in communication with a process control unit, in which it is decided what actions to take as a result of the concentration measurement. These actions include, but are not limited to, stopping the process, increasing the amount of sterilization agent, decreasing the amount of sterilization agent, marking up insufficiently or overly sterilized containers, etc.

In the embodiment illustrated in Fig. 8 the first, inner, electrode 401 has the form of a linear conductor and the second, outer, electrode 402 has the form of a tubular shell. Opening 420 are arranged for facilitating entrainment to the volume between the electrodes. One beneficial feature of this embodiment is that it has rotational symmetry, and a large volume in which charges may be formed. Also, there will be a comparatively large volume having the same electromagnetic field potential, all along the circumference and length of the inner electrode 401 as oppose to only in the area around the tip 301, as illustrated in previous drawings. This results in a stronger signal from the detector. Further, while the embodiments described earlier is sensitive for disturbances affecting the geometry of the sharp tip, which may be beneficial in some instances, the present embodiment is not as sensitive, and as such it offers a more rigid measurement system, which may have a longer worklife between sendees. In Fig. 9 the embodiment of Fig. 8 is illustrated in a side view. It is shown how an insulator 403 extends up to a point just below the upper part of the openings 420, in order to increase the symmetry of the active measurement region of the system. The relation between the radius of the outside of the inner electrode 401 and the inside of the outer electrode 402 have to be dimensioned such that breakthrough is preλ^ented, which is a straightforward task for a skilled person being posed to the problem.

The reference numbers cited in the appended claims are present for an explanatory purpose, and should not be construed as limiting for the scope of the claims.