Gas concentration measurement with temperature compensation

Abstract

The invention relates to a method for measuring the concentration of a gas component in an atmosphere of a packaging which is made from a plastic film and which comprises a gas concentration indicator substance on the side of the plastic film facing the atmosphere.

Claims

1. A method for measuring a concentration of a gas component in an atmosphere of a package, the package being produced from a plastic film in a packaging machine, the package has a gas concentration indicator substance on a side of the plastic film facing the atmosphere, wherein the gas concentration indicator substance is exposed to an electromagnetic radiation of a wavelength, the gas concentration indicator substance is excited with a plurality of pulses, which then emits light at a different wavelength than the wavelength of the electromagnetic radiation, and the concentration of the gas is determined on a basis of a decay curve of the emitted light, and wherein at least one temperature correction parameter is determined for a category of the plastic film that is made of a particular material, the determined at least one temperature correction parameter is stored in a memory of a computer of the packaging machine, and the determined at least one temperature correction parameter is retrieved from the memory when the plastic film is replaced in the packaging machine, the determined at least one temperature correction parameter that is stored in the memory includes a temperature measurement of the plastic film, and the temperature measurement of the plastic film that is stored in the memory and then retrieved from the memory is utilized in determining the concentration of the gas component, wherein the method comprises: i) generating a reference package; ii) measuring and recording one or more temperature parameters of the reference package during cooling of the reference package to determine a cooling curve of the reference package; and iii) determining the at least one temperature correction parameter from the cooling curve of the reference package.

2. The method according to claim 1, wherein a temperature of the plastic film to which the gas concentration indicator substance is attached is measured.

3. The method according to claim 1, wherein the at least one temperature correction parameter is performed on a basis of a false light model for determining the gas concentration.

4. The method according to claim 1, wherein the at least one temperature correction parameter is determined for a certain gas concentration indicator substance.

5. The method according to claim 1, wherein a certain temperature correction parameter is determined on the packaging plastic film currently in use.

6. The method according to claim 1, wherein the gas concentration indicator substance is excited with a green light and emits a red light.

7. The method according to claim 1, wherein fluorophor in the gas concentration indicator substance emits a red light, and an intensity of the red light decays over time.

8. The method according to claim 1, wherein the category of the film is polyethylene plastic film, polypropylene film, or an amorphous polyethylene terephthalate film.

9. A method for measuring a concentration of a gas component in an atmosphere of a package, which is produced from a plastic film in a packaging machine and has a gas concentration indicator substance on a side the plastic film facing the atmosphere, wherein the gas concentration indicator substance is exposed to an electromagnetic radiation of a wavelength, the gas concentration indicator substance is excited with a plurality of pulses, which then emits light at a different wavelength than the wavelength of the electromagnetic radiation, and the gas concentration is determined on a basis of a decay curve of the emitted light, wherein at least one temperature correction parameter is determined for a category of the plastic film, the category of the plastic film is polyethylene film, polypropylene film, or an amorphous polyethylene terephthalate film, wherein the at least one temperature correction parameter is stored in a memory of a computer, and the at least one temperature correction parameter is retrieved from the memory during a change out of the plastic film, the at least one temperature correction parameter is utilized in determining the concentration of the gas component, wherein the at least one temperature correction parameter is determined on the plastic film currently in use, wherein fluorophor in the gas concentration indicator substance emits red light, and an intensity of the red light decays over time, and wherein a gas exchange in the packaging machine is controlled according to the measured concentration of the gas, wherein the method comprises: i) generating a reference package; ii) measuring and recording one or more temperature parameters of the reference package during cooling of the reference package to determine a cooling curve of the reference package; and iii) determining the at least one temperature correction parameter from the cooling curve of the reference package.

10. The method according to claim 1, wherein the packaging machine comprises a deep drawing station, a filling station, a sealing station, and a sensor, which reads out the gas concentration indicator substance inside of the package.

11. The method according to claim 10, wherein the method comprises: generating the package and measuring an intensity of the emitted light as a function of time and temperature, and determining the decay curve of the emitted light and sending the decay curve to the computer; and converting with the computer, the decay curve to the gas concentration using a false light model and the at least one temperature correction parameter determined from the reference package to correct a value of the concentration of the gas component.

12. The method according to claim 11, wherein the packaging machine has a pressure station, where the package is put under pressure to test for leaks.

13. The method according to claim 11, wherein the reference package has a gas content of approximately 0%, and the method includes measuring a temperature of the gas concentration indicator substance with an infrared sensor.

14. The method according to claim 1, wherein the method comprises: i) generating the package and measuring an intensity of the emitted light as a function of time and temperature, and determining the decay curve of the emitted light and sending the decay curve to the computer; and ii) converting with the computer, the decay curve to the gas concentration.

15. The method according to claim 10, wherein the packaging machine has a pressure station, where the package is put under pressure to test for leaks, and wherein the reference package has a gas content of approximately 0%, and the method includes measuring a temperature of the gas concentration indicator substance with an infrared sensor.

Description

(1) The present invention is explained below on the basis of the figures. These explanations are given merely as an example and do not restrict the general scope of the idea on which the present invention is based. These explanations apply equally to all subject matters of the present invention.

(2) FIG. 1 shows the packaging machine according to the invention,

(3) FIG. 2 shows a typical decay curve.

(4) FIG. 3 shows typical measured values at different O.sub.2 concentrations.

(5) FIG. 4 shows the influence of the gas content on TAU, namely the O.sub.2 content on TAU here.

(6) FIG. 5 shows a possible embodiment of a temperature measurement.

(7) FIG. 6 shows the influence of temperature on the decay curve at a constant pressure.

(8) FIG. 7 shows the temperature dependence of TAU.

(9) FIG. 8 shows the calibration parameters τ, K.sub.SV, f as a function of temperature.

(10) FIG. 1 shows a packaging machine 1, comprising a deep drawing station 2, a filling station 7 and a sealing station 15. A plastic film sheet 8, the so-called bottom film sheet, is pulled out from a supply roll, in this case from right to left, and conveyed along the packaging machine according to the invention, preferably in cycles. In one cycle, the film sheet is conveyed further by one format length. For this purpose, the packaging machine has two transport means (not shown), namely two continuous chains in the present case, provided on the right and left of the film sheet. Each continuous chain has holding means, each working together with an edge of the film sheet. At least one gear wheel about which the respective chain is deflected is provided at the beginning and at the end of the packaging machine for each chain. At least one of these gear wheels is driven. The gear wheels in the intake region 19 and/or in the outlet region may be connected to one another, preferably by a rigid shaft. Each transport means has a plurality of clamping means, which grip the bottom film sheet 8 with a clamping effect in the inlet region and transfer the movement of the transport means to the bottom film sheet 8. In the outlet region of the packaging machine, the clamping connection between the transport means and the bottom film sheet is released. The package recesses 6 are molded in the bottom film sheet 8 in the deep-drawing station 2, which has a top die 3 and a bottom die 4, the latter being in the shape of the package recess to be produced. The bottom die 4 is arranged on a lifting table 5, which is vertically adjustable, as represented by the double arrow. Before each film advancement, the bottom die 4 is lowered and then raised again. In the remaining course of the packaging machine, the package recesses are then filled with the packaging material 16 in the filling station 7. A top film sheet is sealed onto the package recess in the following sealing station 15, which also consists of a top die 12 and a vertically adjustable bottom die 11. The top die and/or the bottom die are also raised and/or lowered before and after each time the film is transported in the sealing station. The top film sheet 14 may be deep drawn and/or may be guided in the transport means and/or may be transported by the transport chains, wherein these transport means then extend only from the sealing station and optionally downstream. Otherwise the embodiments utilized for the transport means of the bottom film sheet are applicable. A gas exchange is preferably carried out in the sealing station in order to reduce the oxygen content of the atmosphere in the package, for example. In the remaining course of the packaging machine, the completed packages are also separated, which is accomplished with the cutting tool 17, 18. The cutting tool 18 can also be raised and/or lowered with a lifting device 9 in the present case. Those skilled in the art will recognize that one cycle preferably includes deep drawing, filling and sealing a plurality of package recesses.

(11) The packaging machine has at least one measurement device, for example, a sensor 13, which reads out a spot comprising a gas concentration indicator substance inside the package and thereby reads out the concentration, for example, the oxygen concentration in the package.

(12) The packaging machine may also have a pressure station, with which the package is preferably put under pressure, and if there are any leaks in the package, it will breathe at this point and/or its deformation and/or reformation behavior will change. This change is detected and analyzed in the pressure station itself or with a sensor downstream from it. The leaking “bad package” are sorted out.

(13) FIG. 2 shows a typical decay curve for light emission by a gas concentration indicator substance, comprising a fluorophor which is excited with a light pulse in the present case. The fluorophor in the gas concentration indicator substance then emits red light as a result of excitation, preferably excitation with green light, and the intensity of the red light decays over time. This light is received by the sensor, whereupon the intensity change in this light is subject to an exponential decay curve. The rate of this decay—the lifetime of the red light is used for calculating the O.sub.2 concentration. To do so, the lifetime of the emission is measured as the so-called decay time TAU of the intensity of the fluorescent light, namely the red light here. Tau is defined as the period of time after which the intensity has dropped to the value 1/e. The variable TAU is the primary measurement result and is a measure of the O.sub.2 concentration.

(14) FIG. 3 shows the decay curves according to FIG. 2 for five different gas concentrations, specifically oxygen concentrations here. The curve shown in FIG. 4 can be generated from these curves and those skilled in the art will recognize that different measured values were used for the curve according to FIG. 4. For this curve it is possible to determine a function, which is then stored in the computer of a packaging machine for example, and on the basis of which the gas concentration, i.e., the oxygen concentration in the package here, can be ascertained on the basis of this curve.

(15) Those skilled in the art will recognize that the measured values may be specific for each type of film, for example, the passage of light through the film can be influenced by the optical properties of the film and then must be determined for different types of film.

(16) The following figures relate to temperature compensation. It has surprisingly been found that the decay curves shown in FIG. 6 represent a function of the temperature of the gas concentration indicator substance, and that this is preferably to be taken into account in determination of a gas concentration in the atmosphere of a package.

(17) Therefore, FIG. 5 shows a measurement setup that is preferably to be used. It has one or more sensors, which, first, record the decay curve of the emitted light in order to be able to calculate TAU and, second have a temperature sensor, namely an infrared sensor here, to measure the temperature of the gas concentration indicator substance. However, this measurement is influenced by the temperature of the film to which the indicator substance is attached, so the temperature of this film is preferably also measured, and it is assumed in first approximation that the gas concentration indicator substance has the same temperature.

(18) From the curves obtained in FIG. 6, the curve shown in FIG. 7 for a certain concentration of the component to be determined can be derived. The curve or the concentration can be correlated with the function illustrated.

(19) The temperature compensation of measured TAU values can now be explained as follows:

(20) The decay curves are recorded for a plurality of different concentrations, preferably five to seven different concentrations, in particular the O.sub.2 concentrations as a function of time and temperature, i.e., per temperature, for example, all the curves according to FIG. 3. And TAU is determined for each curve, after which the temperature dependence of the TAU values can be derived and then used for the following determination of the O.sub.2 concentration. The calibration parameters are preferably determined for each film.

(21) These calibration parameters are input into the control unit of the packaging machine when changing the film, and the O.sub.2 concentration values corrected for temperature are determined in this way.

(22) For example, FIG. 8 shows a set of these nine parameters. The gas concentration, namely the O.sub.2 concentration here, is determined on the basis of the false light model, where three parameters τ, K.sub.SV, f represent the O.sub.2 dependence and the a, b, c indexing describes the temperature dependence.

(23) False Light Model:

(24) $\left[O_2\right]=\frac{{\tau }_0-\tau }{K_{\mathrm{SV}}*\tau +K_{\mathrm{SV}}*{\tau }_0*\left(f-1\right)}$

(25) τ here is TAU and is measured

(26) τ.sub.0 is TAU 0 and is a calibration parameter

(27) f is the radius of curvature at high O.sub.2 concentrations

(28) K.sub.SV is the curvature parameter at average O.sub.2 concentrations

(29) FIG. 8 shows the measured temperature dependence of the calibration parameters ascertained on the basis of the equations that are also shown.

(30) Example of the 0 Point Adjustment for a New Roll of Film

(31) On the basis of a curve analysis, the temperature dependence of the parameter TAU 0 with which the function TAU is described at dropping temperatures is determined. FIG. 7 illustrates an example of a TAU function. The TAU 0 values determined for the film replace the values for TAU 0 previously determined for the film in a laboratory, for example. To do so, the following procedure is followed on the packaging machine: 1) A package with approximately 0% O.sub.2 content is generated with the “step vacuum” function in the sealing station. One and the same preference//? and subsequent regassing with O.sub.2-free N.sub.2 or CO.sub.2/N.sub.2 mixture can be performed by evacuating three times. The resulting reference package (null package) has the lowest possible residual oxygen content, and an O.sub.2 content of 0% is assumed for the sake of simplicity. 2) Initially the package is very warm but it cools down according to its own cooling curve. In doing so, many value parameters of temperature and TAU are measured and recorded. This yields the exact cooling curve according to FIG. 7, which represents the temperature dependence of the TAU value (decay time/lifetime). 3) The calibration parameter T.sub.0 is determined from this (the temperature dependence is represented by T.sub.0, T.sub.0b and T.sub.oc). This is used for film-specific calculation of the O.sub.2 concentration by the machine control unit. 4) Next, the actual packages are produced and, more or less at the same time, the intensity is measured as a function of time and temperature (about 10 times). Then TAU values are determined from this, and the average TAU values and the temperature value are sent to the machine control unit. 5) The average TAU values are converted by the machine control unit into O.sub.2 values using the false light model with the help of the temperature-dependent parameters, which are determined as described above (see FIG. 8).

(32) In this way, a value for the O.sub.2 concentration that has been corrected with regard to temperature is obtained for each sensor spot and/or for each package.

(33) Example of the Sequence of Temperature Compensation:

(34) 1) The temperature-dependent calibration parameters are determined for each film on a test stand by determining the temperature dependence at three temperatures with a constant O.sub.2 concentration.

(35) 2) The temperature dependence of the calibration parameters TAU 0, K.sub.SV and f is determined from the data thereby obtained.

(36) Example of the Sequence of 0 Point Adjustment:

(37) 1) Null packages are produced on the packaging machine.

(38) 2) The temperature of the film is equated with the temperature of the sensor spot for the sake of simplicity, amounting to 42° C. at the moment of the first measurement, for example.

(39) 3) The TAU values are determined as a function of temperature, and the calibration parameter TAU 0 is calculated from this.

(40) 4) The measured data yield the values for TAU=65 and T=42° C., for example.

(41) 5) The preceding curve analysis yields the three calibration parameters TAU 0=67, K.sub.SV=0.25 and f=0.75.

(42) O.sub.2=(67-65)/(0.25*65+0.25*67*(0.75-1)=0.185%

LIST OF REFERENCE NUMERALS

(43) 1 Packaging machine

(44) 2 Deep-drawing station

(45) 3 Top die of the deep drawing station

(46) 4 Bottom die of the deep drawing station

(47) 5 Lifting table, support for a die in the sealing station and deep-drawing station and/or the cutting device

(48) 6 Package recess

(49) 7 Filling station

(50) 8 Bottom film sheet

(51) 9 Lifting device

(52) 10 Completed package

(53) 11 Bottom die of the sealing station

(54) 12 Top die of the sealing station

(55) 13 Sensor, oxygen sensor

(56) 14 Top film

(57) 15 Sealing station

(58) 16 Packaging material

(59) 17 Longitudinal cutting device

(60) 18 Transverse cutting device

(61) 19 Inlet region