GB2054841A - Measurement of basis weight by radiation gauge - Google Patents

Abstract

For accurate measurement of the basis weight (mass per unit area) of a material such as paper between a radioactive source 12 and an ionization chamber 18 the apparatus is calibrated by sing a plurality of standards (eg. 34c) of known basis weight to provide a relationship between basis weight and the output current of the chamber which includes at least terms of the second order and preferably terms of higher orders. The major portion of the radiation path is enclosed in airtight chambers (52, 54) which are sufficiently rigid that the density therein is independent of ambient temperature and pressure variations. The accuracy is increased by measuring ambient temperature and pressure fluctuations, and linearly compensating for resultant density variations in the air gap (10) through which the paper web passes. A wheel 32 holding the standards is induced by a motor 40 and a perforated encoding disc 44. <IMAGE>

Description

SPECIFICATION Apparatus for the measurement of densitythickness by use of radiation This invention relates to apparatus for measuring the density-thickness, mass per unit area, or basis weight of materials by introducing them in the path of radioactive particles emitted from a radioisotope source and impinging upon an ionization chamber. One common use of such apparatus is in the manufacture of paper webs.

The output current I of the ionization chamber is governed approximately by the exponential absorption law: (1) I=lOe where I is the current output for a given densitythickness of matter introduced between chamber and source, where 1o is the current output without the said matter present, where r is a constant determined by various parameters of the system, and where w is the density-thickness of the matter introduced. The relationship between I and w is hence non-linear and may also be expressed as: (2) w = -(i/r) In (I/o).

Even if such an instrument is accurately calibrated, there are many factors which cause the calibration to become inaccurate. The strength of the radioisotope source decays with time. With age and usage the geometric shape of the machine may change, altering the length or alignment of the radiation path between source and chamber. Density variations in the air gap are caused by change in ambient pressure or temperature. The air gap windows which protect the chamber and the source from the surrounding environment may wear, decreasing the mass in the radiation path. Pitch from the wood pump may build up on the windows, increasing the mass in the radiation path. Unless the instrument is calibrated at fairly frequent intervals, its measurements are likely to reflect errors induced by one or more of these factors.

Efforts have been made in the prior art to deal with the factors adversely affecting instrument calibration. For example, Leighton et al U.S. Patent No. 2,675,483, and Pepper et al U. S. Patent No.

2,968,729 disclose the use of an additional ionization chamber to measure the decay in strength of the radioisotope source, or of an auxiliary source having similar decay characteristics, so as to compensate for such decay. These systems, however, fail to compensate for the other factors which tend to disturb calibration.

Leighton U.S. Patent No. 3,180,985 shows circuitry for making a linear approximation to the relationship between ionization voltage of the chamber and density-thickness by a calibration based upon a single measurement of chamber output voltage with no subject material in the radiation path. This linear approximation fails to provide the high degree of accuracy which is necessary in many applications.

Changes in ambient temperature and pressure in the air gap occur so frequently that they must be compensated for continuously. Gilman U.S.

Patent No. 2,800,591 and London U.S. Patent No.

2,883,555 disclose the use of bellows, bimetals and other mechanical devices which respond to atmospheric conditions to alter the length of, or partially obscure, the radiation path so as to compensate for the effects of atmospheric changes upon the measured ionization current.

Otis U.S. Patent No. 2,968,127 discloses the use of a thermistor placed near the radiation path of such a machine to measure the temperature of the air in that path and make linear corrections in the current output of the ionization chamber.

However, because of the non-linear, "exponential" relationship between I and w, the linear changes in I provided by Otis produce inaccurate compensation in the measurement of w.

In general, the present invention contemplates the on-site calibration of the apparatus with three or more standard samples of known basis weight, to obtain a power series approximation of the nonlinear relationship between I or chamber signal and w which includes at least terms of the second order. The major portion of the path between source and chamber is sealed against atmospheric density changes. Residual density changes in the small working air gap are compensated for by making linear changes in computed basis weight.

The present invention provides apparatus for calibrating a meter at the site at which said meter is installed, said meter including a source of radiation and a detector mounted with a space therebetween through which a medium is permitted to pass to cause said detector to produce a current affording a measure of a property of said medium, said apparatus comprising a plurality of internal standards preselected to correspond to respective different known values of said measured property, and means mounting said standards for successive movement into the space between said source and said detector to produce a plurality of output currents, said output currents and said known values of said measured property enabling on-site calibration of said meter.

The present invention further provides a method of calibrating a meter at the site at which the meter is installed, in which meter radiation from a source passes through a medium to a detector to produce a current which is a measure of a property of said medium, including the steps of first selecting a plurality of internal standards respectively corresponding to different values of said measured quantity, successively positioning said internal standards between said source and said detector to produce a number of voltages corresponding to the number of said internal standards, and calibrating said meter from said values of said measured quantity and said voltages.

A preferred embodiment of the invention provides a method of an apparatus for accurately calibrating a meter at the site at which it is installed for use on a production line.

Preferably, the apparatus is arranged to measure density-thickness by being calibrated with at least three samples of known mass per unit area. The non-linear relationship with ionization chamber current is approximated by a power series including terms of at least the second order.

In the preferred embodiment, the major portion of the radiation path between source and chamber is substantially sealed against atmospheric density changes.

A further feature of the preferred embodiment is that atmospheric density changes in the working air gap are measured and used to linearly correct the computed basis weight.

In order that the present invention may be more readily understood, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which like reference numerals are used to indicate like parts in the various views, and in which: Figure 1 is a sectional view of the preferred embodiment of the invention; Figure 2 is a fragmentary view of the sample wheel shown sectionally in Figure 1; Figure 3 is a view of the binary coding disk used for positioning the sample wheel; and Figure4 is a graph of ionization chamber output versus basis weight w.

Referring now more particularly to Figure 1 of the drawings, source head 2 and a detector head 6 are both mounted on an external frame, not shown, such that there is a small working air gap 10 therebetween through which material, such as a paper web, can be passed. Mounted within housing 4 of source head is a radioisotope source 12 which is surrounded on all sides but the top by a radiation shield 14. Source 12 and its shield 14 are positioned by source holder 16 so that radiation emitted from the unshielded top of source 1 2 can pass upwardly to ionization chamber 18 mounted within housing 8 of detector head 6. 1 provide my apparatus with a data processing module 22 located in housing 8.

The radiation path extends from source 12, through a sample wheel 32, through source head window 56, across gap 10, through detector head window 80, through chamber wall 90 into the ionization chamber 18 disposed in detector assembly housing 8. Windows 56 and 80 and chamber wall 90 may be made of stainless steel of approximately 0.5 mil to 2 mils thickness. A shutter 24 normally blocks the radiation path, as shown in Figure 2. However, the shutter 24 may be rotated counterclockwise to the alternate position shown in dotted lines in Figure 2 to permit passage of radiation. Shutter 24 is mounted on a shaft 26 which is rotated by a motor 28 receiving electrical energy from conductors 30.When any readings of basis weight are being made, either for calibration purposes or for purposes of measuring the basis weight of materials, shutter 24 is rotated to the open position.

Sample wheel 32 is mounted within housing 4 of source head 2 and is provided with six openings. Five of these openings are provided with different calibration standards 34A through 34E of known basis weight which may be formed of a polyester film of polyethylene terephthalate resin such as Mylar, a registered trademark of E. I.

duPont de Nemours and Company. Standards 34A through 34E correspond respectively to standards 1 through 5 of Figure 4 and may have respective basis weights of 22, 44, 66, 88, and 110 pounds per 3300 square feet.

The sixth opening 36, which is positioned between heads 2 and 6 when the instrument is in use for measuring basis weight of a web, is empty.

Thus, by rotating the sample wheel, each of the five calibration standards 34A through 34E can be placed in the radiation path above source 12.

Sample wheel 32 is mounted on a shaft 38, which is rotated by a stepping motor 40 receiving electrical energy from conductors 42. Also mounted on shaft 38 is an encoding disk having six equally spaced groups of radially aligned apertures 46 disposed in three concentric tracks providing representations of 1 through 6 in the natural binary code wherein the innermost track represents the least significant digit and the outermost track represents the most significant digit, as may be seen by reference to Figure 3. A U-shaped optical reader 48 includes upper and lower legs disposed on either side of disk 44.

Mounted in the lower leg are three radially aligned light sources, one for each track. Mounted in the upper leg are three radially aligned light sensors, one for each track, which provide three corresponding outputs on conductors 50. Thus, 'with sample wheel 32 in the position shown in IFigure 2, sample 34E is in the radiation path; and optical reader 48 provides a binary output of 110--6.

Ionization chamber 18 consists of an outer electrode 1 02 which lines the inner walls of the chamber and a collector electrode 104 which projects into the center of said chamber and which is insulated from the outer electrode by a suitable insulator 106. Conductor 21 applies a D. C.

voltage from source 4 14 of module 22 to electrode 102. Ionization chamber 18 is filled with a normally nonconducting gas so that no current will normally flow through conductor 20 back to module 22. When, however, radiation from source 12 enters ionization chamber 18, it ionizes gas modules therein, causing such gas to conduct and thereby produce a flow of current through conductor 20. The current is processed through a current to voltage converting circuit to produce a voltage proportional to the current flowing from the ionization chamber. This voltage signal is used in data processing as an indication of the current from the ionization chamber.Thus equation (2) may be rewritten as: (3) w=-(1/r)!n (V/Vo) Module 22 is maintained at a constant elevated temperature by a temperature sensing circuit 108 which, through an amplifier 110, drives a heat dissipating load resistor 112 mounted inside module 22.

Airtight chamber 52 encloses that part of the radiation path between source 1 2 and window 56.

It is bounded by source holder 16, a source enclosure 68, a paper web lower guide plate 64, a source head cover 60, and a source window 56.

Source window 56 is supported in a bezel 58 on source head cover 60 in such a manner as to compress resilient sealing ring 62. 1 dispose a sealing ring 66 between source head cover 60 and web guide 64. 1 place respective seals 70 and 76 between source enclosure 68 and web guide 64 and between source holder 1 6 and source enclosure 68. Shutter position shaft 26 and sample wheel shaft 38 are provided with respective seals 72 and 74 to insure that chamber 52 is hermetically sealed. Suitable bolt seals are provided on the source retaining block 16.

Airtight chamber 54 encloses that part of the radiation path which lies between window 80 and ionization chamber 1 8. It is bounded by detector wihdow 80, a detector head cover 84, and a chamber wall 90. Detector window 80 is provided with a circumferential flange 82 which is mounted over a seal 86 on detector head cover 84.

All of the parts which form the boundaries of airtight chambers 52 and 54 are made of sufficiently rigid materials to prevent internal or external pressure variations from changing the density of the gas which lies within them.

Chambers 52 and 54 need not be absolutely airtight. The total air column between source 12 and detector 18 may be 2.5", necessitated by the need for shutter 24 to prevent external radiation for source 12 when not in use. This air column represents a basis weight of approximately 55 pounds per 3300 square feet. Since it is desired to measure basis weight to within +.01 pound per 3300 square feet, it is only required that chambers 52 and 54 be sufficiently sealed that density variations between calibration intervals of one hour, for example, be less than one part in 5500.

The working air gap 10 may be approximately 1/8", or only 5% of the total air column. In the air gap, each 1" Hg change in pressure causes an error in basis weight of (1/29.92j(.05) (55) = .92 Ib/3300 sq. ft.

Typical temperature variations of 500C from offweb to on-web positions of the sensor are encountered in a paper mill. In the air gap, each 1 OC change in temperature (assuming an ambient temperature of 200C= 2930 A) causes an error in basis weight of (1/293) (.05) (55) = .0094 Ib/3300 sq.ft.

A pressure transducer 97 is mounted above an upper guide plate 65. A pressure port 99 in the upper wall of plate 65 provides communication between air gap 10 and transducer 97, which produce an electrical output on conductors 101.

An air gap thermistor 96 providing an output on conductors 100 is mounted in an insulator 98 retained in the lower plate 64. To maintain an accuracy of +.01/-3300 sq. ft., pressure transducer 97 should have a resolution of at least 0.1" Hg; and temperature transducer 98 should have a resolution of at least 1 00.

In practicing my method and in setting up my apparatus, I first select the five standards 34A to 34E corresponding to respective basis weights at spaced points within the range of weights over which the apparatus is to be used. This is achieved by first placing a large number of calibrated standards in the gap 10 between heads 2 and 6 and measuring the resultant output voltages. By way of example, I have used seventeen calibrated standards corresponding to seventeen spaced values of basis weight from 11.70 to 11 7.10. By the method of least squares, using the measured voltages, I determine the coefficients of the second degree polynomial: (4) w = C1 + C2 v + C3 v2 which has been plotted in Figure 4.

As soon as possible after this has been done and under conditions as nearly identical as possible, I select the five standards 34A to 34E by measuring the voltages and calculating the corresponding weights in the desired ranges.

These then become the internal standards of my system.

When my apparatus has been set up in the manner described, it is placed on line and, with opening 36 in the space 10 between the heads 2 and 6 basis weight measurements may be made in the usual manner on the basis of the known relationship between measured voltage and basis weight.

After a period of time in use of the apparatus owing to environmental factors such as temperature changes, pitch buildup, window wear and the like, the "known" relationship between measured voltage and weight no longer is known and the apparatus must be recalibrated. To achieve such recalibration, I move the apparatus "off line" but not "off site". This may be done, for example, in the absence of a web passing through the installation.

Next, I successively position the internal standards 34A to 34E in the space 52 to provide five voltages in the processing unit 22. Unit 22 includes a suitable microprocessor such for example as an Intel 8080 which has been programmed to update the constants of equation (4) in accordance with the voltages produced by the internal standards, thus to recalibrate the system to produce accurate indications of basis weight in response to voltages related to the chamber current, taking into account the factors which affect the accuracy of the system with the passage of time. As an alternative to using the binomial expression (4) in my apparatus, I might use the exponential expression (1) or the logarithmic form (2) and so set my microprocessor as to calibrate in terms of the selected expression in response to voltage produced by the internal standards.

It will be seen that I have accomplished the objects of my invention. I have provided a method of and apparatus for accurately calibrating a meter at the site at which it is installed for use on a production line. My system accurately accounts for changes owing to deterioration of the source, as well as window wear and pitch buildup. i so construct my system as to minimize the effect of atmospheric density changes.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of my claims. It is further obvious that various changes may be made in details within the scope of my claims without departing from the spirit of my invention. It is, therefore, to be understood that my invention is not to be limited to the specific details shown and described.

Claims (20)

1. Apparatus for calibrating a meter at the site at which said meter is installed, said meter including a source of radiation and a detector mounted with a space therebetween through which a medium is permitted to pass to cause said detector to produce a current affording a measure of a property of said medium, said apparatus comprising a plurality of internal standards preselected to correspond to respective different known values of said measured property, and means mounting said standards for successive movement into the space between said source and said detector to produce a plurality of output currents, said output currents and said known values of said measured property enabling on-site calibration of said meter.

2. Apparatus according to claim 1 including means for providing position output signals indicative of the particular standard disposed in said space.

3. Apparatus according to claim 1 or 2 further including a disk, said standards being carried by said disk at circumferentially spaced locations, said disk being provided with an opening between a pair of said locations, said means mounting said standards comprising means mounting said disk for rotary movement adjacent to said meter with the portion thereof constituting said locations adapted to pass through said space, means positioning said disk with said opening in said space, and means for driving said disk successively to position said internal standards in said space to cause said detector to produce said output currents.

4. Apparatus according to claim 3 as dependent on claim 2 wherein said means for providing position output signals is responsive to the position of said disk.

5. Apparatus according to any one of the preceding claims wherein said property to be measured is the mass per unit area of said medium, said source of radiation comprising a radioisotope source, and said detector comprising an ionization chamber.

6. Apparatus according to claim 5 wherein said standards are chosen to have values of mass per unit area which vary in approximately equal intervals throughout the expected range of mass per unit area of the materials to be measured.

7. Apparatus according to claim 6 wherein there are five standards.

8. Apparatus according to claim 5, 6 or 7 further including respective substantially airtight housings for each of said source and said ionizing chamber, said housings being supported with a gap therebetween through which said medium can pass, said substantially airtight housings being of sufficiently rigid construction as to minimize the effect of normal changes in temperature or pressure of the ambient atmosphere.

9. Apparatus according to claim 8 including a linear thermistor in contact with the gas in the gap between said two airtight chambers adjacent to the location at which the radiation path of said source crosses said gap.

10. A method of calibrating a meter at the site at which the meter is installed, in which meter radiation from a source passes through a medium to a detector to produce a current which is a measure of a property of said medium, including the steps of first selecting a plurality of internal standards respectively corresponding to different values of said measured quantity, successively positioning said internal standards between said source and said detector to produce a number of voltages corresponding to the number of said internal standards, and calibrating said meter from said values of said measured quantity and said voltages.

11. Apparatus for the measurement of the mass per unit area of a given material utilizing the output current of an ionization chamber to measure the amount of radiation emitted from a radioisotope source which enters said chamber after being projected onto an area of said material, comprising at least three calibration standards each made of a material of known mass per unit area, means mounting said calibration standards for successive movement into the path of radiation between the radioisotope source and the ionization chamber in the absence of said medium in said path, means for actuating said moving means, and for measuring the output current of the ionization chamber when each of the calibration standards is placed in said radiation path.

12. Apparatus according to claim 11 wherein said calibration standards are chosen to have values of mass per unit area which vary in approximately equal intervals throughout the expected range of mass per unit area of the materials which the apparatus is intended to measure.

13. Apparatus according to claim 12 wherein there are five calibration standards.

14. Apparatus according to claim 12, 13 or 14 further including respective substantially airtight housing for said source and said ionizing chamber, said housings being supported with a gap therebetween through which said material can pass, said airtight housings being of sufficiently rigid construction as to minimize the effect of normal changes in temperature or pressure of the ambient atmosphere.

1 5. Apparatus according to claim 14 including a linear thermistor in contact with the gas in the gap between said two airtight chambers adjacent to the locations at which the radiation path crosses said gap.

1 6. Apparatus for the accurate measurement of the mass per unit area of a given subject material utilizing the output current of an ionization chamber to measure the amount of radiation energy emitted from a radioisotope source which is absorbed when passed through said subject material comprising at least two substantially airtight housings respectively for said radioisotope source and said ionization chamber, means mounting said housings with a narrow space therebetween in the path of radiation passing from said source to said chamber, said material adapted to pass through said space, said substantially airtight chambers being of sufficiently rigid construction that the effect of normal changes in temperature or pressure of the ambient atmosphere is minimized.

17. Apparatus according to claim 16 wherein a linear thermistor is placed in contact with the gas in the gap between said two airtight chambers adjacent to the location at which the radiation path crosses said gap.

18. Apparatus for calibrating a meter substantially as hereinbefore described with reference to the accompanying drawings.

1 9. A method of calibrating a meter substantially as hereinbefore described with reference to the accompanying drawings.

20. Apparatus for the measurement of the mass per unit area of a given material, substantially as herein before described with reference to the accompanving drawings.