RADIOMETER MOISTURE METER

I0605 6 The present invention relates to methods of, and apparatus for, measuring a physical characteristic of a material, and is useful in particular for measuring the moisture content or thickness of a paper or other web or sheet material. In the manufacture of paper, it is important to be able to measure the amount of mois, ture present from the pulp web stage to the final product stage, since a non-uniform distribution of mois£ure within the paper web during the manufacture of the paper causes unacceptable variations in the quality of the finished product. ii:• •" i? /'. ï In order to maintain the moisture content at a predetermined, required level, the paper web is dried by hot air, electrical energy or any other suitable method to the required equilibrium moisture content for the humidity conditions in which it will be used. ï Since organic sheet materials are basically a mixture of solid and water (free or bound), the electrical quantities most sensitive to variations in the moisture content are the effective dielectric constant, which can be measured by active microwave techniques, and the effective radiation temperature, which can be measured by radiometric techniques. However, these sheet materials are electrically lossy (i.e. they absorb electrical energy due to the presence of moisture), and to avoid damaging the sheet materials any attempt to measure their dielectric constant or radiation temperature should preferably not be accompanied by dissipation of energy in the sheet materials. ï . It isknown that all objects in the physical universe which are not at absolute zero temperature radiate energy over a wide spectrum. The microwave portion of this energy can be accurately detected and interpreted to yield information about the l0 source object. It is accordingly an object of the present invention to provide an improved method of measuring a physical characteristic of'a material, e.g. the moisture content or thickness of a paper web, by detecting such energy. According to the present invention, there is provided a method of measuring a physical characteristic, e.g. the moisture content, thickness or temperature, of a sheet material, e.g. a paper web, which comprises the steps of sensing the frequency spectrum of the radiation temperature of the material by use of a radiometer to obtain a radiometer output signal corresponding to the frequency spectrum, and employing the radiometer output signal as a measurement of the physical characteristic. Also according to the present invention, there is provided apparatus for measuring a physical characteristic of a material, which comprises an antenna for sensing the frequency spectrum of the radiation temperature of the material, means for guiding the material past the antenna at a predetermined spacing from the antenna, a radiometer connected to the antenna for providing a radiometer output signal co-responding to the frequency spectrum of the radiation temperature of the material, and means for indicating the signal as a measurement of the physical characteristic. To counteract spurious signals in the radiometer, a screen may be provided around the antenna, and the material advanced through the screen past the antenna. The antenna may be one of a pair of radiometer antennas disposed at opposite sides of a position at which the frequency spectrum of the material is sensed. The signal outputs of the two radiometer antennas are compared to obtain a zero difference "z i i0 1O6O546 signal which is applied to the radiometer, and the radiometer is then .adjusted-to obtain a null radiometer output signal. The invention will be more readily understood from the following description of embodiments thereof given, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 illustrates a paper web moisture content sensing apparatus according to one embodiment of the invention; Figure 2 illustrates a modification of the arrangement of Figure i; and Figure 3 shows a graph illustrating the relationship between the signature and the moisture content of a test piece of newsprint paper. The apparatus illustrated in Figure 1 of the accompanying drawings has two pairs of rollers ii and 12 for guiding a paper web 14. One of each of these pairs of rollers, for example the rollers ii, may be driven by any suitable drive means (not shown) for advancing the paper web 14 along a path of travel. Above the path of travel of the paper web 14, there is provided a radiometer antenna 15, the output of which is connected to a radiometer 16. The rollers ii and 12 serve to guide the paper web 14 past the radiometer antenna 15 at a predetermined spacing from the radiometer antenna The radiometer antenna 15 and the portion of the path of travel of the web 14 at which the web 14 passes the radiometer antenna 15 are surrounded by a screen 17, the purpose of which is to counteract spurious signals in the radiometer 16 due to stray radiation. Beneath the path of travel of the paper web 14, at the ji.. .L '•.'. i.' • i. • . ii EI- .i • " i i0 side thereof opposite from the radiometer antenna 15, there is provided an aluminum plate 18 for supporting the paper web 14. The output of the radiometer 16 isapplied, through an inverter (not shown), to a data logger 20, a strip chart recorder 21 and an X-Y recorder 23. The operation of the above-described apparatus is as follows: As the paper web 14 is advanced through the shield 17 past the radiometer antenna 15, the latter senses the frequency spectrum of the radiation temperature of the paper web. More particularly, paper web radiates energy over a wide freqeuncy spectrum, a limited frequency range of which is detected bY the antenna 15. In the radiometer 16, the antenna signal is mixed with a swept local oscillator signal to provide an IF signal, the local oscillator signal having a rate of frequency sweep which is adjusted in accordance with the time constant of the radiometer receiver. The IF signal is then :t iii ii" i!ii• amplified and recorded by the data logger 20, which provides a digitalindicätion, and also by the strip chart recorder 21 and the X-Y recorder 23, which provide analogue indications. It will be apprecìated that the area of the paper web which is sensed by the radiometer antenna 15 must be sufficiently large to ensure that local variations in the solid density of the paper web are integrated out and do not affect the accuracy of the measurement. Also, the volume of the radiometer antenna must be sufficiently large to avoid physical obstruction and near-field effects of the coupling probes. In addition, the rollers 12 and 13 guide the paper web 14 so as to eliminate errors due to lateral and vertical displacements of the paper web 14 as it is being sensed by the radiometer antenna i: i!i i0 If it is desired to monitor lateral variations of the moisture content across the paper web 14, then a plurality of radiometer antennas may be transversely spaced apart across the path of advance of the paper web. By measuring the frequency spectrum of the radiation temperature by the paper web, which is normally at room temperature corresponding to the usual condition in a paper mill prior to the drying stage, the signature is related to the non-electrical properties of interest, i.e. the moisture content. To a first approximation, it may be assumed that the signature at a single frequency is linearly related to the moisture content provided that the latter is below 20%, which is the case in the usual situations encountered in practice. Since the paper web is continuously visible to the radiometer antenna, the antenna signal represents a continuous perturbation of the reference signature and thus represents the moisture content variations on a continuous and real time basis. The signature may be employed to indicate the moisture content, if the paper weight remains constant, or vice versa. In the modification of the above-described apparatus illustrated in Figure 2, in which like reference numerals have been used to indicate like parts, a second radiometer antenna is disposed at the side of the path of the paper web 14 opposite from the radiometer antenna The radiometer antenna 15 is surrounded by the screen 17, as in the embodiment of Figure 1, but the aluminum plate 18 of Figure 1 is omitted. The second radiometer antenna 25 is likewise surrounded by a screen 27. The two radiometer antennas 15 and 25 are connected to ï.. ,ï ..' ï • ' kI.. .ï . : Á i . i . ii' . .Õo }i .", :" . ñ'•. , .:. • i: iii! • • i! ;i :!'" . .'.. - • "...,.. :, , . ..... . . ... ." 1 546 a magic tee 28, which in turn is connected to the radiometer 16. Two pairs of rollers Ii and 12, of which one of each is driven, by drive means not shown, are provided for advancing the paper web 14 between the radiometer antennas 15 and The advantage of this embodiment is that the reference signature can be set equal to zero, or a constant determined by the desired level of moisture content of the sheet material, regardless of the moisture content of the paper web 14. For this purpose, the radio frequency signal outputs of the two antennas and 25 are fed to the magic tee 28, which is used to subtract one of the signals from the other, the difference signal (i.e. zero radio frequency signal) from the difference arm of the magic tee 28 being fed+to radio frequency amplifiers in the radiometer 16, and the radiometer output then being adjusted to zero. When this nulling procedure has been completed, it is no longer necessary to subtract the two radio frequency signals from the two antennas 15 and 25. Instead, only one of these radio frequency signals can be fed into the radiometer antenna 16 for measurement purposes, or alternatively the magic tee 28 can be employed to sum the two radio frequencies from the antennas and 25, and the summed signal Can be supplied to the radio frequency amplifier of the radiometer 16, thus providing double the signal input of a single antenna. Using the single-sìded measurement apparatus illustrated in Figure l, the signature indicating the incremental percentage moisture content by weight, 6S, (above equilibrium moisture content) for newsprint paper was measured in volts and plotted against 6M, which was measured on a wet weight basis, i.e. the weight of moisture added to the paper for the purpose of experiment to the weight of the paper at the equilibrium or room tem1060546. perature moisture content. More particularly, a microwave radiometer operating in the 12 - 18 GHz band was employed as the radiometer 16, and paper samples were passed over a large aluminum sheet with the paper samples below the shown antenna 15 which has antenna aperture dimensions of 6" X 5", with its larger dimension parallel to the longer dimension of a waveguide 19 extending from the radiometer 16. The axial height of the antenna 15, measured from its throat tO the midpoint of its aperture, was 12.75". The radiometer I0 settings for best sensitivity were found to be 14.32 GHz operating frequency, 180 MH3 IF bandwidth and 1 sec. integration time. The screen 17 was of length 41", width 41" and height 44 1/2", and the paper had a thickness of 0.003" and an approximate weight of 1 oz. per 350.square inches. As can be seen from Figure 3, the signature increased at a linear rate as the paper was moisturized from its equilibrium moisture content at the relative humidity of the room (about 5%) up to almost 20%. The above-described apparatuses have the advantage that they are simple to construct and operate, and also that they enable detection of not only variations in the paper moisture content, but also in the solid content provided that the area being monitored is sufficiently large to allow a reasonable statistical reading, since the radiometer antenna in fact integrates the emission from all the individual particles of the solid and moisture present in the paper web. Also, the signature is directly related to the area and composition of the material being sensed and is not affected by presence of the edges of the material, as is the case in most active techniques, in which higher order modes are excited or 'i A" :"••,'El i" ' " • i " " " ' " ' 1 0546 diffraction effects are encountered. As will be appreciated, more information can be obtained with multi-frequency measurements of the radiation temperature of the sensed material, although the information which can be obtained is limited since the lower layers or portions of a thick material may,. for example, be masked by the overlying portion of the material. If required, the output of the radiometer antenna or antennas can be used directly to regulate a hot..air flow for drying the paper web, which is normally effected beyond the moisture monitoring stage, using conventional proportional, derivative and integral controls. ,••...• . . • .r The embodiments of the invention in which an exclusive