Frequency modulated microwave in combination with a voltage detector and comparator device for measuring a varying moisture content of sheet-like samples
This is a continuation of application Ser. No. 858,893, filed Dec. 8, 1977, now abandoned. 
    
    
     The subject of the invention is a device for simultaneously measuring the moisture content at the sides and in the centre of sheet-like samples or of sample webs (textile webs, paper webs, plastic webs, rubber webs, plastic and wood panels, and the like) with microwaves. 
     In the continuous dyeing and finishing of textile webs a uniform application of liquor is required. For this reason the padding mangle is still, at present, the most widely used machine for applying liquor. The pick-up of the padding mangle can be altered across the width of the textile webs, depending on the design, by pressure on the journals of the slightly convexly ground rolls or, in the case of floating rolls, by pressure on the journals and additional pressure on the fluid in which the one or both rolls float. There are also designs of padding mangles in which pressure can be applied separately on the journals and in the centre of the pair of rolls. With all types of padding mangles it is desirable, and also adequate, to measure the amount of applied moisture at the sides and in the centre of the material web, for monitoring and controlling a uniform application of liquor across the whole width of the material. Microwave moisture-measuring instruments are suitable for determining the amount of applied liquor in the high moisture range. The equipment consists of a microwave oscillator, a microwave transmitter and a microwave receiver. The disadvantage with the known equipments is that with them only the moisture at one particular part, for example at the centre, at the edge, and the like, of the textile web can be sampled at a time. From such a measurement no conclusions can, however, be drawn as to the moisture distribution across the whole width of the web. Traversing microwave instruments which operate by reflection have failed in practice and are therefore not used. The adjustment of the equipment was too difficult and the fluttering of the wet material web caused errors in the measurement results. 
     The object was therefore to provide a device with which the moisture application can be measured simultaneously at the sides and in the centre of sheet-like samples, in particular sample webs, and with which the fluttering of the wet sample web no longer has an effect. 
     For achieving the object a device has now been found for measuring the moisture content of a sheet-like sample with microwaves, which device consists of a microwave oscillator, microwave transmitter and microwave receiver wherein at least three horn transmitter are flanged via an attenuator, directional couplers and attenuators to a frequency modulated microwave oscillator, opposite each of the horn transmitters, which are fixed on a frame, a microwave receiver with detector is arranged on the same frame, and a reference detector is flanged, via an attenuator, to at least one of the directional couplers. 
     The microwave receivers can be arranged so that they can be moved on the frame. 
     Moisture contents from about 30 g of water to about 600 g of water per m 2  can be determined with the device. It is thus possible for differences of water content of 1-3% from measuring position to measuring position to be measured reproducibly. 
     If with other application equipment such as, for example, spraying and foam equipment, or behind pre-drying units such as infra-red pre-driers, three measuring positions across the web width are not adequate for monitoring the uniform moisture content, further measuring positions can be flanged on, via additional directional couplers, in addition to the three described. 
    
    
     In the following text the device is described in an illustrative embodiment. 
     The FIGURE shows a schematic representation of the device for measuring the moisture content on the sides and in the center of webs, such as for example textile webs, paper webs, and the like. 
    
    
     The microwaves of about 2.4 to 3.7 cm wavelength, produced in the frequency modulated microwave oscillator 1, pass, via the attenuator 2, into the directional couplers 3, 3a, 3b which have a coupling factor of, for example, 3 dB. These three directional couplers couple in each case a part of the microwave power into the three measuring paths 11, 12 and 13, which consist of the attenuators 4, 4a, 4b, the horn transmitters 5, 5a, 5b, the receivers 7, 7a, 7b and the detectors 8, 8a, 8b. The sample web 6 runs between the transmitters and receivers. Microwave power is absorbed on penetrating the sample and specifically as a function of the moisture content of the sample. The part of the microwave power which is not coupled out by the three directional couplers 3, 3a, 3b passes through the attenuator 9 into the detector 10. The whole device is arranged on a frame 14 by means of fasteners 15. In the case of wider sample webs, straight waveguides can also be flanged on between the directional couplers 3, 3a, 3b. 
     The voltage resulting at the detector 10 can be calibrated to indicate the absolute amount of water in grams of water per m 2 . The voltages at the detectors 8, 8a, 8b serve to show the difference in the moisture content at the edges and in the centre of the sample web. For this purpose the attenuators 4, 4a, 4b, are set without the sample web, in such a way that the same microwave powers are indicated in the measuring paths 11, 12 and 13 by the detectors 8, 8a, 8b. The voltage value indicated by one detector, for example 8a, behind the material web, is held to the preset voltage value by reduction of the attenuation in the attenuator 2, even when a wet sample web absorbs a part of the microwave power. The attenuation of the microwave power is altered as a function of the moisture content in such a manner that the detector voltage, for example at 8a, is always constant. As a result, at a higher moisture content a higher voltage results at the detector 10. Differing voltage values at the detectors 8 and 8b, compared with 8a, signify differing moisture across the width of the material web. The attenuation of the microwaves at the position 2 can be effected by hand or regulated automatically with an attenuator which can be controlled electronically, for example with the Model 704 from Messrs. Rohde &amp; Schwarz, D-8000 Munich. If diodes with square-law characteristic curves are chosen for the detectors the resulting direct current voltage is proportional to the particular microwave power passing into the detectors. 
     The outputs of the detectors 8, 8a, 8b and 10 are connected to an indicating instrument, for example a pen recorder--which is not represented. The output voltages are suitable in particular for controlling a uniform application of liquor across the width of the material web and also for controlling a uniform drying across the width of the material web.