Patent GB-1359778-A (GB)
Title: Methods of and apparatus for measuring characteristics of materials
IPC: G01N27/02
Filed: 1972-08-18
Abstract: 1359778 Measuring physical characteristics electrically DREXELBROOK CONTROLS Inc 18 Aug 1972 [20 Aug 1971] 38597/72 Heading G1N A first characteristic of a material is measured by deriving first and second signals respectively representative of the susceptive and conductive components of an admittance probe associated with the material (or by deriving a signal having components respectively representing the susceptive and conductive components) said susceptive component varying with said first characteristic and also with a second characteristic of said material, said conductive component varying only with said second characteristic, and combining said first and second components (or phase-detecting said signal) to yield a signal primarily representative of variations in said first characteristic independent of variations in said second characteristic. Measuring dry weight of polyester granules. A probe 20, Fig. 1, immersed in the granules has a susceptive component dependent on the dry weight of the granules and the moisture content represented by capacitance 19 in bridge circuit 17, and a conductive component dependent on the moisture content represented by resistance 18. The bridge is energized from a low impedance high frequency current source comprising an oscillator 10 and a high gain amplifier 12 with substantially 100% negative feedback. The susceptive and conductive components are detected at 31 and 32 respectively, low pass filtered at 36 and 37, and an adjustable fraction (chosen by setting potentiometer 41) of the conductive component is subtracted from the susceptive component in differential amplifier 42, the output of which may be further amplified at 44 and indicated or used for control. Measuring dry weight of corn. The apparatus of Fig. 1 is modified by the addition of a capacitance in series with the probe, e.g. by providing an insulated coating on the probe, so as to alter the relative shapes of the susceptance and conductance curves Fig. 3 (not shown). Alternatively Fig. 1A (not shown), the outputs of filters 36 and 37 may be fed to a combining circuit in which a signal representing the ratio of the conductive and susceptive is formed (at 109) modified (at 110) and multiplied (at 111) with the susceptance component, the resultant being subtracted from the susceptance component (at 112). Measuring moisture content of corn. In the apparatus of Fig. 1, divider 45 may be used, by changing over switches 43 and 47, to give the percentage of water to dry weight, or the total weight of water. Measuring thickness of coating. The thickness of a still wet clay coating on boxboard is measured using two spaced electrodes (93, 94,) Fig. 6 (not shown) in contact with the uncoated side of the board as it leaves the coating plant and connected in the bridge circuit of Fig. 1. The outputs of filters 36 and 37 are fed to a combining circuit Fig. 7 (not shown) in which the susceptive component is squared (at 100), divided by the conductive component (at 101) and the result added to the conductive component (at 102). Measuring liquid level. An immersion probe having an insulating coating (105) Fig.8 (not shown) is used to sense the level of a conductive liquid (109) which tends to produce a coating (106) on the probe equivalent to an infinite transmission line. The circuit of Fig. 1 is used. Arrangements using only one phase-detector. The output of buffer 26, Fig. 1, is amplified and fed to a single phase sensitive detector (81), Fig. 5 (not shown) whose control phase is adjustable to perform the desired subtraction of the subsceptive of conductive components (e.g. 45 degrees for the liquid level example above).