Patent GB-695568-A (GB)
Title: Method and apparatus for the determination of electrical conductivity
IPC: G01N27/22
Filed: 1950-09-13
Abstract: 695,568. Bridge circuits; resistance measurements. DRAGER, O. H. Sept. 13, 1950 [Sept 21, 1949], No. 22535/50. Class 37. The conductivity of a material, particularly an electrolyte, is measured by interposing a dielectric layer between the material and one or more condenser plates and measuring the complex A.C. resistance by means of an instrument connected to these plates. As shown in Fig. 1 the material 2, the conductivity of which is to be measured, is in contact with a layer 4 of substantially loss-free dielectric material, such as porcelain or glass. On this layer are arranged electrodes 6, 8 of tinfoil or other conducting material. The measurement of the complex A.C. resistance, i.e. the capacitance and loss angle, between these electrodes is a measure of the conductivity of the material 2. The displacement current flows between the plates 6, 8 and the material 2, while only the conductivity current flows in the material 2. Alternatively a single condenser plate may be separated from the material 2 by a substantially loss-free dielectric and an electrode embedded in the material. More than two condenser plates may be employed, alternate plates being connected together. The plates 6, 8 may be connected in the grid circuit of a valve 28, Fig. 4, having oscillatory circuits 30, 32 in its anode and grid circuits. A milliammeter 34 is connected in the anode circuit, its deflection being a measure of the conductivity. In a modification, Fig. 5, a high-frequency generator 40 is connected through a resistance 42 and inductance 44 to a condenser 46 connected to the plates 6, 8. The voltage measured by a valve voltmeter 48 is proportional to the ohmic resistance of the material, while the adjustment of the condenser 46 gives the capacitance between the plates 6, 8. Fig. 6 shows a bridge circuit in which the plates 6, 8 are arranged in one arm of the bridge, with a generator 50 supplying the current and a valve voltmeter 52 in the other diagonal. The ratio arms comprise resistances 54, 56 while the fourth arm has a variable resistance 58 and a variable condenser 60. In the circuit shown in Fig. 7 the phase jump method is employed. The plates 6, 8 are connected in an oscillating circuit 62 coupled to a coil 64 in the circuit of a generator 66. Variations in the damping resistance and capacitance of the parts 2, 4, 6, 8 alter the tuning of the circuit fed by the generator 66 and cause corresponding changes of amplitude of the current in the indicator circuit 68. The circuit of the generator 66 is then tuned by a variable condenser 70 to the phase jump condition. Fig. 8 shows an arrangement in which it can be ascertained whether the frequency in the measuring circuit is greater or less than that of maximum damping. The combination 2, 4, 6, 8 affects the frequency and amplitude of the current in an oscillating circuit 72 coupled with the indicator circuit by means of a coil 74. This is connected to two circuits in parallel, one a low-pass circuit 76 and the other a high-pass circuit 78. These include indicators 80, 82 which indicate frequencies above or below the joint limiting frequency of the shunt circuits. Fig. 9 shows an apparatus for holding the material to be tested. This is placed in a cup 90 of dielectric material, which can be slid into or removed from a tube 88 of dielectric material. On the outside or the tube 88 are arranged rings 84, 86 forming electrodes. In a modification, Fig. 10, the material is placed in a dielectric tube 92 bearing ring condenser plates 84, 86. The temperature of the material may be controlled by passing cold or hot fluid through a system comprising an inlet pipe 94 and an outlet pipe 96. In another construction, Fig. 11, the measuring vessel 98 has an enlarged upper part 100 and is arranged within a tube 102 of ceramic or other insulating material. A number of spring rings 104 serve as condenser plates. The tube 102 is surrounded by a tank 106, the temperature of which is kept constant by fluid flowing through coils 108. A burette 112 and a glass rod 114, or a thermometer, are inserted through a glass bell 110. By raising and lowering the rod 114, or by replacing it by a rod of different diameter, variation in the thickness of the body of material can be obtained. The tube shown in Fig. 12 has a narrow central portion 116 and enlarged ends 118, which are closed by plugs and surrounded by condenser plates 120. An arrangement for measuring the conductivity of a flowing fluid is shown in Fig. 14. A tube 122 has inserted in it a solid body of dielectric material having enlarged portions 124 spaced apart by portions 126 of small cross-section. Condenser rings 128 are arranged on the tube 122 opposite the portions 126. The condenser plates may be secured by adhesive or by clamps, and may be formed of metal foil.