Patent GB-827112-A (GB)
Title: Electric current differential measurement system
IPC: G01N27/48
Filed: 1957-08-29
Abstract: 827,112. Electric tests. FOXBORO CO. Aug. 29, 1957 [Sept. 25, 1956], No. 27302/57. Class 37. Apparatus for measuring the concentration of an additive to a liquid, such as that of scaleinhibiting hydrazine to boiler water, comprises an arrangement of three electrodes of which the central one has an applied D.C. reference potential (e.g. zero) and the two outer ones different applied indicator D.C. potentials, the degree of unbalance of the currents flowing in the two circuits 25<SP>1</SP>, 25<SP>11</SP>, Fig. 1, connected to the electrodes A 1 , C; C, A 2  respectively being indicated on a voltage-measuring device 32. The voltages applied to the outer electrodes are respectively above and below the region of maximum gradient in the current-voltage curve so that the difference in their currents is proportional to the concentration of the additive. The measuring circuits each have D.C. sources 26, 26<SP>1</SP>, variable resistors 29, 291 and fixed resistors 27, 27<SP>1</SP>. By operating electrode A 1  at or near the threshold of the breakdown region of the electrical current flowing through the fluid, i.e. where the fluid begins to decompose so that circuit 25<SP>1</SP> carries only fluid conductively current (e.g. at <SP>1</SP>/ 10  volt) and by operating electrode A 2  at a potential (e.g. at <SP>1</SP>/ 2  volt) where decomposition current plus conductivity current is collected (see Fig. VII) the differential current indication given by 32 is of decomposition current only as is desired. If the water or other fluid contains several additives the fluid electrical current-electrode voltage curve is as shown in Fig. VIII with a separate kink X, Y, Z corresponding to each additive so that by giving C, A 1 , A 2  suitable potentials either X, Y or Z can be " bracketed," so that the concentration of any of the additives can be separately determined. If the conductivity of the liquid changes with time, e.g. if the concentration of the fluid changes but the hydrazine concentration remains the same the differential measurement still gives an approximately correct result, Fig. IX (not shown). If the temperature varies the fluid electrical current will change in the same sense, Fig. XI (not shown), so that temperature compensation means must be provided. This is achieved, Fig. X (not shown), by placing a temperature-sensitive resistance in the liquid and connecting it to one arm of a Wheatstone bridge, the bridge being in series with the measuring unit. Thus when the temperature varies the Wheatstone bridge is unbalanced and a correcting voltage is applied to cancel out the temperature variation in fluid electrical current. In an alternative temperature compensating circuit, Fig. XII (not shown), a variable resistance whose value is varied by a temperaturesensitive device is added in the A 2 -C circuit. No temperature compensation arrangements are provided in the A 1 -C circuit in either of Figs. X or XII since the conductivity current component through the fluid is virtually unaltered by temperature variations. The three electrodes A 1'  A 2 , C are of platinum and can be perforated plates or grids, the perforations causing the formation of a uniform diffusion layer in the liquid. In a practical embodiment, Fig. III, the electrodes A 1 , C, A 2  are mounted on washers 43, 44, 45 held between spacers 41, 42 by plates 46, 47 fitting on shoulders 48, 49 and three bolts 50 passing through the plates 46, 47. Radial alignment is achieved by sleeve 51 which has a longitudinal slot for the passage of electrode connection fins 53 and for radial supporting arms integral with the electrodes. Rotation of the assembly is prevented by a key and slot arrangement, Fig. IV (not shown). O sealing. rings 40 in caps 37, 38 prevent escape of water. The complete circulation system is shown in Fig. I in which hydrazine is fed from storage tank 16 with valve adjustable feed 17 to water supply 10 so that a mixture of hydrazine and water collects in storage tank 11 most of which is fed to boiler 13 but samples of which are drawn off for measurement of the hydrazine concentration at 19; the liquid emerging from the measuring apparatus is fed back to storage tank 11 or allowed to go to waste.