Patent GB-675457-A (GB)
Title: Improvements in methods of and apparatus for calibrating measuring systems for constituent potentials
IPC: G01N27/12
Filed: 1950-10-23
Abstract: 675,457. Electric furnace control systems., LEEDS & NORTHRUP CO. Oct. 23, 1950. [Oct. 22, 1949] No. 25769/50. Class 40 (i). [Also in Groups II and XXXV] A method of calibrating the measuring system of apparatus controlling and measuring the degree of carbonization of ferrous material, e.g. as described in Specification 646,078, in which the change in resistance of a ferrous wire is proportional to the change in its carbon content concentration, makes use of the fact that the point of minimum carbon concentration is fixed and can be reproduced for calibrating purposes. Use is also made of the fact that if the carbon-reducing means is capable of oxidising the wire, its resistance will reach a minimum and then begin to rise with initiation of oxidation. A carburizing furnace 10, Fig. 1, having a valve 11 for regulating the supply of carburizing agent, contains a carbon-sensitive resistance element 12, e.g. of iron wire, connected in a bridge circuit comprising resistors 18, 19, 20, a temperature-compensating slidewire resistor 13, automatically regulated by a temperature-responsive element 24 through control device 25, and a network-balancing slidewire resistor 14, automatically controlled by a detector and driving means 15, through gears 21a, 31, the balance point on slide-wire 14 being indicated on scale 14a, and recorded on a chart 23. The bridge is energized by D.C. through a calibrating slide-wire 26 also controlled by control device 15 through a drive connection 28 which can be made to engage with gear 31 by a movable lever 30. To calibrate the bridge, the furnace is fed with a known carbon-reducing atmosphere, when the carbon content of element 12 is reduced to a known fixed minimum, having a known corresponding resistance, the slide wire 14 is brought to its known calibrating scale-point, and slide-wire 26 is adjusted by the drive 15 to rebalance the bridge. The calibrating procedure is initiated by pressing button switch 40, thereby energizing relay 42 to complete its holding circuit, energizing a signal lamp 48, the operating coil 49 of a valve 50, and a timing motor 51. The valve 50 admits air to the housing 53 containing the element 12, so that a normally operating pump 54 draws this air through the housing 53 at such a rate that it is first heated and then allowed to decarburize the element 12. The resistance of element 12 falls to its minimum value, causing the detector and drive 15 to move its shaft 32 to the left, taking with it a nut 56 which is in frictional engagement with a carriage 59, so that a reversal in direction of movement of the carriage rotates a crank-arm 62 to close contacts 63, or the nut 56 strikes a crank-arm 58 on the extreme left to close contacts 57. Either of contacts 57, 63 together with contacts 55 closed by the timing device 51 after a short delay, close the energizing circuit of relay 66, which completes its holding circuit and energizes coil 69 to attract armature 70 to cause the drive connection 28 to engage with gear 31, and also to operate change-over switch 73, thus replacing the upper branch of the bridge by another branch containing fixed resistors 33, 34. If at the moment of transfer the slide-wire 14 is not at its calibrating scale point, the bridge is not balanced, and the detector and control drive 15 moves the point of contact to rebalance, simultaneously adjusting slidewire 26 a corresponding amount. A. timing device 76, connected in parallel with relay 66, short-circuits this relay after a predetermined interval, thereby causing the return of the circuit to normal. In a modification, Fig. 6, the supply of air or carburizing agent is controlled by a three-way valve 100 operated by a solenoid 101 and joined to the pump 54 so as to draw the carburizing medium into the housing 53 during normal processing, and to pump air into the housing during calibration. In this modification, the bridge 122 which is fed with - A.C., contains two calibrating slide wires 128, 132, a temperaturecompensating. slide wire 126, and a balancing slide wire 130. The unbalance diagonal of the bridge contains an A.C. amplifier 120 which, through relay 114, controls the valve 112 regulating the supply of carburizing agent to the furnace 10, and a parallel diagonal includes indicating apparatus such as a galvanometer 152, and a recording chart 155. To initiate the calibration operation, push button 156 is . operated to energize relay 144, which completes its holding circuit, and energizes solenoid 101- to change over the three-way valve 100 to decarbonization of the element 12-and lights .a lamp 173. The unbalance voltage of the bridge is amplified in 120 and causes relay 119 to close the energizing circuit of a motor 180 via field winding 178 or 179 to drive the calibrating slide wires 128, 132 in the appropriate direction. The bridge connection to the slide wire 130 is also transferred to the junction of fixed resistors 134, 135 corresponding to the known minimum resistance value of the element 12. When balance has been regained with slide wires 128, 132 in their corrected positions the circuit returns to normal, and the three-way valve 100 is operated to allow carburization in the furnace 10, and of the element 12 to continue. Values of the various resistors are given by way of example for particular concentration of carbon content, and also sizes and materials for suitable sensitive elements.