Patent GB-913659-A (GB)
Title: Improvements in or relating to the recording of freeness of paper making stock
IPC: G01N33/34
Filed: 1959-02-18
Abstract: 913,659. Paper manufacture. AUSTRALIAN PAPER MANUFACTURERS Ltd. Feb. 18, 1959 [Feb. 21, 1958], No. 5650/59. Class 96. [Also in Group XXXVIII] The freeness of paper-making stock is recorded by taking successive samples in freeness testing vessels and measuring the amount of water which drains in a given time, the measurement being corrected for temperature and consistency variations in the stock and the testing vessels subsequently emptied in readiness for further samples. In the form shown in Fig. 1 a flow of pulp from the head-box passes over a weir in the tank 1 at the rear of the machine. Under the head so formed stock is fed to the tank 5 where it is diluted and flows into a delivery channel 2 containing a float coupled to a differential transformer 9. The stock then runs into container 7 normally closed by a valve formed by an apertured disc 6 carried on a slowly-rotating vertical shaft 49 which brings the freeness testing vessels 14, 14a under the container 7 periodically, the valve 6 opening as each passes. The container 7 has an overflow, not shown, which is adjustable so that the accumulated amount of stock fills each vessel 14, 14a without undue spilling. The bottoms of the vessels 14, 14a are closed by standard freeness drainage plates and have wear-resistant rings sliding on a track 45 which is cut away in one sector to allow each vessel to drain for a standard time into a measuring tank, not shown, with a solenoid-operated valve operated by a switch controlled by a cam 61, Fig. 7, driven by the shaft 49. The tank contains a float 49K coupled to a differential transformer by which the volume of water collected is measured. After measurement the valve opens to drain the water and then closes in readiness for the next test. The vessels 14, 14a are carried on shafts projecting from the shaft 49 and each carries a gear which, after the stock has drained, engages a fixed rack which inverts the vessel to discharge the tested sample. The vessel is then rinsed by jets of water before being erected to receive the next charge of stock. The electrical circuit which provides a corrected measure of the freeness of the stock sample is shown in Fig. 7. The float 49K in the measuring tank carries a steel rod which forms the core of a differential transformer having two sections side-by-side with primary windings 50 connected in parallel and fed with A.C., the secondary windings being series-opposed. The output voltage produced in dependence on the position of the float is fed to an amplifier comprising valves V 1 , V 2  and is then rectified by one of the diodes in V 3 , the rectified voltage at the cathode of which charges a capacitor 57 in the cathode circuit of a diode V 5 . The peak value of the voltage produced on this capacitor represents the freeness value to be recorded. This voltage is applied to one valve V 15 , of a pair V 14 , V 15  which are adjusted, in the absence of signal, to equality of cathode potential by a bias control 109. Any difference of cathode potential is fed to a recorder through resistances 90, 91. This output is corrected for variations of temperature in the measuring tank by a thermistor 63 in the tank, coupled in a bridge circuit 62. Variation in temperature of the thermistor produces a voltage, depending in magnitude and phase on the magnitude and direction of the variation, which is amplified by valves V 6 , V 7  and then fed to a phase discriminator rectifier including valves V 7 , V 8  to produce a D.C. voltage output of corresponding magnitude and polarity. A fraction of this voltage, adjustable by a potentiometer 70, is fed to the grid of the valve V 14  to vary the cathode potential difference fed to the recorder. Another correction is derived from the differential transformer 9 which has a magnetic core coupled by a non-magnetic rod to a float 7 in the channel 2 which, when the consistency of the stock in the channel has an optimum value, maintains the core midway between the two sets of transformer windings so that no output is fed to the valve V 10 . Any movement from this position produces a voltage, depending in magnitude and phase on the size and direction of the movement, which is amplified by valves V 10 , V 11  and then fed to a phase discriminator rectifier including valves V 12 , V 13  to produce a D.C. voltage output of corresponding magnitude and polarity. A fraction of this voltage adjustable by the potentiometer 110, is also fed to the grid of the valve V 14 . In order that the system may respond rapidly to lower freeness values a switch operated by a cam 60 connects a leakage resistance 59 across the capacitor 57 for a period in each operating cycle.