Patent GB-757850-A (GB)
Title: Moisture measuring apparatus
IPC: A24B9/00
Filed: 1954-08-13
Abstract: 757,850. Treating tobacco. AMERICAN MACHINE & FOUNDRY CO. Aug. 13, 1954 [Sept. 24, 1953], No. 23548/54. Class 130. [Also in Groups XXXV and XXXVI] Apparatus for measuring and controlling the moisture content of tobacco comprises a pair of electrodes 12, 14, Fig. 1, placed on either side of a conveyer belt 10 for determining the initial moisture content as the tobacco T enters a predryer 24 to establish the conditions in the predryer and a further pair of electrodes 104, 106 measuring the final moisture content of the tobacco, to give signals which retroactively control the conditions in a main dryer 70 to maintain the final moisture content at a pre, determined value. The inside of the predryer 24 is fed with heat by steam supplied through a pipe 30 controlled by a valve 34. The air inside the predryer is made to circulate by a blower 38, the air flow being heated at 42 to an extent automatically controlled at a valve 44 by thermostat 46, and fed past a damper 48 controlled by the initial moisture content of the tobacco as measured by electrodes 12, 14. The inside of the main dryer 70 is fed with moist, heated air by a blower 76, the heat being supplied by. steam coils 80 via a valve 86 automatically controlled by device 84 in accordance with the temperature measured by a thermometer 82, and the moisture being provided by a humidifier 88 via a valve 92 automatically controlled by device 90 in accordance with the humidity measured by a hygrometer 89. The settings of control devices 84, 90 are adjustable manually, or automatically by the readings of final moisture content obtained from electrodes 104, 106 and automatic weighing devices 110. Indications are finally obtained on a control box 118 of instantaneous weight of tobacco, total weight, total moisture content, and the ratio of moisture to total weight. Moisture measuring circuit (Fig. 2). The capacitor formed by the electrodes 12, 14 with the tobacco T as dielectric is connected in a bridge circuit fed at centre-tapped coil 208 from an H.F. oscillator MO, and balance adjustable by variable condenser 214 and differential condenser 212. Any unbalance signal is mixed in one half of a double-triode valve 228 with a signal obtained from the other half connected as an oscillator, to give a signal proportional to the unbalance voltage, which after amplification at valves 244, 246, 248 is rectified at voltage doubler 250 to be indicated at meter 17 and on magic eye valve 252, and to provide a voltage at plug 299 for control purposes. The bridge is normally unbalanced to discriminate between high and low values of capacity between electrodes 12, 14. Servo-amplifier circuit (Fig. 3).-The measuring signal at plug 299 is integrated and fed to one half of double-triode valve 310, the two valve halves being connected in a bridge circuit with the centre-tapped primary of AC fed transformer 312. After initial balance at potentiometer 332 to a predetermined value, any unbalance voltage from the bridge alters the bias voltage of the grid of valve 314 or 316 to change the current in field winding 320 of motor 66, causing it to rebalance the bridge by moving the arm 326 of potentiometer 328, at the same time rotating cam 62 thereby adjusting the damper 47, Fig. 1, in the path of blower 38. A similar servo-amplifier is provided in control box 118, the position of arm 334 of potentiometer 332 being adjusted in accordance with the total weight of tobacco flowing between electrodes 104, 106 and the servo-motor 66 adjusting the arm of a further potentiometer to give a signal indicative of the ratio total moisture to total weight. This signal is sent through a further servo-amplifier 130 of the same type, whereby control device 90 adjusts the steam input to dryer 70 in accordance with this ratio.