Patent GB-737804-A (GB)
Title: Improvements in remote electrical indicating or/and control systems
Inventor: WHITEHEAD CYRIL CRYER
IPC: G01F23/292
Filed: 1952-12-09
Abstract: 737,804. Electric indicating systems. BAILEY METERS & CONTROLS, Ltd Dec. 9, 1953 [Dec. 9, 1952], No. 31199/52. Class 40 (1). [Also in Group. XL (b)] A device for indicating liquid level at a distance or for operating warning devices if the level falls and rises too far comprises a flying spot scanner traversing the filled and unfilled parts of the gauge and affecting a light-sensitive device differently depending on what part of the gauge it is traversing. As shown, Fig. 1, a blue rectangular raster produced on the screen, of a cathode-ray tube 13 traverses vertically the face of the gauge 1 which is provided with a slot behind which is the gauge glass. Behind the gauge glass is a mirror strip 21. The section of the gauge glass is shown in Fig. 2, and the arrangement is such that if the light beam is traversing a part of the gauge glass filled with liquid it is refracted as to miss the mirror. If the beam traverses an empty part or a part filled with vapour such as steam the light beam impinges on mirror 21 which is so shaped as to direct the light back to a photocell 21. This cell may be of the caesiumantimony type with a good blue response. Synchronizing pulses generated by the timebase 14 forming the raster on the screen of the cathode-ray tube are combined with the output of photo-cell 17 and fed to a wide-band amplifier and synchronizing pulse mixer 27. The receiver comprises a synchronizing pulse separator 30 a local time-base generator 31 and a cathode-ray tube 32. Thus a further raster is produced in tube 32 which is in effect a visual representation of the gauge face and of the length of the liquid in the gauge. Further receivers 29<SP>1</SP> may be provided. The flying spot scanning may be effected mechanically by a Nipkow disc or mirror screw. In a modification (Figs. 5 to 7, not shown), a photo-cell of elongated construction is arranged behind the gauge so as to receive light transmitted by it and not refracted while another cell situated in front receives light reflected by the gauge plate. The two photo-cells are parallelled and feed a common indicator. In the arrangement of Fig. 8, a slot 46 in the gauge plate beside the slot for the gauge glass is traversed by bars 47 corresponding to the gauge plate indications which are reproduced in the receiving cathoderay tube thus enabling the level of the liquid to be read directly. Instead of an elongated photo-cell a reflector and a normal cell may be used so that the cell receives all light traversing but not refracted by the gauge glass and its contents. Fig. 14 shows a system in which the output of the elongated photo-cell 35 is used to operate warning devices if the liquid level falls or rises too much. The outputs of the two cells 17, 35 are applied through pentode amplifiers 55<SP>1</SP>, 58<SP>1</SP>. The output from valve 551 is transmitted to a triode cathode follower valve 57<SP>1</SP> and from there to the synchronizing pulse mixer to operate the remote indicator. Valve 58<SP>1</SP> also feeds a cathode follower 561 of pentode-type, the output of which is also fed to triode cathode follower 57<SP>1</SP>. Pentode 58<SP>1</SP> feeds a further cathode follower 59<SP>1</SP> and an amplifying pentode 60<SP>1</SP>a and 60<SP>1</SP>b, and a clipping pentode 61<SP>1</SP> which levels the tops of the pulses produced by photo-cell 35. These pulses are now of uniform amplitude but of length depending on the level of liquid in the gauge glass. The clipped pulses are rectified by valve 62<SP>1</SP>b and develop a voltage representing liquid level across a condenser 62<SP>1</SP>d. This voltage is used to trigger A.C. fed thyratrons 63<SP>1</SP>a and 67<SP>1</SP>, each having an anode relay the contact of one of which is biased " on " by a spring and the other " off ". The thyratrons fire at different voltages and the arrangement is thus that one relay closes its contacts to operate an alarm if the level is too high and the other if it is too low. In a modification of this system (Fig. 15, not shown), each relay controls a two-way switch the contacts of which are so interconnected that as to operate a single warning device when the level is too high or too low. In a further modification (Figs. 16 to 18, not shown) a glass plate is positioned between the gauge glass and the rear photo-cell to divert light to additional photo-cells which are vertically adjustable to operate warning devices at any chosen liquid level. The circuit of the photo-cell is arranged to differentiate the pulses which it produces at line frequency and to keep a circuit tuned to line frequency in oscillation developing a voltage which maintains a gas-filled relay conducting and thus holding open an electromagnetic relay, two such devices one operating a relay biased to close and the other biased to open operate indicators for the top and bottom levels being provided. Specification 392,642, [Group XVIII], is referred to.