Patent GB-1182137-A (GB)
Title: Improvements in Manometers or Pressure Transducers
Inventor: GREER JAMES ROBERTSON BOUVARD
IPC: G01F1/38
Filed: 1967-07-04
Abstract: 1,182,137. Spirometers. MERCURY ELECTRONICS (SCOTLAND) Ltd. 4 July, 1968 [4 July, 1967], No. 30756/67 Heading A5K. [Also in Divisions G1 and H3] Pressure measuring apparatus comprises a pressure measuring transducer developing an electrical signal related to a fluid pressure or pressure difference, valve means which are operated in order to establish a datum condition in said transducer and means operated synchronously with the valve means so as to establish a feedback path in the measuring circuit to which the transducer output signal is applied, thereby to correct for errors in measurement due to drift in the system. The invention is described with reference to a photoelectric manometer for measuring animal respiration. The manometer shown is a differential device comprising a measuring chamber 5 divided in half by a light reflecting membrane 6. The two halves of the chamber are connected by conduits 3, 4 to a spirometric head 1, having a gauze member 2, and being connected by conduits 1a, 1b to the respiratory path of the animal. The pressure difference between the two halves of chamber 5 is sensed by a differential arrangement of photoresistors 10, 11 receiving light from lamp 7 reflected off the sides of the membrane 6, via mirrors 8, 9 and the transparent end walls 5b, 5a of the chamber. The potential appearing a junction 12 is applied via potentiometer 13 to an operational amplifier 16, the output of which is indicated by a meter 19. The output of the amplifier 16 may also be connected to a feedback network including a potentiometric and resistive network and an amplifier 29, the output of which is indicative of any discrepancy due to drift, between the output of amplifier 16 and the median potential of the d.c. supply, the manometer at this time being in a datum condition. This condition is brought about by the valving system 60, 64 being controlled, by closing of relay contact A2 and operation of solenoid 58, to equalise the pressures in the two halves of the chamber 5. At the same time as contact A2 closes, the contact A3 in the feedback loop is closed. This allows for any correction necessary to be applied to the amplifier 16. At times when contact A3 is open, any correction required to be applied to the amplifier 16 is stored by capacitor 32. The contacts A2, A3 may be closed by operation of relay A/3, and the datum check thereby carried out, periodically or at will. In the latter case, a push button 55 is closed to connect relay A/3 through resistor 56 to the supply. One periodic arrangement utilises capacitor 46 unijunction transistor 49, the capacitor being periodically charged, via resistor 47, and rapidly discharged, to produce a voltage pulse across resistor 50 which briefly switches on transistor 51, causing energisation of relay A/3. A relay holding circuit 52, 53 including changeover contact A1 also controlled by relay A/3, is provided, together with transistor protection diode 54. The second periodic arrangement is operated in time with a periodically occurring signal from amplifier 16 e.g. caused by rhythmic breathing. A threshold switch 62 either senses the arrival of a signal to prevent passage of a pulse from unijunction 49 from actuating the relay A/3 (thereby preventing a datum check), or senses the termination of such a signal and causes direct operation of the relay (thereby initiating a datum check). Other circuit features include capacitors 17, 31 which prevent oscillation and switching effects, a Zener diode 45 connected in series with the lamp 7, and the stabilised half-wave rectified supply. Instead a differential manometer, a simple manometer may be provided in which case the valving arrangement is adapted so that during datum checks the manometer is opened to the atmosphere. A single, complex valve may replace valving arrangement 60, 64, 65.