Patent GB-689066-A (GB)
Title: Improvements in apparatus for measuring the velocity of a gaseous fluid and in particular the air speed of an aircraft
IPC: G01P5/14
Filed: 1950-11-14
Abstract: 689,066. Electric indicating systems. DION, G. H. Nov. 14, 1950 [Nov. 15, 1949], No. 27855/50. Class 40 (i) [Also in Group XXXVI] In apparatus for measuring the velocity of a gaseous fluid, the static pressure p, the dynamic pressure P and the temperature T of the fluid are respectively measured and translated into variable electrical parameters which are supplied to an electric analogue calculator wherein they are combined (according to the law V = K #T #P/α#P+#p) to effect an automatic balance against a variable electrical parameter operating a measuring instrument to indicate the velocity V. In Fig. 1 an electrodynamic pressure sensitive device 34 of the kind described in Specification 671,083, comprises a coil energizing the centre leg of a closed magnetic circuit 28 forming the walls of two chambers 24, 25 separated by an elastic diaphragm 23 carrying a disc 26 of magnetic material capable of attraction by the magnetized centre leg and normally earthing a contact 30 connected to the grid of a thermionic tube 32P, whose anode current flows through the coil and a measuring instrument 33P. The electromechanical characteristics are such that the diaphragm oscillates continuously making and breaking the grid earthing circuit and consequently interrupting the anode current to maintain the oscillation continuously. The chambers 24 and 25 are respectively connected to a Pitot tube so that the latter is held at the sum of the static and dynamic pressures P + p and the former is held at the static pressure p whereby the mean anode current of tube 32P is proportional to #P. Similar electrodynamic devices 37 and 39 have their two internal chambers connected respectively to vacuum and static pressure p, and to vacuum and a hydrogen filled gas thermometer 38, so that the mean anode currents of the associated thermionic tubes 32p and 32T measured by instruments 33p, 33T are proportional to #P and #T respectively. An electrodynamic balance comprises an oscillating member 1, pivoted about an axis 2 which carries moving coils 14, 15 inter-acting with fixed coils 20, 21 about the core of a magnetic circuit 18 and with fixed coil 22 about the core of a magnetic circuit 19. The member 1 carries an earthed contact 3 arranged to make contact 4 or 5 on unbalance in either direction to earth respectively the grid 12 or 13 of a normally cut off double triode 11 allowing the appropriate triode element to conduct and energize motor 10 to drive the earthed arm 8 over a potentiometer 7 carrying a constant current from source 40. The voltage tapped off between points 8 and 9 causes a proportionate current I to flow through the coil 14 of the electrodynamic balance. Coil 15 is energized by the mean energizing current through the coil 27 of device 34 (i, = k#P), the coil 20 by the mean current through the coil 27 of device 37 (i, = k #P), the coil 21 by the current from a variable tap on potentiometer 36 (i, = k a #P) and coil 22 by the mean current through the coil 27 of device 39 (i 2  = k #T). Unbalance of the member 1 makes contacts 3, 4 or 3, 5 and drives the motor to adjust I, rebalance the member and stop the motor. Then I (i, + i 4 ) = Ki 1  i 2  or I (α #P + #P) = 1 2 = K #T #P and since from the above law V=K#T#P/α#P+#P where V = gas velocity T temperature P = dynamic pressure p = static pressure α = a constant K = a constant the value of 1, and the setting of potentiometer 7 indicated on a dial represents the velocity V of the gaseous fluid. In modifications (Figs. 3, 4 not shown) the representative electrical parameters and the potentiometer rebalance current are combined in a resistance bridge network or in a magnetic amplifier.