Patent GB-1018697-A (GB)
Title: Non-destructive thickness measuring device
Inventor: BARNES AL-G
IPC: G01B17/02
Filed: 1964-08-04
Abstract: 1,018,697. Ultrasonic gauging. CONTINENTAL OIL CO. Aug. 4, 1964 [Sept. 3, 1963], No. 30543/64. Heading H4D. In an ultrasonic thickness gauge in which the time of travel of an ultrasonic pulse transmitted by a transducer 16 through the front surface 44a (Fig. 3) of a specimen 44 and detected by the same transducer after reflection from the back surface 44b of the specimen is determined by a digital counter 64 counting clock pulses from a generator 34, the echo pulses resulting from first and second reflections at the back surface 44b are used to respectively start and stop the counting, the transmitted pulse and the pulse reflected from the front surface 44a being prevented from actuating the counter in order to reduce errors due to the coupling between the transducer 16 and the specimen 44. In the embodiment of Fig. 3, the transmitted pulse, followed by an echo pulse from the interface 44a between a coupling fluid 46 and the specimen 44, then by echo pulses resulting from multiple internal reflections in the specimen 44, are amplified and shaped by a receiver 52 and used to trigger bi-stable flip-flops 58, 60, 62. Output signals from these flip-flops are suitably combined and control " AND " gates 54, 66 such that clock pulses from the generator 34 are fed to decade counters 68, 70, 72 only during the interval between the third and fourth pulses received by the receiver 52. The frequency of the generator 34 may be made proportional to the velocity of ultrasonic waves in the specimen 44 such that the counters 68, 70, 72 provide a direct read-out of specimen thickness in hundredths of an inch. A control generator 12 is used to periodically trigger a transmission pulse generator 14 and to simultaneously reset the flip-flops 58, 60, 62 and counters 68, 70, 72, so that repeated measurements of specimen thickness may be made automatically and sufficiently rapidly for a visual read-out to appear continuous when luminous indicators are used in the counter 68, 70, 72. In the embodiment of Fig. 1 (not shown) no fluid coupling 46 is used: the echo from the specimenliquid interface is therefore absent and the digital counter is actuated between the second and third received pulses, only two bi-stable flip-flops 22, 26 being used. In an alternative embodiment shown in Fig. 5, the blocking of pulses received prior to the required first and second echo pulses from the back surface 120b of a specimen 120 is achieved by the use of a monostable flip-flop 126 which opens a gate 125 after a predetermined time delay. Echo pulses passing through gate 125 to a receiver 142 trigger bi-stable flip-flops 152, 160 whose outputs are combined and, in conjunction with pulses obtained directly from receiver 142, control " AND " gates 148 and 164 such that clock pulses from a generator 168 are fed to decade counters 172a, 172b only during the interval between the first and second echoes from surface 120b of the specimen 120. If the thickness of the specimen being measured is beyond the range of the counters 172a, 1726, a signal from counter 1726 is transmitted through a conductor 184 to the logic circuitry to shut off the gate 148 and thereby stop the operation of the counters 172a, 172b. Examples of ultrasonic pulse frequencies and pulse repetition frequencies are given.