Patent GB-771637-A (GB)
Title: Continuously indicating electroacoustic densitometer
IPC: G01N29/34
Filed: 1955-06-13
Abstract: 771,637. Measuring physical properties by acoustic waves. KRITZ, J. June 13, 1955 [June 17, 1954], No. 16940/55. Class 118 (2). [Also in Groups XXXVI and XL (c)] The density p of a fluid medium is determined by producing a control D.C. voltage having a component of amplitude A proportional to the acoustic impedance pv of the fluid, generating a recurrent rectangular wave 33 having a period T inversely proportional to the propagation velocity v of acoustic waves in said medium, producing a corresponding wave 182, Fig. 4, having a peak-to-peak amplitude A equal to said component of said control D.C. voltage, integrating said corresponding wave 182 to give a triangular wave 48 having a peak-to-peak amplitude which is proportional to the product of A and T and thus is directly proportional to p and deriving an indicating D.C. voltage proportional to said peak-to-peak amplitude which is applied to a meter indicating density. As shown in Fig. 1, the acoustic impedance is determined by applying the output from a crystal-controlled oscillator 16 through a variable inductor 17 to a piezo-electric transducer 13 located in a lateral extension of the duct 11 containing the fluid 10 with one face 14 of the crystal in contact with the fluid. When inductor 17 is adjusted to resonate the load on oscillator 16 the amplitude of the electrical oscillations across the transducer 13 is proportional to the acoustic impedance of the fluid 10 plus a constant factor due to the impedance of the crystal holder and the required control D.C. voltage is produced by rectifying said electrical oscillations in detector 18. In order to compensate for losses in the output circuit of oscillator 16, chiefly in inductor 17, an adjustable portion of the output voltage from detector 18, Fig. 2 (not shown), is applied to vary the amplitude of the output from oscillator 16 in the same or opposite sense depending on the nature of the fluid under test. The rectangular wave 33 having a period inversely proportional to the fluid wave propagation velocity v is produced by coupling a high-gain amplifier 28 to two crystal transducers 20, 24 mounted in a similar manner to transducer 13 on opposite sides of the duct 11 such that an electroacoustic closed oscillatory loop is established. The transducers 13, 20 and 24 are inclined at equal angles a and b to the axis of the duct so that errors are not introduced by flow of the fluid within the duct. The rectangular wave 33 from amplifier 28 and the D.C. control voltage from detector 18 are applied to a clamp circuit 37 to give a corresponding rectangular wave 47 whose positive amplitude is proportional to the control voltage and the positive-going portions of this wave having an amplitude greater than a fixed threshold value corresponding to the constant component of the control voltage due to the impedance of the crystal holder are selected by a limiter valve 132, Fig. 4 (see below), and integrated at 46 to give a triangular wave 48 which is applied to a peak rectifier 55 giving a D.C. voltage proportional to density which is applied to indicator 60. Circuit details, Fig. 4. The oscillator 16, Fig. 1, comprises a pentode 66 with a tank circuit 83, 84, 85 across which is connected a limiting diode 87 cathode coupled to a cathode follower 91 such that the peak-to-peak amplitude of the oscillator output is proportional to the positive bias applied to the grid of valve 91. The detector 18, Fig. 1, comprises a cathode follower 100 coupled to a cathode follower 102, the D.C. control voltage from which is applied to the grid of valve 91 to control the output of oscillator 66 and to the anode of the triode clamp 111 (37, Fig. 1). The cathode of clamp 111 is maintained at a negative voltage and the rectangular wave 33 from amplifier 28 is applied to its grid via valve 119 such that during the positive half-cycles of wave 33 valve 111 is conductive and its anode is clamped to the negative cathode potential but during the negative half-cycles the valve is cut off and the potential of its anode is equal to that of the control voltage from cathode follower 102. The resultant output wave 47 from the clamp 111 is applied to a cathode follower limiter 132 which passes those portions of wave 47 of amplitude greater than the fixed positive potential of its cathode and the resultant wave 182 is integrated to produce the triangular wave 48 in a Miller integrator comprising a pentode 141 with its anode coupled via cathode follower 142 both to its grid and its cathode via capacitors 160 and 161 respectively. The negative peaks of the output wave 48 are clamped to earth by diode 165 and the resultant wave 184 is peak rectified by diode 166, the resultant D.C. voltage 185 being applied to meter 60 through cathode-follower 170. Meter 60 can be calibrated when using a fluid of known density by adjustment of the input resistor 140 of the Miller valve 141 and the output tapping 176 in the cathode circuit of cathode follower 170.