GB565461A - Improvements in or relating to electrical peak strain meters - Google Patents

Abstract

565,461. Thermionic valve voltmeters; determining dimensions of materials. AUGHTIE, F. April 22, 1943, No. 9842/44. Divided out of 565,378. [Class 37] A peak strain meter comprises the combination with an electrical strain gauge of equipment having a voltage output related to the strain and a peak voltmeter so arranged as to give an enduring, but not necessarily permanent, record or indication corresponding to the maximum strain applied to the gauge. Fig. 1 is a diagram of a circuit in which peak voltage is determined by charging a condenser and using a thermionic valve as an electrometer. A condenser C1 is connected to the grid of a valve V1, which is preferably an electrometer triode having a high grid insulation. The condenser and valve are enclosed in a lighttight electrical screen S. The strain gauge resistance is connected across terminals T1, T2. When a key K is depressed the condenser C1 becomes charged from the anode battery B1 through a resistance R6, so that the grid of the valve V1 is positive. On opening the switch K, this charge leaks away by the passage of electrons from the filament of the valve V1 to its grid until an equilibrium potential is attained. When a positive potential is applied momentarily to the terminal T1 a similar flow of electrons takes place so that when the terminal T1 subsequently reaches the same potential as the terminal T2 the grid of the valve V1 is left negative with respect to its initial value. This causes a corresponding change in the anode current of the valve V1 which is measured by a valve V2 and meter M. A balancing valve V3 enables the circuit to be adjusted initially by means of a slider resistance R4 so that no current flows through the meter M. The valve V1 may be a screened grid valve .to minimize capacitative reaction back from the anode to the grid circuit. Alternatively, this reaction can be minimized by a neutralizing or bridge connection using a resistance R5 and auxiliary condenser C2, so that the condenser is supplied with a potential proportional but opposite in sign to that of the anode. If the charging time constant is not sufficiently short, a separate diode may be used as the unidirectional path. To correct for errors due to the filament of the valve V1 not having an equipotential surface and having different temperatures along its length, the filament may be supplied with high frequency current, or the filament supply may be switched off a small fraction of a second before the peak occurs. Errors due to the charging time constant may be minimized by applying a voltage wave to the peak voltmeter which, while retaining the correct peak value, is of modified shape as compared with the strain-time record. This may be effected by a controllable switching circuit operated by a circuit which provides a time differential of the strain-time record, as shown in Fig. 2. The output of an amplifier 1 following the strain gauge resistance 2 is applied to the electrometer 3 through a normally open switch 4, which is closed for controlled intervals by a trigger and timing circuit 5, which in turn is controlled or initiated through a differentiating circuit 6 from the amplifier circuit. The differentiating circuit produces a voltage proportional to the slope of the strain-time curve, passing through zero at the maximum value and then becoming negative. The trigger circuit is made insensitive to a positive but sensitive to a small negative voltage, so that it operates on passing the maximum. The timing and switching circuit then connects the electrometer triode to the amplifier for a short time so that the voltage applied to it is a small time section of the complete wave. When there is more than one peak value, extra trigger and switching units may be used so that the second trigger circuit is inoperative until after the first has operated. Strains occurring at a known interval of time after the peak value may be read by inserting a time delay circuit between the trigger and switching units. To determine the strain at a known interval before the peak 'value, a delay network or its equivalent is inserted between the amplifier and the switch. The time delay devices may be in the form of a long cable, an equivalent delay circuit, or the conversion of the voltage into a mechanical movement which is propagated through an elastic medium as a wave and then reconverted to a voltage delayed by the time of propagation of the wave through the medium. The switching operation by which the voltage wave is applied to the voltmeter may be effected in a part of the circuit where the wave form has been modified, thereby minimizing errors arising from voltage disturbances caused by the switching operation. A number of peak voltmeters may be arranged in cascade so that the effective duration of the indication of peak value is progressively increased to a value sufficient to permit easy visual observation. Specification 517,486 is referred to.