GB1238172A - - Google Patents

Abstract

1,238,172. Electrical measurements. NATIONAL RESEARCH DEVELOPMENT CORP. Nov.11, 1968 [Aug.11, 1967], No.37103/67. Heading G1U. In a measuring system in which output signals from an impedance network - e.g. unbalance signals from bridge 11 - are indicative of the variable being measured and are amplified by an amplifier of selectable gain, provision is made for sequence of reference output signals of predetermined magnitudes to be produced, e.g. for calibrating a pen recorder fed by the amplifer, a different sequence being produced for each value of amplifier gain. In the embodiment shown the bridge comprises four strain gauges 12-15. The reference output signals, representative of particular strains, are produced by altering the impedance ratio of a pair of supplementary bridge arms so as to unbalance the bridge. The supplementary arms comprises resistances 37, 38, initially made equal by zeroadjuster 26, so that provided transistors 43, 46, 49 are blocked the bridge is balanced in the absence of a strain to be measured. By unblocking any one of these transistors, however, one of three reference signals is produced at the bridge output, respectively corresponding to the unbalance caused by shunting resistance 38 by resistance 28 alone or in series with resistance 30 or 29. Switching circuits control the transistors 43, 46, 49 so that on momentary depression of switch 35 a sequence of all three, two or one of the reference signals is produced Fig.4 (not shown), depending on the position of switch 34, ganged to the amplifier gain (sensitivity) selector switch 36. In this way three, two or one reference levels are recorded by the pen recorder, the number of levels so recorded also serving to identify the sensitivity range in use. The reference signal sequences may alternatively differ in their order rather than in the number of signals they contain.