Patent GB-871818-A (GB)
Title: Improvements in electrical bridge measuring networks
IPC: G01R17/10
Filed: 1959-03-17
Abstract: 871,818. Measuring-bridges; electrolytic testing. TRANS-WEIGH CO. March 17, 1959 [March 18, 1958], No. 9233/59. Class 37. A measuring network comprising a Wheatstone bridge with input and output terminals and a condition-responsive impedance, e.g. a strain gauge resistance thermometer, or conductivity cell, has a first impedance constituting a fixedly tapped potential divider connected across one pair of bridge terminals through a second impedance, and a potential dividing third impedance connected between the tap of the first impedance and one of the other pair of bridge terminals; the outer terminals of the first impedance being connected as a first conjugate pair of network terminals and the tap of the third impedance and the other bridge terminal being connected as a second conjugate pair of network terminals; while one conjugate pair is connected to a current supply and the other is connected to a detector responsive to the difference between the bridge unbalance voltage and a voltage derived from the supply by the interconnection of the first, second and third impedance with the bridge. In Fig. 1A a Wheatstone bridge comprises equal resistances RA in arms 4, 9 and 9, 6, resistance Ro in series with lead resistance RL in arm 4, 10, 10<1>; and resistance Rx (which may be a strain gauge, resistance thermometer or conductivity cell) in series with lead resistance R<1>L in arm 10<1>, 61, 6. A resistance potential divider RK, R<1>K shunted across source 17 is connected to diagonal terminals 4, 6 through series resistance R1 while the junction of RK, R<1>K is connected to diagonal terminal 9 over variable resistance potential divider RT, whose slider is connected through indicating or recording detector 12 of resistance RK and lead resistance RL to the remaining' diagonal terminal ; the bridge unbalance current being annulled in the detector by a further current derived from the potential divider RK, R<1>K and adjustable e.g. by the slider of RT. It is shown that, for balance n<1> is the fraction of R<1>T between point 9 and the slider 14 of RT, so that the balance point is independent of lead resistance RL and the supply voltage. Similar conditions are stated to hold for reactive impedance networks, in which the supply is derivable from a transformer having a tapped secondary also serving as potential divider RK,R<1>K. A further resistance R2 may be inserted between potential divider RK, R<1>K and diagonal terminal 6, when for equation 1 to hold. The supply and the detector are interchangeable without affecting the above relations, and impedances Ro, RT or n<1> are adjustable for balance and may be calibrated to indicate Rx or a condition to which it is responsive. It is shown that n<1> is a linear function of the measured variable. The detector is replaceable by a rebalancing servo operating an indicator, recorder, integrator, or control mechanism for the measured condition; or the network may be manually balanced to a reference value so that deviation unbalance signals may operate control relays. For temperature compensation, a portion R<1>0 of the reference resistance R0 having the same rate of resistance temperature change as Rx may be located adjacently to the conditionresponsive resistance Rx, with a variable calibrating resistance of valve Rp shunted across the series combination (Fig. 3A, not shown) and by conversion to an equivalent Y-network (Fig. 3B, not shown) the balance condition is given by Plural condition-responsive Rx resistances may be switched into a single measuring network or multi-point ganged switches selecting alternative points 10<1> and 4 of the resistances Rx for connection to the indicator and to resistance RA of the bridge; reference resistance Ro being incorporated in each lead resistance RL and the resistances Rx being returned directly to the common point 6 of the bridge (Fig. 4, not shown). The bridge 15A may be located remotely from the remainder of the network (Fig. 5A) and the arms may comprise equal resistances R variable in response to strain so that opposed pairs respectively increase and decrease by equal amounts #R. Points 4, 6, 9, 10 are respectively connected over leads of resistance RL to R1' R2, RT and indicator 12 respectively, and it is shown that for balance where n is the fraction of RT included between point 9 and slider 14. As before the slider may be automatically adjusted for balance to operate an indicator, recorder, or control of the measured condition. Temperature compensation may be achieved by giving a portion of R1 a positive and a portion of R2 a negative temperature coefficient.