Patent GB-693368-A (GB)
Title: Improvements in or relating to electrical bridge measuring instruments employing ratiometers
Inventor: MCBRIDE MAURICE GRAHAM
IPC: G01R27/12
Filed: 1950-06-23
Abstract: 693,368. Electric indicating systems. SANGAMO WESTON, Ltd. Jan. 1, 1951 [June 23, 1950], No. 15775/50. Class 40 (i). [Also in Group XXXVI] In an electrical bridge measuring apparatus providing an indication dependent on two variables, e.g. for differential thermometry or for hygrometry by comparison of wet and dry bulb temperatures, the operating elements of a ratiometer are respectively connected across the output terminals of a double Wheatstone bridge network energized from a common current source, having common ratio arms connected to the source terminals and to the common point of the ratiometer coils and in which two conjugate arms of the network comprise elements which are respectively variable as known functions of the controlling variables; thebridge and ratiometer constants being such as to satisfy two different predetermined current ratios in the coils for two different extreme sets of conditions delimiting the working range of the instrument e.g. so that in differential thermometry the temperature difference may be measured independently of absolute temperature, and in hygrometry the relative humidity may be measured according to a known law connecting temperature difference and absolute temperature, irrespectively of the non- linear responses of the temperature variable resistance elements. The latter (Fig. 1) are connected at A, C in a double bridge having fixed arms F, H and common ratio arms B, G, the unbalance currents ID, IE, of the bridges traversing coils D, E of a ratiometer whose indication is independent of the supply current I; and the ratio ID/IR is related to the variations of the temperature sensitive resistances by a given complex equation involving the values of all the resistances constituting the networks, from which the constants for a required response law may be calculated. In a psychrometer (Fig. 2) identical wet and dry thermometric resistances RD, RW are connected in double bridge as above with equal resistance arms T approximating to the thermometric resistance values at a mean temperature, common ratio arms B, G, and equal ratiometer coils S. For 100% humidity RD=RW and iD/IE =1 and for the minimum humidity to be indicated ID/IE = <. A mathematical analysis is given whereby values of ratio arms B and G are computed to give a correct calibration for relative humidity at both ends of the instrument scale irrespectively of the ambient temperature and non-linear response of the resistance thermometer. The current ratios for maximum and minimum humidity may be assimilated to the indication range of the ratiometer by adding shunt and/or series resistances to one or both of the ratiometer coils Fig: 3 (not shown). Resistance values for a differential thermometer are determined similarly to those for a psychrometer. Measurement errors due to temperature variations of the copper ratiometer windings are compensated by winding resistances B, G, and T partly of copper wire and partly of wire having a low temperature coefficient.