Improvements in direct reading ph meters

摘要

612,155. Measuring physical qualities. WILKINS, S. R., and TUNNICLIFFE, E. J. May 13, 1946, No. 14385. [Class 37] A direct reading pH meter comprises a cathode coupled bridge type valve voltmeter to which a glass cell electrode is connected via Sc. Triode A functions as a millivolt measuring valve, while triode B provides the off-set voltage for reference, the out-of-balance current being read on a meter M direct in pH units. Two separate potential dividers are provided across the stabilised D.C. supply for the grids of the two valves A and B. The grid of A is normally returned to a point x via a switch SW2, x being closed to give a linear operation of the valve A without grid current flowing. The grid of valve B is returned to a point y of the second potential divider via a switch SW3, y being on a potentiometer Z2 so that the two anode currents may be balanced for a nil reading of the meter M. A further potentiometer T2 provides a backing-off voltage when the ganged switches are in position R2, such that the reading of meter M is reduced to zero when the voltage input corresponds to 7 pH, thus extending the range of the meter to read 7-15 pH on position R2. The position Z is provided for zero calibration when the cell is replaced in the grid circuit of valve A by a high resistance H.