Eine neuartige, frequenzunabhängige Wechselstrom-brückenschaltung für Präzisionsmessungen mit rein Ohm'schen Widerständen

Abstract

To measure seawater parametersin situ a dc or an ac bridge is applied in general. The first type of bridge cannot be used directly in greater depths. Therefore the author has proposed for more than twenty years to apply an ac bridge for such purposes and in such a way that the parameters to be measured are transformed through the bridge either into a frequency of an oscillator, which depends on the parameter values or in a time interval which can be counted out. In both cases there are no difficulties to transport the parameter values via a cable from any depth. In ac bridges inductive or capacitive resistors are needed and the accuracy of the measurement is limited by the drift phenomenon of such resistances. An ac bridge with real resistances, being superior to those resistors, is proposed. It uses two ac voltages with constant frequency. In this case it is necessary to get an alternating voltageU for the one path of the bridge and an alternating voltagej· U with a phase angle of 90° in the other path. These bridges and some possible realizations of circuits are described in this paper. It could be shown that it is possible to reach an accuracy in measuring, for instance a temperature dependent resistor, of at least 20000:1 in one range. So a temperature value can be measured over a range of 40°C with an accuracy of at least 0,002°C, with a time interval of about 1 msec.