An Inductive Conductivity Meter for Monitoring the Salinity of Dialysis Water

An inductive conductivity meter is described, especially adapted as a salinity monitor for dialysis water. Salinity are given. The principal problems of the inductive conductivity meter result from the low conductivity of electrolytes. The weak coupling due to the electrolyte means that stray coupling must be reduced to a very low level. This has been accomplished by means of a heavy copper eddy-current shield, which reduces the unwanted coupling to a level corresponding to a conductivity increment of the order of 10
^-8
Ω
^-1
. cm
^-1
. The effect of parasitic impedances in the receiver core are minimized by designing this core as a current transformer, rather than a voltage transformer. The operating frequency is 5600 Hz. The circuits are described, including the oscillator, current-sensitive preamplifier, phase-sensitive detector, dc amplifiers, and alarm circuits. The phase-sensitive detector uses ordinary planar transistors and does not require a square wave reference signal.