A newly developed low drift fast response salinity meter

Abstract

Here a new conductivity probe, which is basically a stainless steel tube with a plastic plug with a hole, plugged in one end, is described. The tube is externally insulated and the other electrode is (usually earthed) a metallic slab fixed in the fluid. The electronic used is of a conventional type and uses a 1 kHz signal whose amplitude is modulated by the conductance across the probes terminals. The signal is amplified, rectified, and filtered. The probe is different from the conventional probes, in that the fluid is sucked through the small hole in the probe tip, and only through the fluid in the tip can an electric current flow. Hence, the output and the dynamic response depend, respectively, on the conductivity of the small volume of the fluid in the tip, and how fast it can be replaced. Tests on this and other types of probes, show that this probe has almost no drift and has a satisfactory dynamic response. Probes with different tip geometries have also been tested and show that the frequency response of the probe (typically 200 Hz) does not change significantly. Possible adaptation of such a probe for oceanographic measurements will be discussed. The author has successfully used such a system for laboratory study of turbulence in stratified fluids.