A Handy Time-of-Wetness Meter Using Hg Coulometer

Abstract

A simple and compact wetness meter has been developed. The instrument uses an Fe-Fe couple or humidity sensor, and a mercury coulometer which records the wetness time. It is driven by a SUM-3 type 1.5V dry cell which lasts for over one month on continuous use. Time-of-wetness was 31.2% of total exposure time when the sensor was exposed to outdoor atmosphere in Tokyo. Time-of-wetness was longer and critical relative humidity was lower when the sensor surface was contaminated by NaCl than by Na₂SO₄. When Fe-Fe couple was immersed in a solution, wetness time was 92%. It is also demonstrated that the wetness meter can be applied successfully for the measurement of permeation rate of electrolytes through paint film, when coated sensor is used.