Abstract: A vapor electrical conductivity cell with sensing plates mounted on insulrs exposed to the vapor. Vapor condensation on those insulators cause errors in the conductivity measurements. The improvement of the invention keeps the insulators at a temperature higher than the vapor. The leakage errors are therefore reduced considerably.

Abstract: An apparatus and method are disclosed for measuring the conductivity or ner of water ion clusters present in a moist air environment. A test chamber is utilized for operatively simultaneously holding a "dummy" reference cell and a "large" conductivity cell having substantially the same leakage resistance. The cells are designed to have "cell factor" ratios substantially different from each other. A D.C. power source with an in series vacuum-tube voltmeter is used to alternately measure the voltage drop of each cell under the similar variable ambient conditions. The conductivity or number of ion cluster present in the test environment is determined by a calculation using voltage compensated values and by graphical extrapolation.