A conductivity detector detects the electrical conductivity of a fluid under analysis for determining chemical or physical properties of the fluid that are related its electrical properties. Such conductivity detectors may find use in, for example, hemodialysis systems for analyzing the effectiveness of the hemodialysis treatment. In an aspect, to improve accuracy of the conductivity measurements, the detector utilizes four-wire resistance measurement methods. In another aspect, to avoid fouling or contamination of the electrodes, the detector utilizes capacitively-coupled contactless conductivity detection (C4D) methods so that the electrodes are physically unconnected to the fluid contained in a fluid chamber. In a possible further aspect, the fluid chamber may be a disposable component removable from the electrodes. The conductivity detector can include other features such as calibration circuits and features for electrically isolating the fluid under detection from the fluid in the rest of the system.