How does a transformer oil moisture analyzer work?

A transformer oil moisture analyzer measures water content either as dissolved moisture in parts per million, or as water activity which correlates with saturation. Most portable instruments use capacitive or polymer sensors plus temperature compensation, because moisture solubility changes strongly with oil temperature. Good practice is to circulate oil through a closed sampling loop, purge the line, then wait for a stable reading. Results are used together with oil temperature and insulation paper condition, because wet paper can re release moisture back into the oil during load cycling.