Abstract: The oil water testing equipment is designed for field testing on board. The practical experience on ocean ships has inspired the development work. The principle of measurement is that when CaH₂ is put into lubricating oil sample, it will react with and only with water and generate gas soluble neither in water nor in oil. The measurement of the pressure of gas reflects the moisture content of the lubricating oil. The design is verified through comparing its test results to those with typical conventional laboratory testing equipment.