Abstract: A portable instrument for testing at the wellhead continuously, or a stationary instrument for use at a production battery, the outflow of crude hydrocarbons from the well or wells. The invention utilizes a simple relationship of density and thermal coefficients of expansion of the oil and the water, together with improved instrumentation to measure mass flow rate of the crude mixture, to produce accurate continuous readings of oil and water contents in the crude. The invention depends upon state of the art technology both as to the crude handling and the electronics portions of its apparatus.