A new technique is proposed to make a contribution to improving the accuracy of measuring low attenuation coefficients of liquids, such as pure water. It is a differential technique with one folded light path and two alternative positions of a retroreflector, creating the measuring path length. Air calibration is needed for maximum adjustment. The method is described and its advantages and disadvantages are discussed. Experimental results based on a laboratory set-up are considered and the expected accuracy is compared with the data obtained.