I will have to agree with roydm on this one and thanks for quick specific gravity lesson. Zdas04 the DP meter is the common choice for interface levels. The other common choice are displacer levels, the old mechanical prehistoric reliable means of interface measurement. Radars are percieved as reliable, technically advanced and of course best for any type of level measurement. Radars are so sensitive to their environment. At times if the the water di-electric constants change to less then 2.5, your radar is useless. You have to tweek your radar most time to operate beyond its capability in order for it to work, this all depends on the di-electric constant of the product. Radar folks are still working out the gaps in there latest and greatest. All a DP requires is a full vessel, the media that will be measured, such as the oil for a oil and water interface and a control board operator that will not upset the environment of the vessel. All these devices have the same weakness. The weekness is an emulsified if there is such a word, vessel (Unit upset, oil and water mixture thru out the vessel). The calculation for DP interface measurement is easy. If you are able to use actual media. You would fill your chamber with oil, set your LRV. Fill the vessel with water, set your URV and you are done.
SP1 (Oil) X Measuring range = Upper Range Value (URV)
Sp2 (H2O) X Measuring Range = Lower Range Value (LRV)
URV-LRV = Span
Hopes this Helps