• The Heron H.OIL & SM.Oil interface meters are used to measure the thickness of floating or sinking hydrocarbon products (LNAPL, PSH, DNAPL), in groundwater accurately, reliably, safely & economically.The SM.Oil is essentially a smaller, lighter and mini-version of the industry standard H.Oil. Unlike other interface meters, the H.Oil and SM.Oil come with fully submersible probes.