Under fixed (and many times, not continuous) conditions, the answer is a qualified yes; DP can provide an inferred interface level reading.
In reality, a DP measurement is the ratio of the head pressure (heavier/lighter) that each of the two fluids contributes to the total head pressure. That ratio, as a percentage, indicates the interface elevation above the lower tap.
The required fixed conditions are
- known and stable densities of the heavier and lighter fluids.
- level must remain at or above the upper tap, or at a fixed level below the upper tap (sure . . .)
The taps are a fixed elevation distance apart which constitutes the 100% range for the interface level.
When the level between high and low taps is 100% lighter fluid, the head pressure span*lighter fluid Specific Gravity (SG) becomes interface LRV/minimum/zero %/4mA value.
When the level between high and low taps is 100% heavier fluid, the head pressure span*heavy fluid SG becomes the interface URV/maximum/100%/20mA value.
When the interface of the two liquids is half way between the taps, then 50% of the head pressure is contributed by the lighter fluid, 50% by the heavier fluid and the resulting output is 50%/12mA.
Issues are impulse leg fill fluid contamination or diaphragm seal temperature response.