Interface Level Calculator
Measure the interface between two immiscible liquids, such as oil over water, with a DP transmitter and remote seals. Both tappings must stay covered by liquid.
- LRV = g × H × (SGlight − SGfill) = 1.5 m × (0.85 − 0.93) of water column
- URV = g × H × (SGheavy − SGfill) = 1.5 m × (1 − 0.93)
- Span = g × H × (SGheavy − SGlight) = 22.065 mbar
Interface Level Formula
With both tappings submerged, the DP depends on how much of the column between the taps is heavy liquid. With the interface x metres above the lower tap:
LRV = g × H × (SGl − SGfill) URV = g × H × (SGh − SGfill)
Worked example
Taps 1.5 m apart, oil SG 0.85 over water SG 1.0, silicone fill SG 0.93. LRV = 1500 × (0.85 − 0.93) = −120 mmH₂O, URV = 1500 × (1.0 − 0.93) = 105 mmH₂O. The span is only 225 mmH₂O (about 22 mbar).
Frequently Asked Questions
Can a DP transmitter measure interface level?
Yes, as long as both tappings are always covered by liquid and the two liquid densities are known and stable.
Why does the fill fluid SG matter?
The LP capillary is filled with seal fluid up to the upper tap, so its head is subtracted from every reading and shifts the LRV and URV.
What if the upper tap is uncovered?
Then the transmitter measures total level and interface together and the reading is meaningless. Interface measurement needs the upper tap below the minimum total level.
Learn more on the blog
Types of level transmitters
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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