A DP transmitter measuring the interface between two immiscible liquids of different density — an oil-water separator, a boot, a decanter — needs its own calibration math, not the single-liquid DP level formula. This gives the 0%/100% calibration span and, if you have a live reading, the current interface position.
Verified against published standard
Liquids & Geometry
Span is the vertical distance between the LP tap (above all liquid) and the HP tap (at the bottom of the measured range) — the same span you'd use for a single-liquid DP level range.
Enter a live or test DP reading to back-calculate the current interface position; leave blank to just get the 0%/100% calibration values.
Enter liquid and geometry data to calculate.
About This Calculation
A DP transmitter measuring the interface between two immiscible liquids of different density — an oil-water separator, a boot, a decanter — needs different calibration math than a single-liquid level measurement, because the transmitter's differential pressure now depends on BOTH the total span AND the current thickness of each liquid layer, derived from a hydrostatic balance between the two liquids' different densities. This calculator derives both the 0%/100% calibration span for a given service and, given a measured DP, the current interface position.
Frequently Asked Questions
Why does interface level calibration need BOTH liquid densities, not just the heavier one?
The DP transmitter’s reading changes as the ratio of light-to-heavy liquid above the low-pressure tap shifts — the SAME total liquid volume produces a different pressure reading depending on where the interface sits, since light and heavy liquid contribute differently to the hydrostatic head, which is exactly why both densities enter the calibration math.
What assumption does this calculation make about the vapor space above the liquids?
It assumes any vapor-space pressure is common to both the high and low pressure taps and so cancels out of the differential — the same assumption a plain single-liquid DP level measurement makes. A pressurized vessel with an uncompensated or asymmetric vapor space needs an additional correction this basic calculation doesn’t include.
Does a wet reference leg change the interface level calculation?
Yes — a wet leg deliberately fills the low-pressure impulse line with a known reference fluid, whose own hydrostatic contribution must be subtracted from the calculation, similar to how a wet-leg single-liquid level transmitter needs its own additional correction term beyond the basic dry-leg case this tool covers.