DP Level Calculator
Calculate the calibration range of a differential pressure level transmitter, including zero suppression and wet-leg elevation, and convert a measured DP back to level.
Level from a measured differential pressure
ΔP = ρ·g·(h + d). Assumes the high-pressure impulse line is full of process liquid and g = 9.80665 m/s². inH2O and mmH2O are referenced to water at 4 °C.
How DP level measurement works
A column of liquid exerts a hydrostatic pressure proportional to its height and density. A differential pressure transmitter measures the difference between the high-pressure (HP) side, connected near the bottom of the vessel, and the low-pressure (LP) reference side.
ΔP = ρ × g × h → h = ΔP ÷ (ρ × g)In an open tank, the LP side is vented to atmosphere. In a closed tank, the LP side is connected to the vapor space so that the vessel pressure cancels out.
Zero suppression
If the transmitter is mounted below the lower tap, the liquid in the HP impulse line adds a constant pressure. At 0% level the transmitter already sees a positive ΔP, so the lower range value is raised. This is called zero suppression.
Zero elevation and wet legs
When vapors condense (steam drums, for example), the LP line is deliberately filled with liquid to keep its pressure constant. This wet leg pushes on the LP side with a full column of liquid, so at 0% level the differential pressure is negative. The range is "elevated", and both the LRV and URV are often negative values.
Worked example
A closed water tank has a 5 m span. The taps are 5.5 m apart and the wet leg is filled with water (SG 1.0). The transmitter is level with the lower tap.
- LRV (0%) = 0 − 1.0 × 9.807 × 5.5 ≈ −53.9 kPa (−539 mbar)
- URV (100%) = 1.0 × 9.807 × 5 − 53.9 ≈ −4.9 kPa (−49 mbar)
- Calibrated span = 49.0 kPa (490 mbar)
Enter these values in the calculator above to check the result.
Common mistakes
- Using the design density instead of the density at operating temperature.
- Forgetting the transmitter mounting height when calculating suppression.
- Letting a wet leg evaporate or partially drain, which causes slow drift.
- Ignoring the fill fluid and capillaries of remote diaphragm seals, which behave like a wet leg.