Formula used
Basic hydrostatic head: ΔP = ρgh
Wet leg: ΔP = ρprocessgh + high-side correction − ρwetgHwet
Interface level: ΔP = g[ρlowerh + ρupper(H − h)] + correction
Transmitter span: Span = URV − LRV
4–20 mA output: mA = 4 + 16[(DP − LRV)/(URV − LRV)]
A differential-pressure transmitter measures the pressure difference between its high and low ports. In an open tank, the low side is usually vented while liquid head acts on the high side. Correct elevation inputs prevent common commissioning errors and unexpected zero shifts.
Closed tanks route vapor pressure to the low side so common vessel pressure cancels. A dry leg should remain gas filled, while a wet leg deliberately supplies a constant reference head. The calculator keeps negative LRV values because wet-leg ranges often require them.
Remote seals add fill-fluid columns between process connections and the transmitter sensor. Unequal seal elevations create a fixed differential pressure that must be included during calibration. Temperature correction estimates density change, but manufacturer data remains the preferred source.
How to use this calculator
- Select the calculation mode and actual vessel installation.
- Choose common units, then enter calibrated and current levels.
- Enter process density or enable the specific-gravity option.
- Use one elevation datum for taps, seals, and transmitter position.
- Complete wet-leg, remote-seal, or interface fields when displayed.
- Select output scaling, tank geometry, and calibration points.
- Review warnings, results, uncertainty, and the calibration table.
- Export the table to CSV or create a PDF report.
Worked example
Consider a closed tank containing a liquid with density 900 kg/m³ and a five-metre level span. The transmitter sits one metre below the bottom tap, while a six-metre water wet leg acts on the low side. At 50% level, the process head is combined with the elevation correction and reduced by the wet-leg head.
| Input | Example value | Purpose |
|---|---|---|
| Installation | Closed tank with wet leg | Applies constant low-side reference head |
| Process density | 900 kg/m³ | Converts liquid height into pressure |
| Calibrated range | 0 to 5 m | Defines LRV, URV, and span |
| Wet-leg height | 6 m | Creates zero elevation |
| Wet-leg density | 1000 kg/m³ | Defines constant low-side head |
| Current level | 2.5 m | Produces the expected transmitter output |
Common mistakes
Use elevations from one datum and keep positive direction consistent. Confirm whether the entered liquid level begins at the high-pressure tapping point or another reference. Reversing transmitter ports changes pressure polarity and can produce misleading negative levels.
Do not treat wet-leg density as process density unless both fluids are identical. Verify that remote-seal fill density and temperature behavior match manufacturer documentation. Square-root extraction is for flow applications and normally remains disabled here.
Frequently asked questions
What is LRV in DP level measurement?
LRV is the differential pressure expected at the minimum calibrated liquid or interface level.
What is URV?
URV is the differential pressure expected at the maximum calibrated level.
Why can a wet-leg LRV be negative?
The constant low-side wet-leg head may exceed the high-side process head at minimum level.
Does vessel pressure affect a closed-tank DP reading?
Equal vapor pressure reaches both sides and ideally cancels, although impulse-line and seal effects can remain.
What is zero suppression?
Zero suppression occurs when the minimum-level differential pressure is positive because of installation head.
What is zero elevation?
Zero elevation occurs when the minimum-level differential pressure is negative, commonly with wet legs.
Can this calculator handle interface level?
Yes. It combines lower-liquid and upper-liquid hydrostatic heads over the entered total height.
Why include transmitter elevation?
Liquid-filled impulse lines create additional static head when the transmitter is above or below a tap.
Should square-root extraction be enabled?
Usually no. Hydrostatic level is linear with pressure, while square-root extraction is generally associated with differential-pressure flow.
Can results replace a transmitter datasheet?
No. Use verified process data, transmitter limits, seal specifications, and site procedures before final calibration.
Engineering disclaimer
This calculator supports preliminary sizing, calibration planning, learning, and field checks. Final settings must be verified against approved process data, transmitter datasheets, hazardous-area requirements, local procedures, and applicable engineering standards.