The setup
An open (atmospheric) tank is the simplest level application a DP or gauge-pressure transmitter sees: the high-pressure side connects to a tapping near the tank bottom, the low-pressure side vents to atmosphere, and the measured pressure is just the hydrostatic head of liquid above the tapping:
The only wrinkle is where the transmitter sits. If it
mounts below the tapping (common — transmitters like being accessible
at grade), the liquid-filled impulse line adds a constant head
d that exists even when the tank is empty. The range
must be shifted up by that constant — zero suppression:
Worked example
Diesel tank (SG 0.85), 8 m maximum level, transmitter mounted 0.5 m below the tapping:
- LRV = 0.85 × 9.807 × 0.5 = 4.17 kPa
- URV = 0.85 × 9.807 × 8.5 = 70.85 kPa
- Span = 66.68 kPa (≈ 6800 mmWC)
- At 4 m level: P = 0.85 × 9.807 × 4.5 = 37.51 kPa → 50% → 12 mA
Configure LRV/URV exactly like this and the 4–20 mA output reads true level with no offset — empty tank = 4 mA despite the transmitter seeing 4.17 kPa.
Field notes
- Suppression is not elevation. Transmitter below the tapping → suppress (shift range up). LP-side wet leg on a closed tank → elevate (shift range down, often to a negative LRV). Mixing the two is the classic DP-level configuration error.
- Use the operating SG, not the datasheet 15 °C value — hot hydrocarbons are several percent lighter.
- The tapping is your zero. Liquid below the tapping is invisible to the measurement; if the usable volume extends below it, the DCS must add that offset.
- Closed tanks are different animals — the vapour-space pressure must be subtracted via the LP side, and wet-leg condensate changes the math. That deserves its own calculator (coming soon).
Frequently asked questions
How do I calculate the range of a DP transmitter for open tank level?
LRV = SG × 9.807 × d and URV = SG × 9.807 × (H + d) in kPa, where H is the maximum level above the tapping and d the distance the transmitter sits below the tapping (zero suppression). With the transmitter at the tapping, d = 0.
What is zero suppression in level measurement?
When the transmitter is mounted below the tank tapping, the liquid in the impulse line adds a constant pressure even at zero level. Suppression shifts the LRV up by that constant so 4 mA still means empty tank.
Why does an open tank not need a wet leg?
Because the low-pressure side simply references atmosphere, which also pushes on the liquid surface — the atmospheric contribution cancels. Closed (pressurised) tanks need the LP side connected to the vapour space, wet or dry leg.
What happens to the calibration if the liquid density changes?
The transmitter measures pressure, not level — it infers level assuming a fixed SG. A 5% density change reads as a 5% level error at full scale. Ranging must use the operating density.
Provided for reference and education. Verify independently before use in safety-critical work. See our disclaimer.