DP Level Measurement Explained: Open and Closed Tanks
9 min read · updated 2026-07-12
Radar gets the glamour, but differential pressure still measures more tank levels than any other technology. It works beautifully — provided you get the ranging right. This guide walks through the whole logic, open tanks to wet legs.
The principle: a pressure gauge is a ruler
A column of liquid presses down with a pressure proportional to its height and density:
Ten metres of water is ~98 kPa; ten metres of diesel (SG 0.85), ~83 kPa. Tank shape and width are irrelevant — only vertical height above the sensing point counts. So a pressure transmitter at the bottom of a tank is really a height gauge with a density assumption baked in. Play with the numbers in the hydrostatic pressure calculator.
Open tanks: the easy case
On an atmospheric tank, the transmitter's low side simply vents to air; atmosphere pushes on the liquid surface too, so it cancels. The range is set by the level span: a 0–8 m diesel tank needs 0–66.7 kPa. The one wrinkle is zero suppression: if the transmitter mounts below the tapping (they usually do, for access), the liquid-filled impulse line adds a constant head that exists even at empty tank — so both LRV and URV shift up by that constant. The open tank DP calculator computes the full range with suppression and shows the live mA at any level.
Closed tanks: cancelling the vapour space
Pressurise the tank and a single tapping reads liquid head plus vapour pressure — useless. The fix is a true DP measurement: high side at the bottom tapping, low side connected to the vapour space, so the vapour pressure appears on both sides and cancels. What fills that low-side impulse line defines the two variants:
Dry leg — the line stays gas-filled (works when the vapour never condenses). The math is open-tank-simple. Wet leg — with condensing vapours (steam is the classic), the line is deliberately filled with liquid and kept full. That full leg puts a constant head on the low side, making the differential negative at empty tank — so the range shifts down: zero elevation, typically a negative LRV. The closed tank DP calculator handles both legs and the elevation arithmetic.
Suppression vs elevation is the classic confusion, so here is the memory hook: extra liquid on the high side (transmitter below tapping) → suppress, range moves up. Extra liquid on the low side (wet leg) → elevate, range moves down.
The density assumption — DP level's honest weakness
The transmitter measures pressure and infers level through an assumed SG. If the product density drifts — temperature, product change, stratification — the level reading drifts with it, proportionally: range for SG 0.85, receive SG 0.80, and full scale reads ~6% high. Use the operating density when ranging (the density/SG/°API converter helps translate lab values), and treat unexplained level offsets as a density question before a transmitter question.
From level to volume
Level is rarely the end goal — inventory is. On a vertical cylindrical tank the relationship is linear (every cm holds the same litres — see the vertical tank calculator). On a horizontal cylinder it is strongly nonlinear: at 25% depth the tank holds only 19.6% of its volume, which is why DCS level-to-volume blocks use a 20-point characterisation curve. The horizontal tank calculator computes the exact segment volume and generates a mini dip chart.
Common failure modes, briefly
A wet leg that partially boils off or drains reads high and creeps. A blocked impulse line freezes the reading. A suppressed range entered as elevated (or vice versa) offsets everything by exactly the leg head — instantly recognisable once you know to look. And a perfect transmitter with yesterday's SG still reads wrong. DP level rewards engineers who respect the hydrostatic arithmetic — which is exactly what the calculators here are for.
Calculators used in this guide: hydrostatic pressure · DP level open tank · DP level closed tank · vertical tank volume · horizontal tank volume