About level & tank calculations
Differential pressure still measures more tank levels than any other technology, because it is robust, well understood and works on liquids that defeat radar and ultrasonic instruments. It also has more configuration traps than any other measurement, and almost all of them come down to two ideas: suppression and elevation.
When the transmitter sits below the tapping, the liquid in the impulse line adds constant head that exists even at empty tank, so the range shifts up — zero suppression. When a wet leg puts constant head on the low-pressure side instead, the differential goes negative at empty and the range shifts down — zero elevation. Confusing the two offsets every reading by exactly the leg head, and it is the single most common DP level error.
Level itself is rarely the end goal. Inventory is, and converting level to volume is linear in a vertical cylinder but strongly non-linear in a horizontal one — at quarter depth a horizontal tank holds under 20% of its volume.
How these tools fit together
- Identify the tank type. Open and atmospheric, or closed and pressurised? A closed tank needs true differential measurement so the vapour pressure cancels.
- Determine the leg arrangement. For closed tanks, establish whether the low-side impulse line is dry or wet. A wet leg means the range will be elevated and typically fully negative.
- Calculate LRV and URV. Work out the transmitter range including any suppression or elevation, using the actual operating density rather than a nominal value.
- Convert level to volume. Apply the tank geometry — and for horizontal cylinders, expect a characterisation curve rather than a straight line.
Frequently asked questions
What is zero suppression in level measurement?
When the transmitter is mounted below the tank tapping, liquid in the impulse line adds constant pressure even at zero level. Zero suppression shifts the calibrated range upward by that constant so 4 mA still represents an empty tank.
Why does the density of the liquid matter?
A DP transmitter measures pressure and infers level through an assumed specific gravity. If the actual density differs from the configured value, the level reading is wrong in direct proportion — which is why unexplained level offsets should be treated as a density question first.
All calculators are provided for reference and education. Verify independently before use in safety-critical work — see our disclaimer.