Instrumentation
Differential Pressure & Level
DP-flow square-root relationship, hydrostatic level, and open/closed tank level from DP.
Standard method — stated scope limits apply
DP ⇄ Flow
Hydrostatic Level
Open Tank Level (from DP)
Closed Tank Level (from DP)
Enter values and calculate.
About This Calculation
Measuring liquid level with a differential pressure transmitter works because a column of liquid exerts hydrostatic pressure proportional to its height and density — mounting the transmitter's high-pressure side at the bottom of the vessel (or via an impulse line) and its low-pressure side at the top (open to atmosphere for a vented tank, or connected to the vapor space for a closed/pressurized vessel) measures exactly that hydrostatic head, independent of the vessel's absolute pressure. This is the standard, most common level measurement technique in industrial process plants precisely because it needs no moving parts and directly measures a physical quantity (pressure) that scales predictably with level.
Frequently Asked Questions
Why does a CLOSED, pressurized vessel need the low-pressure side connected to vapor space, not atmosphere?
Because the vessel’s own internal pressure (above the liquid) would otherwise show up as a large, level-independent offset on the high-pressure side reading — connecting the low-pressure side to the SAME vapor space cancels that common pressure out of the differential, leaving only the true hydrostatic head due to liquid level.
What is a wet leg and when is it needed?
On a closed vessel where the low-pressure impulse line could see condensation collecting in it over time, a wet leg deliberately fills that line with a known reference fluid instead — the calculation then has to subtract that reference column’s own hydrostatic contribution, which is why wet-leg level transmitters need their calibration to account for it explicitly, not just the vessel’s own liquid.
Does liquid density changing (e.g. with temperature) affect a DP level reading?
Yes, directly — the transmitter genuinely measures pressure, and pressure = density × g × height, so if the calibrated density assumption no longer matches actual process density, the SAME true level reads as a different pressure, and the transmitter reports an incorrect level unless recalibrated or density-compensated.
Why is DP level still used when radar and ultrasonic level exist?
It has no moving parts, works reliably in dirty, foaming, or vapor-obscured services where non-contact methods can struggle, and is well-proven at high pressure and temperature — the trade-off is a genuine dependency on knowing liquid density accurately, which radar and similar non-contact techniques don’t share.