DP level measurement — using hydrostatic head to infer tank level, including sealed-tank and density-compensated arrangements.
A related differential pressure transmitters topic.
A related differential pressure transmitters topic.
A related differential pressure transmitters topic.
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Differential Pressure Transmitters — in depth
Differential pressure measures level through hydrostatic head: the pressure at the base is proportional to liquid height and density. Open tanks use a single DP; sealed or pressurised tanks reference the vapour space; remote seals and density compensation handle hot, viscous or varying-density fluids.
What matters in practice
Base pressure ∝ level × density.
LP side references vapour space.
Corrects for varying density.
For hot, viscous or fouling fluids.
| Setup | Use | Note |
|---|---|---|
| Open tank | Vented | Single DP |
| Sealed tank | Pressurised | LP reference |
| Remote seal | Hot/viscous | Diaphragm |
| Compensated | Variable density | Corrected |
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Fundamentals, design drivers and practical guidance
DP level measurement — using hydrostatic head to infer tank level, including sealed-tank and density-compensated arrangements.
For level, the transmitter measures hydrostatic head, with the high side on the vessel and the low side referenced to atmosphere (open tank) or the vapour space (closed tank, requiring wet- or dry-leg compensation). Installation discipline dominates performance: impulse lines must be sloped, kept free of gas pockets on liquid service and condensate on gas service, and zeroed with the correct elevation and suppression so the calibrated span matches the real process.
Reynolds & Bauhm specifies, installs and calibrates DP instrumentation with the impulse-piping detail, manifold valving and compensation that decide whether the reading is trustworthy — integrating the signal into the control and alarm system with verified scaling.
The differential-pressure (DP) transmitter is one of the most versatile instruments in water and process plant: by measuring the pressure difference across two points it can infer flow, level, density and interface, all from a single, well-understood physical principle. Its enduring popularity comes from ruggedness, the absence of moving parts in the wetted path, and a deep base of engineering practice for sizing and installation.
What our engineers assess on every scope of this type
| Parameter | Typical basis | Why it matters |
|---|---|---|
| Manifold | 3- or 5-valve | Safe zeroing and isolation |
| Calibration | Zero + span verified | Reading matches true process |
| Flow | DP across primary element | Square-root law infers flow rate |
| Level | Hydrostatic head | DP equals liquid column height |
| Closed tank | Wet/dry-leg comp | Cancels vapour-space pressure |
| Impulse lines | Sloped, trap-free | Prevents gas/condensate errors |
Common questions on differential-pressure measurement
By sensing hydrostatic head — the pressure exerted by the liquid column. On a closed tank the vapour-space pressure is cancelled using a wet or dry reference leg, so the transmitter reports true level regardless of headspace pressure.
Because trapped gas on liquid service, or condensate on gas service, shifts the measured differential and corrupts the reading. Correct slope, routing and a proper valve manifold are what make DP Level Measurement reliable in practice.
They reposition the calibrated zero to account for the transmitter being mounted above or below the tapping point, or for a constant reference leg. Setting them correctly aligns the calibrated span with the real process range.
Zero and span are verified against a reference, the signal path is loop-checked into the control system, and the engineering scaling is confirmed — so the displayed value, alarms and control all act on a trustworthy measurement.
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