DP flow measurement — how an orifice, venturi or nozzle creates a differential proportional to the square of flow.
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 is the most common flow-measurement principle. A primary element — orifice plate, venturi or nozzle — creates a pressure drop that varies with the square of flow; the transmitter measures it and a square-root extraction linearises the signal, giving a robust flow reading with no moving parts.
What matters in practice
Orifice, venturi or nozzle restricts flow.
ΔP ∝ flow².
Square-root extraction for flow output.
Robust, widely-applicable.
| Element | Loss | Best for |
|---|---|---|
| Orifice | Higher | General |
| Venturi | Low | Low-loss duty |
| Nozzle | Medium | High velocity |
| Output | √ΔP | Linear flow |
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Fundamentals, design drivers and practical guidance
DP flow measurement — how an orifice, venturi or nozzle creates a differential proportional to the square of flow.
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
It reads the pressure drop across a primary element such as an orifice plate; because flow is proportional to the square root of that differential pressure, the transmitter applies square-root extraction to output flow. DP Flow Measurement depends on the primary element being sized for the design range.
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 Flow 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.
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