How a differential-pressure transmitter works — a sensing diaphragm converts the pressure difference across two ports into a calibrated signal.
Differential Pressure Transmitters — in depth
A DP transmitter measures the difference between two pressures. Process pressure on each side deflects a sensing diaphragm; capacitance, strain-gauge or resonant sensing converts that tiny deflection into a linear 4–20 mA or digital signal — the basis for flow, level, density and filter-condition measurement.
What matters in practice
Pressure compared across high/low ports.
Difference deflects the sensing element.
Capacitive/strain sensing to 4–20 mA.
Zero and span set the range.
| Element | Function | Note |
|---|---|---|
| HP/LP ports | Apply pressures | Two sides |
| Diaphragm | Sense difference | Deflects |
| Transducer | Convert | To signal |
| Output | 4–20 mA | /digital |
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Fundamentals, design drivers and practical guidance
How a differential-pressure transmitter works — a sensing diaphragm converts the pressure difference across two ports into a calibrated signal.
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.
For flow, a DP transmitter reads the pressure drop across a primary element — orifice plate, venturi, nozzle or averaging pitot — and applies the square-root relationship between differential pressure and flow rate. Correct sizing of the primary element for the design flow range, plus square-root extraction and low-flow cut-off in the transmitter, set the achievable turndown and accuracy.
What our engineers assess on every scope of this type
| Parameter | Typical basis | Why it matters |
|---|---|---|
| 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 |
| 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 |
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 Transmitter Working Principle 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 Transmitter Working Principle 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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