Direct answer: Process pressure deflects a metal diaphragm by a tiny amount. A sensing element—often piezoresistive, capacitive or strain-gauge based—converts that deformation into an electrical change. Transmitter electronics compensate, linearise and scale it into a robust signal such as 4–20 mA for a PLC analog input.
The useful causal chain is pressure → force on area → diaphragm deflection → electrical change → conditioned signal → engineering value. Understanding it explains why a blocked impulse line, damaged diaphragm and open current loop produce different symptoms.
Stage 1: pressure reaches the diaphragm
The process connection exposes a wetted diaphragm or transmits pressure through fill fluid to an internal sensing diaphragm. Pressure is force per unit area, so the diaphragm deflects predictably within its elastic range.
The movement is extremely small. A transmitter is not a pressure switch with a large mechanical snap. It continuously measures deformation across a range.
Diaphragm material, process seal, fill fluid and connection must match pressure, temperature, corrosion, hygiene and overpressure requirements. A chemically incompatible but electrically correct transmitter is still a wrong selection.
Stage 2: deformation becomes an electrical signal
Common sensing methods include:
- Piezoresistive/strain gauge: resistance changes as the element stretches or compresses, commonly arranged as a bridge.
- Capacitive: diaphragm movement changes the spacing/capacitance between electrodes.
- Resonant or other specialised methods: pressure changes a measured physical property suited to high accuracy or demanding service.
In a Wheatstone-bridge style sensor, tiny resistance changes create a differential voltage. Temperature also affects materials, so the electronics apply compensation based on calibration data.
Stage 3: electronics linearise and scale
The electronics amplify the small sensor signal, correct non-linearity and temperature effects, apply range/zero calibration, and drive the output. Smart transmitters may add HART or another protocol over/alongside the analog signal.
Important terms:
| Term | Meaning |
|---|---|
| LRV | lower range value; process value represented by 4 mA |
| URV | upper range value; process value represented by 20 mA |
| Span | URV − LRV |
| Zero | configured lower endpoint, not always zero pressure |
| Turndown | ratio between sensor capability and configured span; excessive turndown can affect performance |
A 0–10 bar transmitter ranged 0–10 bar maps 0 bar to 4 mA and 10 bar to 20 mA. A differential-pressure transmitter could instead map −100 to +100 mbar; 4 mA then represents −100 mbar, not physical zero.
Why industry uses 4–20 mA
Unlike a 0–20 mA loop, 4 mA is a live zero. A valid minimum measurement still carries current, while a broken wire can fall near 0 mA. Current is also comparatively robust over long cable runs because the receiver measures loop current rather than relying solely on a remote voltage level.
Linear scaling is:
Engineering value = LRV + (mA − 4) × (URV − LRV) / 16
For 0–10 bar:
- 4 mA = 0 bar
- 8 mA = 2.5 bar
- 12 mA = 5 bar
- 16 mA = 7.5 bar
- 20 mA = 10 bar
Signals outside the normal range may represent underrange/overrange or a configured fault indication. Follow the transmitter and PLC module conventions; do not hardcode one universal threshold.
Two-wire loop to a PLC analog input
A common two-wire transmitter uses the same loop for power and signal:
+24 V supply → transmitter +
transmitter − → PLC analog input +
PLC analog input −/return → 0 V supply
The exact path depends on whether the analog input is active/passive, isolated/shared and how the common is referenced. A loop calibrator can source or simulate current, but must be configured correctly to avoid putting two power sources into the same loop.
Check the loop's voltage budget: supply voltage must cover transmitter minimum voltage, input burden, barriers/isolators, cable drop and any other series device at 20 mA.
Shielding and earth practice follow the system design. Connecting a shield at arbitrary multiple points can create unwanted currents; leaving it floating against the drawing can reduce noise protection.
Gauge, absolute and differential pressure
- Gauge pressure: referenced to local atmosphere. A tyre gauge is a familiar example.
- Absolute pressure: referenced to an internal vacuum reference.
- Differential pressure: measures the difference between two process connections.
Differential pressure transmitters can infer flow across a primary element, level from hydrostatic head, or filter condition from pressure drop. The transmitter measures differential pressure; PLC/configuration turns that into the desired process variable.
PLC scaling and diagnostics
The analog module converts current to raw counts. Scale those counts from the module's configured raw range—not a copied constant from another brand.
A robust analog-input function should produce:
- raw input;
- scaled engineering value and units;
- underrange/overrange quality;
- wire-break/bad-channel state;
- filtered value if process dynamics justify it; and
- simulation/maintenance state that is clearly visible.
Alarm logic should use engineering units and appropriate hysteresis/delay. Filtering a noisy signal too aggressively can hide a real fast process change.
What common failures look like
| Symptom | Likely zones |
|---|---|
| Near 0 mA | open loop, lost supply, failed transmitter |
| Fixed 4 mA | true LRV, blocked impulse path, frozen configuration/simulation |
| Fixed high value | overpressure, plugged low side on DP, wiring/configuration issue |
| Noisy reading | process pulsation, grounding/shielding, loose connection, cavitation |
| Slow response | impulse-line restriction, damping setting, viscous fill/process |
| Offset | zero shift, mounting/head effect, temperature, diaphragm damage |
Calibration compares applied reference pressure to transmitter output at controlled points. It is different from ranging, which changes LRV/URV, and from trimming, which adjusts sensor/output calibration. Record as-found and as-left data under the site's instrument procedure.
Safety and process isolation
Pressure systems can release hazardous fluid or stored energy. Isolate, depressurise, drain/vent and verify according to the approved process before opening impulse lines or removing a transmitter. Chemical compatibility, temperature and hazardous-area requirements are part of the instrument design.
Learn the mechanism and the PLC value together
Use the Pressure Transmitter lesson to change process pressure, watch the sensing story and inspect the PLC signal. The contextual Pro tutor can explain the specific sensor and current scenario without leaving the lesson.
Then practise engineering-unit handling in the analog input examples and 4–20 mA scaling calculator.
Frequently asked questions
What does a pressure transmitter do in one sentence?
It converts physical fluid/gas pressure into a standardized electrical value that a PLC, DCS or indicator can read.
Why does 4 mA represent zero?
It creates a live zero: the minimum valid measurement still proves loop current exists, while an open circuit can fall below the valid measurement range.
What is the difference between a pressure switch and transmitter?
A switch changes a discrete contact around a setpoint. A transmitter continuously reports pressure across a range, usually as 4–20 mA or digital data.
Can a pressure transmitter measure tank level?
Yes. In a known-density liquid, hydrostatic pressure corresponds to liquid head. Mounting elevation, density, vessel pressure and DP reference arrangement must be handled correctly.
Why can the PLC show pressure when the process is isolated?
Pressure may remain trapped, an impulse line may hold a hydraulic head, the input may be simulated, or the transmitter/scale may be offset. Verify the entire measurement chain safely.
