A pressure transmitter converts a pressure measurement into a signal for a display, PLC or control system. A sensing element responds to pressure, and electronics condition the response into an output such as 4–20 mA or a digital value. To select and use one correctly, define the pressure reference, range, process connection, medium and receiving instrument.
How does a pressure transmitter work?
Pressure reaches the sensing element directly or through a suitable isolation arrangement.
Deformation changes an electrical property such as resistance or capacitance.
Calibration and compensation relate the response to pressure.
The configured electrical or digital signal is sent to the receiving system.
This is a conceptual chain. Sensing technology, isolation and output vary by model; a pressure transmitter is not defined by a particular diaphragm material or output alone.
For example, a piezoresistive design measures a resistance change associated with deformation, while a capacitive design measures a capacitance change. These electrical responses must be calibrated to pressure. Temperature compensation reduces specified instrument effects; it does not remove the need to respect process temperature and pressure limits.
Gauge, absolute or differential pressure?
| Reference type | What is measured | Typical selection question |
|---|---|---|
| Gauge pressure | Pressure relative to local atmospheric pressure. | Is the process requirement expressed above or below ambient pressure? Check whether the model is vented gauge or another reference arrangement. |
| Absolute pressure | Pressure relative to an absolute vacuum reference. | Does the application require absolute pressure, such as a vacuum process or a calculation based on gas properties? |
| Differential pressure (DP) | The difference between two applied pressures. | Are you measuring a filter pressure drop, liquid head with a reference, or another difference? Check both differential and static-pressure ratings. |
A gauge and an absolute transmitter can show different values on the same process because their references differ. Do not select a sensor using “bar” or “psi” alone; record the reference as well as the unit. For a DP transmitter, the permissible pressure on each port and the allowed differential are separate limits.
Sensor, transducer and transmitter: what is the difference?
A pressure sensor is the pressure-responsive element or assembly. A transducer converts pressure into an electrical signal; a transmitter generally includes a conditioned output suitable for transmission to another instrument. Manufacturers use these terms with some overlap. Check the actual power, signal, wiring, range and accuracy specification instead of assuming that a product name guarantees one output type.
Pressure transmitter selection checklist
| Item | What to specify | Why it matters |
|---|---|---|
| Pressure reference and range | Gauge, absolute or DP; lower and upper configured values; expected vacuum and overloads. | A correct numeric range with the wrong reference still gives the wrong measurement. |
| Medium and wetted materials | Liquid, gas or vapor; chemical composition; solids; seals and diaphragm materials. | Chemical incompatibility or blockage can damage the instrument or isolate it from the process. |
| Temperature and pressure limits | Normal and upset conditions, ambient temperature and any isolation requirements. | The measuring range is not necessarily the overload or temperature rating. |
| Process connection | Thread or flange, sealing method, mounting and service access. | A matching nominal size does not establish pressure rating or seal compatibility. |
| Output and receiving input | 4–20 mA, voltage or digital protocol; active/passive input and power supply. | The transmitter and PLC must use compatible wiring and signal interpretation. |
| Performance and environment | Required accuracy basis, response, enclosure protection and approvals. | Percent of span and percent of reading are different specifications; approvals must match the actual installation. |
The Apure PCM300T absolute pressure transmitter is listed as an absolute-pressure option. If the process requirement is gauge pressure or differential pressure, choose the appropriate model and reference rather than assuming a unit conversion can change the sensor reference. Review the Apure pressure measurement range with the process data above.
Calculate a linear 4–20 mA pressure signal
For a transmitter configured with a linear pressure output, let LRV be the lower range value and URV the upper range value. With pressure P in the same unit and current I in mA:
I = 4 + 16 × (P − LRV) / (URV − LRV)
P = LRV + (I − 4) / 16 × (URV − LRV)
Example configuration: LRV = 0 bar, URV = 10 bar. At 5 bar, I = 12 mA. At 12 mA, P = 5 bar.
| Pressure for a theoretical 0–10 bar range | Expected ideal current | Percent of configured span |
|---|---|---|
| 0 bar | 4 mA | 0% |
| 2.5 bar | 8 mA | 25% |
| 5 bar | 12 mA | 50% |
| 7.5 bar | 16 mA | 75% |
| 10 bar | 20 mA | 100% |
These values illustrate ideal linear scaling, not an acceptance tolerance or test result for an Apure model. They do not apply unchanged to an output configured with square-root extraction for DP-based flow. Out-of-range and diagnostic currents must be interpreted using the model manual; do not automatically treat every value below 4 mA as a valid pressure.
Conceptual two-wire 4–20 mA wiring
Connect to transmitter +.
Connect to passive analog input +.
Return to supply 0 V to complete the series loop.
This example assumes a passive current input and a compatible loop-powered two-wire transmitter. An active input may supply loop power itself. It is not a terminal-specific installation drawing; follow both device manuals.
The supply must provide the transmitter’s minimum operating voltage plus the voltage drops across the receiving input, wiring and any barriers or isolators, without exceeding device ratings. Confirm polarity, loop burden, grounding and shielding from the actual drawings. Do not add an external supply to an active input without verifying the circuit design.
How to calibrate and verify a pressure transmitter
- Review the transmitter manual, pressure reference, configured range, output mode and required tolerance. Use a suitable traceable pressure reference and current measurement arrangement.
- Arrange safe process isolation and depressurization under the site procedure before disconnecting pressure equipment. Check fittings, media compatibility and pressure ratings.
- Apply stable reference pressures across the required interval, commonly including lower, intermediate and upper points. Compare applied pressure, indicated pressure and current.
- Check increasing and decreasing pressure where the procedure calls for it. Record error, repeatability and hysteresis rather than judging only the zero point.
- If adjustment is permitted, follow the model procedure. Sensor trim, analog-output trim and range configuration are different operations; changing the range is not a substitute for checking accuracy.
- Repeat the checks after adjustment and record the final configuration, reference equipment and results before returning to service.
Troubleshoot a wrong pressure reading
| Symptom | Checks before adjustment |
|---|---|
| PLC value disagrees with local display | Confirm both use the same LRV, URV, unit and output mode. Measure loop current and check analog-input scaling. |
| No current or implausibly low signal | Check power, polarity, open circuit, input type and available loop voltage. Interpret fault currents using the manual. |
| Offset at zero | Confirm the pressure reference, trapped pressure, mounting effects and zero procedure; absolute zero is not atmospheric pressure. |
| Slow or fixed reading | Check blocked ports, closed valves, impulse lines, dampening settings and a genuinely stable process. |
| Unstable reading | Check pulsation, vibration, gas or liquid in impulse lines, wiring and process disturbances. |
Related measurement tasks
A pressure transmitter may also support liquid-level or flow measurement, but those applications add assumptions. See level transmitter types and hydrostatic selection for density and closed-tank considerations. The pressure reading alone is not automatically a level or flow value.
Technical reference
Emerson: Pressure transmitter measurement types and selection provides an overview of gauge, absolute and differential references. Wiring, calibration limits and approvals must come from the manual for the installed device.