How a Pressure Transmitter Works

A pressure transmitter is a process instrument that converts the force exerted by a fluid or gas on a sensing element into a standardized electrical signal — most commonly 4-20 mA DC — that a PLC or DCS can read, scale, and act on. It is the link between the physical world of pascals and bar and the digital world of analog input counts and engineering-unit variables.

Pressure transmitters appear in nearly every process industry: water and wastewater treatment, oil and gas, chemical processing, HVAC, food and beverage, and pharmaceuticals. Understanding how one works — and how to wire and scale it correctly — is a fundamental skill for any automation engineer.

What Is a Pressure Transmitter?

A pressure transmitter is a two-part device:

Sensing element — a mechanical or solid-state structure that deforms, deflects, or changes its electrical properties in proportion to applied pressure.
Transmitter electronics — signal conditioning circuitry that amplifies, linearizes, temperature-compensates, and converts the raw sensor output into a 4-20 mA loop current (or, in smart transmitters, a digital HART or fieldbus signal riding on top of that loop).

The word transmitter is important. A pressure sensor outputs a raw millivolt or resistance signal that needs further conditioning. A pressure transducer outputs a conditioned voltage (typically 0-5 V or 0-10 V). A pressure transmitter outputs a current loop signal suitable for long cable runs without signal degradation — a critical advantage in industrial plants where field instruments may be hundreds of metres from the control room.

Quick definition: A pressure transmitter measures fluid or gas pressure and transmits a proportional 4-20 mA signal to a controller.

Live Zero: 4 mA = valid zero pressure | 0 mA = broken wire / fault

HART-enabled: FSK digital signal rides on 4-20 mA loop for remote configuration and diagnostics

Signal path inside a pressure transmitter: process pressure deforms the sensing diaphragm, producing a millivolt bridge signal that is amplified, temperature-compensated, and converted to a 4-20 mA loop current the PLC reads across a 250 Ω burden resistor.

The Sensing Element

The heart of a pressure transmitter is its sensing element. Two technologies dominate modern industrial transmitters:

Piezoresistive (strain gauge)

A thin silicon diaphragm has four resistors diffused into it in a Wheatstone bridge configuration. When pressure deflects the diaphragm, the resistors change value in proportion to the strain. The bridge produces a differential millivolt output that the electronics amplify and convert to 4-20 mA. Piezoresistive sensors are highly accurate, fast-responding, and available across a wide pressure range — from a few millibar to thousands of bar.

Capacitive

Two capacitor plates are separated by a deflecting diaphragm. Pressure moves the diaphragm closer to one plate, changing the capacitance. The electronics convert the capacitance change to a 4-20 mA output. Capacitive sensing is inherently stable, drifts less over time, and is the preferred technology in high-accuracy differential pressure transmitters used for flow and level measurement.

Other technologies include piezoelectric (used for dynamic/high-frequency measurements, not suitable for static process pressure), resonant frequency, and optical sensing — all less common in standard PLC-connected process instrumentation.

From Sensing Element to 4-20 mA

The transmitter electronics perform three essential functions:

Step	What happens
Amplification	The millivolt bridge signal is amplified to a workable voltage level
Compensation	Temperature coefficients are corrected using an on-board temperature sensor
Signal conversion	A voltage-to-current (V/I) converter drives the 4-20 mA loop

The result is a live-zero output: 4 mA represents 0% of span (lower range value), and 20 mA represents 100% of span (upper range value). The 4 mA live zero allows the PLC to distinguish a valid "zero pressure" reading from a broken wire (which produces 0 mA). This is a key reason the 4-20mA current loop became the dominant industrial signal standard.

HART-enabled transmitters superimpose a digital frequency-shift-keyed (FSK) signal onto the 4-20 mA analog loop. This allows a HART communicator or asset management system to read diagnostic data, reconfigure the transmitter, and perform remote calibration — without interrupting the analog signal.

Gauge vs Absolute vs Differential Pressure

Choosing the right reference type is the single most common selection mistake in pressure measurement. The three types measure different physical quantities:

Type	Reference	Symbol	Typical use
Gauge (PG)	Local atmospheric pressure	barg, psig	Tank level, pump discharge, line pressure
Absolute (PA)	Perfect vacuum	bara, psia	Vacuum systems, vapor pressure, altitude compensation
Differential (DP)	Difference between two process points	bar, psi (no suffix)	Flow, filter condition, liquid level in closed tanks

Gauge Pressure

Gauge pressure is measured relative to the atmosphere surrounding the instrument. At sea level, atmospheric pressure is approximately 1.013 bar (14.7 psi). A gauge transmitter on a pipeline reading 5 barg means the actual pressure inside the pipe is 6.013 bara. Gauge transmitters are used wherever you want to know pressure relative to the surrounding environment — the most common case in process plants.

Sealed-gauge transmitters use a fixed reference pressure sealed into the sensor at manufacture. They behave like gauge transmitters at or near the reference altitude but will read slightly differently at greatly different elevations or in pressurized enclosures.

Absolute Pressure

Absolute pressure is referenced to a perfect vacuum (0 bar absolute). Absolute transmitters are essential when:

The process involves vacuum (below atmospheric pressure)
Accurate vapor pressure calculations are needed
The installation altitude changes significantly (aircraft, high-altitude facilities)
You need to compensate other measurements for barometric variation

Differential Pressure

A differential pressure (DP) transmitter has two process connections — a high-pressure (HP) side and a low-pressure (LP) side — and measures only the difference between them. This makes it uniquely useful for:

Flow measurement across an orifice plate, venturi, or nozzle (see flow meter types)
Liquid level in closed or pressurized vessels (hydrostatic head method)
Filter or heat exchanger fouling monitoring (increasing DP = increasing blockage)

The three pressure reference types: gauge references local atmosphere (most common), absolute references perfect vacuum (required for vacuum and vapor-pressure measurements), and differential measures between two process points (flow, level, filter monitoring).

Range, Span, and Turndown

These three terms appear on every data sheet and cause persistent confusion.

Range is the pair of values that define the measurement limits: the Lower Range Value (LRV) and Upper Range Value (URV). A transmitter configured for a range of 0–10 bar will output 4 mA at 0 bar and 20 mA at 10 bar.

Span is simply URV minus LRV. A 0–10 bar range has a span of 10 bar. A 2–8 bar range also has a span of 6 bar but is a suppressed-zero range (LRV = 2 bar, not 0).

Turndown (also called rangedown) is the ratio of the maximum configurable span to the minimum configurable span for a given sensor module. A transmitter with a maximum span of 100 bar and a minimum span of 10 bar has a 10:1 turndown. High turndown allows a single physical device to be re-ranged across a wide measurement window without changing the sensor module — reducing spare parts inventory and simplifying procurement.

Practical rule: Never range a transmitter so close to its minimum span that normal process noise drives the output erratically. Leave at least 20–25% headroom above the expected maximum process pressure when setting the URV.

Suppressed and Elevated Zeros

When the LRV is greater than zero, it is a suppressed zero configuration (common in DP level measurement where a wet-leg hydrostatic head offsets the zero). When the LRV is less than zero — such as a range of -1 to +5 barg — it is an elevated zero (common in vacuum applications). Both are normal and configurable in the transmitter's electronics.

DP Transmitter Applications: Flow and Level

The differential pressure transmitter is one of the most versatile instruments in process automation because its two-port design enables indirect measurement of several process variables.

Flow Measurement with a DP Transmitter

When fluid flows through a restriction (orifice plate, venturi tube, or annubar), the Bernoulli principle creates a pressure drop proportional to the square of the flow velocity. The DP transmitter measures this pressure drop. A PLC or flow computer then applies the square-root extraction:

Q = K × √(ΔP)

where Q is volumetric flow and K is a calculated coefficient based on pipe diameter, orifice diameter, fluid density, and viscosity. Most modern PLC analog input modules or SCADA packages include a dedicated DP-flow function block that handles this calculation automatically.

Important: Because of the square-root relationship, DP flow meters have a practical turndown of approximately 3:1 to 4:1 before signal-to-noise ratio degrades. Below about 10% of maximum flow, the ΔP becomes very small and measurement uncertainty rises sharply.

Liquid Level in Closed Vessels

A DP transmitter on a pressurized vessel measures the hydrostatic head of liquid above the LP tap. The HP port connects to the vessel bottom; the LP port connects to the vapor space at the top. The transmitter reads the weight of the liquid column:

ΔP = ρ × g × h

where ρ is liquid density, g is gravitational acceleration, and h is liquid height. Configuring the correct LRV (empty vessel) and URV (full vessel) in engineering units produces a direct level readout in metres or percent. This technique is covered in practical detail in the water treatment PLC programming guide.

Selection Criteria

Selecting the wrong pressure transmitter is a common and costly mistake. Use this checklist:

Process conditions

Maximum process pressure (include pressure spikes and water hammer)
Minimum and maximum process temperature
Fluid compatibility with wetted materials (316L stainless, Hastelloy, PTFE-lined)
Presence of viscous, crystallizing, or corrosive media (may require diaphragm seals)

Measurement requirements

Required accuracy (typically ±0.1% to ±0.5% of span)
Required response time
Range and turndown needed
Gauge, absolute, or differential reference

Electrical and installation

Power supply available (typically 24 V DC loop power)
Output signal: 4-20 mA (two-wire), HART, FOUNDATION Fieldbus, PROFIBUS PA
Hazardous area classification (ATEX/IECEx zone, FM/CSA class/division)
Process connection standard (IEC, ASME, flanged, threaded)
Enclosure protection (IP66/67 as a minimum for outdoor installations)

Communication and diagnostics

Is HART bidirectional communication needed for remote configuration?
Does the asset management system support the transmitter's DD/DTM?

Calibration Basics

A pressure transmitter calibration verifies — and if necessary corrects — the relationship between applied pressure and output signal. The essential steps are:

Isolate the transmitter from the process using block valves. Vent any trapped pressure safely.
Connect a calibrated pressure source (dead-weight tester, digital pressure calibrator) to the process connection.
Apply the LRV (e.g., 0 bar) and verify the output is 4.000 mA ±tolerance.
Apply the URV (e.g., 10 bar) and verify the output is 20.000 mA ±tolerance.
Check midpoints at 25%, 50%, and 75% of span to confirm linearity.
Adjust zero and span trims in the transmitter if output is out of tolerance.
Document as-found and as-left values against the calibration due date.

HART-enabled transmitters allow trim adjustments via a handheld HART communicator without removing the instrument or breaking the loop — a significant time saving during routine calibration cycles. Never perform a sensor trim (which modifies the internal characterization) without a traceable reference pressure standard; use only the output trim (zero and span) for routine calibration.

Wiring a Pressure Transmitter to a PLC Analog Input

This is where theory meets practice. A standard two-wire, loop-powered pressure transmitter requires only two conductors. The loop is powered by the PLC analog input module (or an external 24 V DC supply) and the transmitter modulates the current flowing in that loop.

Two-Wire Loop Circuit

+24 V DC Supply
      │
      ├──── (+) Terminal on AI Module
      │
    [Shielded cable, twisted pair]
      │
      ├──── (+) Terminal on Transmitter
      │
   [Transmitter]  (modulates 4-20 mA)
      │
      └──── (-) Terminal on Transmitter
      │
    [Shielded cable, twisted pair]
      │
      └──── (-) Terminal on AI Module (0 V / signal return)
      │
     (GND / 0 V DC return to supply)

Key wiring points:

Use shielded twisted-pair cable (minimum 0.5 mm²). Ground the shield at one end only (typically the control panel end) to prevent ground loops.
The transmitter has only two terminals — (+) and (-). There is no separate power connection; it draws its operating power from the loop current itself.
The AI module's input impedance is typically 250 Ω. The voltage drop across this resistor (1–5 V DC) is what the module's ADC actually reads; the current is inferred from Ohm's law. Some modules have the burden resistor internally; others require an external 250 Ω precision resistor.
Maximum loop resistance (cable + burden) must be within the transmitter's load driving capability — check the supply voltage vs. load curve in the data sheet. A 24 V supply, 250 Ω burden, and typical cable resistance leaves adequate headroom for most runs under 500 m.

Always refer to the specific types of industrial sensors guide for broader wiring conventions across analog and digital sensor types, and the PLC programming basics guide for AI module configuration in common PLC platforms.

Scaling Raw Counts to Engineering Units

A PLC analog input module converts the 4-20 mA loop current to a raw integer count. A typical 12-bit module maps 4 mA to count 0 and 20 mA to count 4095 (0–4095 range). Some platforms use 0–27648 (Siemens S7) or 3277–16383. You must scale this raw value to engineering units in your PLC program.

Linear scaling formula:

EU = EU_LRV + [(Raw - Raw_LRV) / (Raw_URV - Raw_LRV)] × (EU_URV - EU_LRV)

Example — Siemens S7-1200 with a 0–10 bar transmitter:

Parameter	Value
Raw at 4 mA (LRV)	0 counts
Raw at 20 mA (URV)	27648 counts
EU LRV	0.0 bar
EU URV	10.0 bar

A raw reading of 13,824 (exactly 50% of span) scales to:

EU = 0 + [(13824 - 0) / (27648 - 0)] × (10.0 - 0.0) = 5.0 bar

Most PLC platforms provide a dedicated SCALE or NORM/SCALE instruction that handles this arithmetic. In Siemens TIA Portal, use NORM_X followed by SCALE_X. In Allen-Bradley Studio 5000, use the SCL instruction or configure the channel scaling directly in the I/O module properties. In Mitsubishi GX Works, use the D/A scaling parameters in the module setup.

0.0 bar 5.0 bar 10.0 bar

EU = EU_LRV + [(Raw − Raw_LRV) / (Raw_URV − Raw_LRV)] × Span Use NORM_X then SCALE_X in TIA Portal · SCL instruction in Studio 5000 Wire break

Linear scaling from raw ADC counts to engineering units on a Siemens S7-1200: 0 counts maps to 0.0 bar (4 mA), 27,648 counts maps to 10.0 bar (20 mA). Any reading below the wire-break threshold should trigger a fault alarm.

Out-of-range detection: A reading below ~3.8 mA (typically flagged as <0 counts or <3277 counts depending on the platform) indicates a broken wire or failed transmitter. Program an alarm for any raw reading outside the 3.8–20.5 mA window to catch faults early.

Shielded twisted-pair cable — shield grounded at panel end only (prevents ground loop)

Broken wire fault: 0 mA (not a valid process reading) Alarm on any reading below 3.8 mA or above 20.5 mA in PLC program

Two-wire 4-20 mA loop wiring: the transmitter, supply, and PLC analog input module form a simple series loop. The live-zero 4 mA minimum current allows the PLC to distinguish a genuine zero-pressure reading from a wiring fault.

Choosing Range and Span from Process Data

When commissioning, select the transmitter range based on actual process conditions — not the pipe or vessel's rated maximum:

URV: Set to 110–120% of the expected maximum normal operating pressure. This leaves measurement headroom without compressing the useful span unnecessarily.
LRV: Set to 0 for gauge measurements unless the process never drops below a known non-zero minimum (suppressed zero).
Alarm setpoints in the PLC should sit below the URV by enough margin that a rising pressure trend is caught before the transmitter saturates.

FAQ

How does a pressure transmitter work?

A pressure transmitter works by applying process pressure to a sensing element (typically piezoresistive or capacitive), which produces a small electrical signal proportional to the applied pressure. Signal conditioning electronics amplify and linearize this signal and output a 4-20 mA DC current that represents the pressure across the configured range. The transmitter draws its operating power directly from the two-wire loop supply — no separate power cable is needed.

What is the difference between gauge and absolute pressure?

Gauge pressure is measured relative to local atmospheric pressure — a gauge transmitter reads zero at atmospheric pressure. Absolute pressure is measured relative to a perfect vacuum — a transmitter reading 1.013 bara at sea level is in fact reading the atmosphere itself. The choice matters whenever the process can go below atmospheric pressure (use absolute) or when barometric changes would affect the reading (use absolute with compensation). For most open-to-atmosphere process measurements, gauge is correct.

What is turndown in a pressure transmitter?

Turndown (or rangedown) is the ratio of the maximum configurable span to the minimum configurable span for a given sensor module. A 10:1 turndown transmitter with a 0-100 bar maximum span can also be configured down to a 10 bar span. High turndown reduces the number of different transmitter models you need to stock and allows a single device to cover a wider range of process conditions.

How do you wire a pressure transmitter to a PLC?

A two-wire pressure transmitter connects in series with a 24 V DC supply and the PLC analog input module using a shielded twisted-pair cable. The (+) terminal of the transmitter connects toward the (+) supply side of the loop, and the (-) terminal connects toward the AI module return. The transmitter modulates the loop current between 4 mA (LRV) and 20 mA (URV). The AI module reads this current — via an internal or external 250 Ω burden resistor — and converts it to a raw count that the PLC program scales to engineering units.

How a Pressure Transmitter Works: Types, Ranges, and PLC Wiring

What Is a Pressure Transmitter?