Pressure measurement in instrumentation

Pressure is one of the four process variables — alongside temperature, flow, and level — that shows up in nearly every industrial process. Understanding how it's measured, what instrument to pick for a given job, and how to calibrate that instrument correctly is core knowledge for any instrumentation technician. This article walks through the fundamentals: what pressure actually is, the scales used to express it, the sensing technologies behind pressure gauges and transmitters, and a practical calibration procedure you can use in the field.

What Is Pressure?

Pressure is force applied per unit area. In a process plant, pressure shows up almost everywhere — fluid flowing through a pipe, gas in a vessel, differential pressure across an orifice plate used for flow measurement, and so on.

Pressure readings are affected by more than just the process itself. Altitude, weather, and atmospheric conditions all influence a pressure measurement, which is why pressure is expressed using a few different reference scales depending on the application.

Pressure Scales: Gauge, Absolute, and Differential

Gauge Pressure

Gauge pressure is the scale most people are familiar with from everyday life — tire pressure ratings, for example, are given in gauge pressure. It's referenced to atmospheric pressure, meaning a gauge-pressure instrument reads zero when vented to atmosphere. Gauge pressure is written with a "(g)" suffix: kPa(g), bar(g), mmWC(g).

Absolute Pressure

Absolute pressure is referenced to a full vacuum rather than to atmospheric pressure. An absolute-pressure instrument will not read zero when vented to atmosphere — it reads the local atmospheric pressure instead. It's written with an "(a)" suffix: kPa(a), bar(a).

Absolute Pressure = Gauge Pressure + Atmospheric Pressure

Differential Pressure (DP)

Differential pressure is the difference between two pressure points:

ΔP = P1 − P2

DP is proportional to flow rate, which is why differential-pressure measurement is the basis for orifice plate

Vacuum and Total Vacuum

Total vacuum is the complete absence of pressure — the condition found in outer space. A vacuum reading falls anywhere between total vacuum and normal atmospheric pressure, and by definition can't go below −1 bar(g).

Atmospheric Pressure

Atmospheric pressure is the pressure at the earth's surface due to the weight of the atmosphere above it, typically 14.7 psi (101.3 kPa) at sea level. It drops with altitude — roughly 2.5 psi per 5,000 feet of elevation — and also varies with weather and local climate.

Pressure Units and Conversions

Unit	Description
Pa (Pascal)	SI unit of pressure
Bar	Common metric unit in process industries
PSI	Pounds per square inch
mmWC	Millimeters of water column
atm	Atmospheres
Torr	1 Torr = 1 mmHg
kg/cm²	Common in older mechanical gauges

Quick conversions:

1 bar = 100 kPa = 100,000 Pa = 14.504 psi = 0.987 atm = 750 mmHg
1 atm = 101.3 kPa = 14.696 psi = 760 mmHg
1 kg/cm² = 14.223 psi = 0.98 bar
1 kPa = 0.145 psi = 4.02 in H&sub2;O

Types of Pressure Sensors

There are two broad categories of pressure-sensing technology: mechanical (deflection-based) elements that give a direct visual reading, and electronic sensors that convert pressure into a proportional electrical signal for transmission to a control system.

1. Bourdon Tube

The Bourdon tube is the classic mechanical pressure gauge element. It's a curved, flattened tube — sealed at one end and connected to the process at the other — that straightens slightly under internal pressure. That small deflection is mechanically amplified through a gear-and-linkage assembly to move a pointer across a dial.

Three common Bourdon tube geometries:

C-type — the standard curved-tube design used in most general-purpose gauges
Helical — coiled into a spring shape for greater sensitivity at higher pressures
Spiral — flattened into a spiral for improved sensitivity in a compact housing

Advantages: Simple, rugged, low-cost, accurate to about ±2% of span (0.1% achievable with precision designs), safe for high-pressure service, easy field installation.

Limitations: Slow response — not suited to dynamic or rapidly changing pressure; sensitive to shock and vibration; subject to hysteresis; temperature-sensitive, so not ideal for high-temperature service.

Common Bourdon tube materials: Phosphor bronze (non-corrosive service), brass (low pressure), beryllium copper (medium range), SS316 (corrosive fluids, high pressure), with monel, tantalum, and titanium used for specialty applications.

2. McLeod Gauge

The McLeod gauge is a reference-standard instrument used to measure very low (vacuum) pressures by compressing a known volume of gas and applying Boyle's Law (P₁V₁ = P₂V₂) to back-calculate the original pressure. It's not used for continuous process monitoring — it's a calibration-reference instrument, valued because its reading doesn't depend on the gas composition and requires no correction factors. Its main limitation is that it only gives a single-point, sampled reading rather than continuous output.

3. Manometers

Manometers use a column of liquid to balance an applied pressure and are mostly a legacy technology now, replaced in the field by smaller and more rugged electronic sensors — though they're still useful as a simple calibration check.

U-tube manometer — the pressure difference between the two legs is read directly as a height difference: ΔP = ρg × Δh
Inclined manometer — the low-pressure leg is angled to stretch the liquid travel distance, giving a magnified scale for measuring small, low pressures
Well manometer — one leg is much wider than the other, giving a compact single-scale reading for low-pressure applications

4. Diaphragm

A diaphragm sensor uses a flat or corrugated metal membrane that deflects under applied pressure; that deflection is mechanically linked to a pointer or converted to an electrical signal.

Advantages: Good linearity, compact, moderate cost, and — because the diaphragm itself seals the process — no leak path.

Limitations: Limited to low-pressure applications, difficult to repair, and susceptible to cracking under overpressure or fatigue.

Typical use: Processes where product purity matters (food, pharma, chemical) since the diaphragm isolates the sensing element from direct contact with the process fluid.

5. Bellows

A bellows element is a convoluted tubular membrane, sealed at the process end and connected to an indicator or sensor at the other. It can produce a longer mechanical stroke than a diaphragm for a given pressure change, making it useful where more linkage travel is needed to drive an indicator.

6. Capsule

A capsule is built from two diaphragms welded together into a sealed chamber. Applied pressure expands the capsule similarly to how a balloon expands under internal pressure. Capsules are typically stainless steel and can handle static pressures up to roughly 2,000 psi, but the usable differential-pressure range is limited to a few hundred kPa — pushing well beyond that range risks permanent damage.

7. Strain Gauge

A strain gauge is a fine metal wire or foil element bonded to a mechanical structure (a diaphragm, force bar, or similar). As the structure flexes under load, the gauge's length and cross-section change slightly, which changes its electrical resistance. That resistance change — measured through a Wheatstone bridge — produces an output proportional to applied pressure, typically converted to a standard 4–20 mA signal. This is the same working principle used in load cells for weight measurement.

8. Piezoelectric

Piezoelectric sensors generate their own electrical charge when the sensing crystal (natural quartz, tourmaline, or engineered ceramics) is mechanically stressed — no external excitation voltage is needed. Some designs include a built-in preamplifier (known as IEPE — Integrated Electronic Piezoelectric transducers), which simplifies signal conditioning.

Advantages: Self-generating, no external excitation required.

Limitations: High output impedance and low raw signal level mean special low-noise coaxial cable and a charge amplifier are usually required in the measurement chain.

9. Capacitive

A capacitive sensor consists of a flexible sensing diaphragm positioned between two fixed capacitor plates. Applied pressure deflects the diaphragm, changing the capacitance on each side proportionally. This differential capacitance is converted electronically into a standard 4–20 mA or 1–5 V DC output. Capacitive sensing is the operating principle behind most modern differential-pressure transmitters.

10. Inductive

Inductive-type sensors work on the same general principle but vary inductance instead of capacitance as pressure is applied; measuring that inductance change gives the pressure reading. Less common than capacitive designs in modern process instrumentation, but still found in some specialty applications.

Pressure Transmitters

A pressure transmitter converts a pressure input into a standard 4–20 mA signal for transmission to a PLC or DCS. Most modern transmitters are two-wire, loop-powered devices running on a 24 VDC supply, with the process signal riding on the same pair of wires as the power.

Pressure Switches

A pressure switch changes contact state (open to closed, or vice versa) once the measured pressure crosses a set trip point. The contact can be wired normally open or normally closed depending on the fail-safe requirements of the system, and is typically used to drive an alarm or interlock rather than provide continuous measurement.

Wet Leg and Dry Leg Conditions

These terms come up specifically in DP-based level measurement using a pressure transmitter connected to a vessel by impulse piping.

Dry leg: The low-pressure side piping stays empty because the vapor above the liquid doesn't condense. Range calculations follow the standard formulas used for bottom-mounted transmitters on open vessels.

Wet leg: Vapor above the liquid slowly condenses and fills the low-side piping with liquid. To eliminate the resulting measurement error, the low side is deliberately pre-filled with a reference fluid, and the transmitter's zero point must be elevated to compensate for the added head pressure from that fluid column.

Common Faults in Pressure Transmitters

No power reaching the transmitter
Impulse line leaking or blocked
Transmitter itself faulty
mA output cable damaged or loose
Calibration drifted out of tolerance
Analog input channel fault at the PLC/DCS side

Calibration Procedure

Zero adjustment — with the transmitter vented to atmosphere (or both sides at equal pressure for a DP unit), adjust the zero output.
Static pressure test — apply equal pressure to both sides of a DP transmitter. The zero reading should not shift; if it does, perform static alignment.
Vacuum test — apply equal vacuum to both sides. Again, zero should hold steady.
Span calibration — power the transmitter (typically 24 VDC), vent the low-pressure side to atmosphere, connect the output to a calibrator or multimeter, and confirm/adjust zero.
Apply the required span pressure to the high-pressure side and adjust the span to match the reference value.
Re-check zero after span adjustment, since span trims can shift the zero point slightly.

The Bottom Line

Picking the right pressure-sensing technology comes down to the application: mechanical gauges like the Bourdon tube are fine for local visual indication on stable, non-critical services, while strain-gauge and capacitive sensors dominate anywhere the reading needs to feed a control system. Whatever the technology, the same fault-tracing logic applies — check power and wiring first, then impulse lines and process connections, and only then suspect the sensing element itself.

industrial instrumentation basics

Search This Blog