Pressure · 2.1

Pressure sensors: strain gauge, capacitive, piezoresistive and resonant

26 October 2026 · 9 min read · IEC 60770-1, IEC 61298

In short

Almost every pressure sensor works the same way. A diaphragm deflects under pressure, and something measures the deflection.
The four common ways to measure it are strain gauge (resistance), piezoresistive silicon (resistance), capacitive (gap) and resonant (frequency).
The technology sets the stability, the temperature behaviour and the overpressure tolerance. These matter more than the brochure accuracy.
The wetted diaphragm and fill fluid are chosen for the process. The sensing element is chosen for the measurement.

Open any pressure transmitter and you find a small sensor behind a thin metal diaphragm. How that sensor turns deflection into a signal explains most of the differences between transmitter models. It shows why one drifts and another doesn't, and why one survives a pressure spike that ruins another.

Where this matters

Long calibration intervals: sensor stability sets how often the transmitter needs checking.
Hot or changing ambient: temperature effects differ widely between technologies.
Pressure spikes and water hammer: overpressure tolerance differs between designs.
Hydrogen and corrosive service: the isolating diaphragm and fill must suit the fluid, whatever the sensor.
Low ranges: some technologies resolve a few mbar far better than others.

How it works

The common front end

Process pressure acts on a thin isolating diaphragm, usually 316L stainless, Hastelloy C-276, Monel, tantalum or gold-plated. Behind it, a sealed fill fluid, usually silicone oil or an inert fluorocarbon for oxygen service, carries the pressure to the sensing element. The isolating diaphragm keeps the process away from the delicate sensor.

1. Metal-foil or thin-film strain gauge

Strain gauges are bonded or deposited on a metal diaphragm. As it flexes, their resistance changes. Four gauges form a Wheatstone bridge:

V_out = GF × ε × V_ex (full bridge, four active arms)

GF is the gauge factor (about 2 for metal foil), ε is the strain (m/m) and V_ex is the excitation voltage. The signal is small: at 1,000 µε and 10 V excitation, V_out = 2 × 0.001 × 10 = 20 mV. Strain gauges are robust and common in pressure gauges, OEM sensors and high-pressure transmitters.

2. Piezoresistive silicon

The resistors are diffused into a silicon diaphragm. The gauge factor is 50–100 times that of metal foil, so the signal is large and low ranges are easy. Silicon is temperature-sensitive, so the electronics compensate using an on-chip temperature measurement. This is the most common technology in modern transmitters.

3. Capacitive

The diaphragm forms one plate of a capacitor. Pressure changes the gap d, and the capacitance follows:

C = ε₀ ε_r A / d

Here ε₀ is the permittivity of vacuum, ε_r the relative permittivity of the fill, and A the plate area. In a differential cell, the sensing diaphragm sits between two fixed plates, so pressure increases one capacitance and decreases the other. The ratio is very stable and gives good overpressure tolerance, because the diaphragm bottoms out on a backstop. This is widely used for DP transmitters.

4. Resonant (silicon or quartz)

A tiny beam is driven at its natural frequency, which changes with the stress the diaphragm puts on it. Frequency is measured digitally, so there is no analogue drift. This gives the best long-term stability and is used for high-accuracy and reference work.

Comparison

Technology	Strengths	Watch out for	Typical use
Metal strain gauge	Rugged, high pressure, simple	Small signal, more drift	Gauges, OEM, very high pressure
Piezoresistive silicon	Large signal, low ranges, low cost	Needs temperature compensation	General-purpose transmitters
Capacitive	Stable, good overpressure, good DP	Sensitive to fill loss	DP flow and level, gauge and absolute
Resonant	Best stability, digital output	Cost	High accuracy, fiscal, reference

What the standards and practice say

IEC 60770-1 and IEC 61298 set how accuracy, temperature effects, overpressure and stability are tested and reported. This makes datasheets comparable, but only line by line.
Vendors state an overpressure limit (the most the sensor survives without damage) and a burst pressure (containment). Both must exceed the design and test pressure.
For oxygen service, the fill must be an inert fluid and the instrument cleaned for oxygen.
For hydrogen, gold-plated diaphragms reduce hydrogen permeation into the fill, which otherwise causes slow drift.
Under SIL rules, a technology choice should come with the vendor's certified failure data (article 11.5).

Worked example: choosing a sensor for three services

Service	Key need	Good fit	Why
Hydraulic power unit, 0–400 bar, pulsating	Robustness	Metal strain gauge	Thick metal diaphragm survives spikes. The signal is ample at high range.
Draft on a fired heater, −5 to +5 mbar	Low range	Piezoresistive or capacitive DP	Large signal at low pressure.
Gas sales line, 70 barg, 5-year calibration interval	Stability	Resonant	Frequency output with very low drift.

Signal size check. A strain-gauge bridge at 10 V excitation and 1,000 µε gives 20 mV. A silicon bridge with a gauge factor around 100 would give about 1 V for the same strain. That is why silicon suits the draft measurement, while metal foil is fine at 400 bar where strain is not the problem.

What this example shows

Match the technology to the service's real demand: spikes, low range or stability.
The wetted diaphragm and fill are a separate choice, driven by the process fluid.
Most project transmitters will be piezoresistive or capacitive. The others are for special cases.

When it fails: the fixes in order

Drift: check for hydrogen permeation or a temperature cycle first. Then consider a more stable technology or a gold-plated diaphragm.
Damage after spikes: fit a snubber, check the overpressure rating, and consider a sensor with a mechanical backstop.
Noisy low-range reading: use a lower-URL sensor rather than ranging down a high one (article 1.4).
Corroded diaphragm: change the wetted material, or add a diaphragm seal (article 2.4).

Common mistakes

Choosing on reference accuracy alone and ignoring stability and temperature effects.
Silicone fill in oxygen service.
A standard 316L diaphragm in wet hydrogen or high-temperature hydrogen service.
Assuming the overpressure limit equals the burst pressure.
Ranging a high-URL sensor far down for a low-pressure service.
Specifying the sensor technology when the requirement is really a performance figure.

Checklist

The performance needed is stated: total probable error, stability and calibration interval.
The range and URL suit each other (turndown).
The overpressure limit exceeds the design and test pressure.
The wetted diaphragm material suits the fluid.
The fill fluid suits the service (oxygen, vacuum, temperature).
Hydrogen service has a gold-plated or equivalent diaphragm.
Pulsating service has a snubber or a robust sensor.
SIL service has certified failure data.

Quick reference

Bridge: V_out = GF × ε × V_ex. GF ≈ 2 for metal foil and roughly 100 for silicon.
Capacitive: C = ε₀ε_rA/d. In a differential cell, the ratio of the two capacitances.
Stability ranking (typical): resonant > capacitive ≈ piezoresistive > metal strain gauge.
Fill: silicone for general use, inert fluorocarbon for oxygen.