How is pressure measured?

How pressure is measured with gauges, sensors and pressure transmitters

Pressure can be measured with a manometer, pressure gauge, barometer, electronic pressure sensor or pressure transmitter. The correct method depends on the medium being measured, the pressure range, required accuracy, temperature, process connection and whether the application requires gauge, absolute or differential pressure.

In industrial automation, a pressure value is often needed for more than local indication. A sensor or transmitter can convert the measured pressure into a 4–20 mA, 0–10 V or digital signal for a PLC, controller, display or monitoring system. This allows pressure to become part of the machine's alarm, recording and control functions.

If you want to start with the physical definition, see what pressure is and how it is defined.

Which units are used to measure pressure?

The SI unit of pressure is the pascal (Pa):

1 Pa = 1 N/m²

A pascal is small for many industrial applications, so pressure is commonly expressed in kPa, MPa or bar. Depending on the industry and region, instruments may also use psi, mbar, mmHg or kgf/cm².

A commonly used conversion is:

1 bar = 100,000 Pa = 100 kPa

For unit definitions and conversions, see pressure units including Pa, bar and psi.

Gauge, absolute or differential pressure?

Before choosing an instrument, it is necessary to establish what the measurement is referenced to. A 10 bar sensor is not fully specified simply by saying “10 bar”.

Gauge pressure is referenced to the surrounding atmospheric pressure. Many hydraulic, pneumatic and compressor pressure readings are expressed this way.

Absolute pressure is referenced to an ideal vacuum. This is useful where the measurement must remain independent of changes in local atmospheric pressure or where vacuum conditions are involved.

Differential pressure measures the difference between two pressure points. Typical applications include filter condition monitoring, some flow measurements and process supervision.

For applications where the difference between two points matters, see what differential pressure means in industrial measurement.

How does a manometer measure pressure?

A manometer measures pressure or pressure difference by balancing it against a known reference.

The classic example is the U-tube manometer. A liquid inside the U-shaped tube moves when the pressures at the two ends are different. The difference in liquid height can then be used to determine the pressure difference.

U-tube manometers are especially useful for understanding basic pressure measurement principles. On industrial machinery, however, continuous measurement is more commonly handled by mechanical gauges or electronic sensors.

How does a Bourdon pressure gauge work?

Many familiar circular industrial pressure gauges use a Bourdon tube.

The curved metal tube changes shape slightly as internal pressure increases. A mechanical linkage transfers this small movement to the pointer on the dial.

Bourdon gauges are widely used on hydraulic power units, compressors, pumps, tanks and process piping. Their major advantage is simple local indication without requiring an electrical power supply.

If the pressure value must also be transmitted to a PLC or control panel, an electronic sensor or transmitter is generally more useful.

How does a diaphragm pressure sensor work?

Another common measuring principle uses a flexible diaphragm. Pressure acting on the diaphragm causes a very small deformation.

That movement can be transferred mechanically to an indicator or detected electronically and converted into a usable signal.

Diaphragm-based measurement is particularly useful in low-pressure, differential-pressure and certain process applications where the sensing element needs to be isolated from the process medium.

How does an electronic pressure sensor measure pressure?

An electronic pressure sensor contains a pressure-sensitive element. Process pressure causes a minute mechanical change in that element, which the electronics detect and convert into an electrical value.

Depending on the design, the sensing technology may be piezoresistive, strain-gauge, thin-film or capacitive. These technologies have different strengths, so sensor selection should consider pressure range, temperature, accuracy, overload resistance and compatibility with the process medium.

Common industrial outputs include:

• 4–20 mA current output
• 0–10 V and other analogue voltage outputs
• Switching outputs
• Digital communication interfaces

The resulting signal can be processed by pressure indicators and controllers, PLC analogue inputs or a wider automation system.

Pressure sensor or pressure transmitter: what is the difference?

The terms pressure sensor and pressure transmitter are often used loosely in everyday industrial language.

A transmitter normally refers to a device that not only senses pressure but also conditions the measurement and provides a standardised signal suitable for transmission to another device.

For example, a 4–20 mA pressure transmitter can convert its measuring range into a current signal that is read directly by a PLC analogue input.

For a closer look at this type of instrument, see what a pressure transmitter is and how it is used.

How is pressure read from a 4–20 mA signal?

Consider a pressure transmitter with a measuring range of 0–10 bar and a linear 4–20 mA output.

Its scaling could be:

4 mA = 0 bar
12 mA = 5 bar
20 mA = 10 bar

The PLC, indicator or controller scales the incoming current back into engineering units such as bar.

In an actual installation, the manufacturer's datasheet must be checked because the measuring range, output configuration and behaviour outside the nominal range can differ between devices.

One reason 4–20 mA is widely used in industrial systems is that the signal is well suited to long cable runs, and the 4 mA live-zero can help distinguish a valid zero-pressure reading from certain wiring or signal faults.

What does a barometer measure?

A barometer measures atmospheric pressure.

Traditional mercury barometers balance atmospheric pressure against a column of mercury, while aneroid barometers use the deformation of an evacuated or partially evacuated metal capsule. Modern atmospheric measurements may also use electronic pressure sensors.

For the meteorological side of the subject, see how atmospheric pressure is measured and what barometric pressure indicates.

How is vacuum measured?

When working with vacuum, it is essential to know whether the reading is expressed as absolute pressure or gauge pressure relative to atmosphere.

For example, a display showing -0.8 bar usually represents a gauge value below atmospheric pressure. It does not mean that absolute pressure has fallen below zero.

As vacuum becomes deeper, different measuring technologies may be required. A sensor designed for ordinary pneumatic or hydraulic pressure is not automatically suitable for every vacuum range.

The correct instrument therefore depends not only on the numerical pressure but also on the vacuum range and measuring principle.

How is hydraulic pressure measured?

In a hydraulic system, the measuring point determines what the reading actually tells you. Pressure at the pump outlet, valve block, accumulator line or cylinder port may represent very different operating conditions.

A mechanical pressure gauge can provide local indication, while an electronic sensor is better suited when the value must be logged, alarmed or used in automatic control.

For devices intended for these applications, see hydraulic pressure sensors and transmitters.

How is compressed-air pressure measured?

In compressed-air systems, pressure at the compressor receiver is not necessarily the same as pressure at the machine.

Filters, regulators, valves, pipe length, pipe diameter and changing air demand can all create pressure loss. For that reason, a sensor intended to monitor the actual machine supply should be installed at a process-relevant measuring point, not simply next to the compressor.

See air and compressor pressure measurement for related applications.

Does sensor location affect the pressure reading?

Yes. Even an accurate sensor can give a misleading picture of the process if it is installed at the wrong point.

A sensor mounted before a pump does not measure the same condition as one installed after the pump. Measuring only one side of a filter does not directly give the pressure drop across that filter; two pressure points must be compared.

Liquid-filled impulse lines, trapped gas pockets, blocked connections, vibration, pulsation and excessive process temperature can also influence measurement quality or sensor life.

In industrial pressure measurement, where the pressure is measured can be just as important as which sensor is used.

How do you choose the correct pressure range?

A sensor should not be selected from normal operating pressure alone. The application may also produce pressure spikes, pump-start transients, rapid valve closures or other short-duration overpressure events.

At the same time, choosing a range that is unnecessarily large is not always an advantage. A very wide-range sensor on a low-pressure process may provide poorer practical resolution or accuracy than a properly selected device.

The measuring range should therefore be selected by considering normal pressure, maximum expected pressure, overload capability and the manufacturer's specifications together.

What does pressure-sensor accuracy mean?

A label such as “0–10 bar” only describes the nominal measuring range. It does not describe the quality of the measurement.

Depending on the application, specifications such as accuracy, repeatability, hysteresis, temperature influence and long-term stability may also matter.

A general compressor monitoring system and a reference measurement setup do not require the same performance. The required accuracy should therefore come from the needs of the process, not simply from choosing the sensor with the most impressive specification.

How is a pressure instrument calibrated?

Calibration compares the indication or output of the instrument with a known reference pressure or reference instrument.

Pressure can be applied at several points across the measuring range and the results compared with the reference values. Depending on the instrument, zero and span adjustments or digital calibration parameters may then be corrected.

Calibration does not mean that the instrument will remain accurate indefinitely. Overpressure, temperature cycling, vibration, ageing and mechanical wear can affect measurement performance over time.

Common mistakes in pressure measurement

Using the wrong pressure reference: confusing gauge, absolute and differential pressure.

Choosing the wrong measuring range: operating constantly near the upper limit or choosing an unnecessarily wide range.

Ignoring process temperature: excessive temperature can cause measurement error or damage the sensing element.

Measuring at the wrong point: pressure can differ significantly across pumps, filters, regulators, valves and long pipelines.

Ignoring pressure spikes: fast valve movement and pump operation can create short transients well above the normal operating pressure.

Mixing pressure units: values in bar, psi, kPa and MPa should not be compared without proper conversion.

How is pressure measured? In short

Start by identifying the type of pressure and the expected range, then select a suitable gauge, sensor or transmitter for the process.

If local indication is enough, a mechanical gauge may be the simplest solution. If the measurement must be sent to a PLC, recorded, alarmed or used for automatic control, an electronic sensor or transmitter is generally more suitable.

For a reliable installation, do not look only at the bar range. Also check the pressure reference, accuracy, process temperature, media compatibility, process connection, electrical output, overload rating and measuring point.

Related pressure topics

Further reading:
Changing pressure in a container | How materials behave under pressure | Pressure energy | Car tire pressure | Other meanings of pressure

For a real industrial installation, always verify the manufacturer's specifications for measuring range, pressure reference, overload capability, accuracy, process temperature, media compatibility and process connection.

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