What are Ways to Measure Pressure in Industrial and OEM Applications?

Pressure can be measured mechanically with a pressure gauge or electronically with a pressure transducer or transmitter. The right method depends on whether an operator needs a local visual reading or a control system needs an electronic pressure signal.

Mechanical pressure gauges provide visual indications of pressure levels throughout a process. Pressure transducers and transmitters detect pressure changes and send electrical signals to monitoring or control systems.

Ashcroft has extensive experience developing pressure measurement solutions for industrial and OEM applications. In this article, you will learn how pressure is measured, the differences among common pressure types, and where pressure measurement instruments are used.

How is pressure measured?

Pressure is measured by determining how much force is applied over a specific area. This principle is commonly expressed as pressure equals force divided by area.

Figure 1: Pressure equation

In the United States, pressure is commonly expressed in pounds-force per square inch, or psi. Other units are used throughout the world depending on the industry, application, and system of measurement.

The pascal (Pa) is the International System of Units (SI) unit of pressure. However, units such as bar, millibar, kilograms-force per square centimeter, inches of water column, and psi are also commonly used in industrial applications.

What units are used to measure pressure?

Pressure units vary by region, industry, and application. The chart below shows units commonly associated with SI, MKSA, and English measurement systems.

Figure 2. Pressure system of units

The added letters identify the pressure reference:

• psia means pounds per square inch absolute.
• psid means pounds per square inch differential.
• psig means pounds per square inch gauge.

What is considered high or low pressure?

At Ashcroft, we generally define low pressure as pressure below 15 psi, and high pressure is generally defined as pressure above 15 psi.

Figure 3. PSI ranges

However, high or low pressure also depends on the application and the instrument’s available range.

For example, 100 psi may be considered high pressure in one application but relatively low in a hydraulic system operating at several thousand psi. Instrument selection should always be based on the actual operating range, pressure reference, process conditions, and potential overpressure events.

How does a manometer measure low pressure?

What types of pressure can be measured?

Pressure may be measured as absolute, gauge, vacuum, compound, or differential pressure. Each type uses a different reference point, so understanding the required pressure reference is an important part of selecting an instrument.

What is absolute pressure?

Absolute pressure is measured in relation to a perfect vacuum, or zero absolute pressure. Because absolute pressure uses a perfect vacuum as its reference point, there is no negative absolute pressure.

Figure 5. Absolute pressure

Absolute pressure measurement is useful when changes in atmospheric pressure could affect the accuracy of the reading.

What is gauge pressure?

Gauge pressure is measured in relation to the surrounding atmospheric or barometric pressure. A gauge pressure reading of zero means the process pressure is equal to the local atmospheric pressure.

Positive gauge pressure is greater than atmospheric pressure. This is one of the most common pressure references used in industrial processes.

What is vacuum pressure?

Vacuum pressure is pressure below the surrounding atmospheric or barometric pressure. It is commonly displayed as negative gauge pressure or measured in units such as inches of mercury.

Figure 6. Gauge and vacuum pressure

What is compound pressure?

Compound pressure includes both positive pressure and vacuum on the same instrument range or scale. A compound pressure gauge is useful when a process may operate above atmospheric pressure at one point and below atmospheric pressure at another.

What is differential pressure?

Differential pressure is the difference between two pressure points in a system. It is often abbreviated as DP, ΔP, or Delta P.

Figure 7. Differential pressure

Differential pressure measurement may be used to monitor filters, determine flow, measure liquid level, or compare pressure across process equipment.

What pressure ratings should you understand when selecting an instrument?

Static pressure, maximum working pressure, proof pressure, and burst pressure describe different operating or safety limits. These terms should not be treated as interchangeable when selecting a pressure instrument.

What is static pressure?

Static pressure is the pressure exerted by a fluid independent of its motion. In a differential pressure instrument, static or line pressure may be present on both sides of the sensing element even when the differential pressure between the two sides is low.

The instrument must be rated to withstand the system’s static or line pressure in addition to its specified differential pressure range.

What is maximum working pressure?

Maximum working pressure, or MWP, is the highest pressure at which an instrument is designed to operate continuously under its specified conditions. It is an operating limit and should not be confused with proof or burst pressure.

What is single-sided static pressure?

Single-sided static pressure is the maximum pressure that can be applied to one side of a differential pressure instrument while the other side is open to the atmosphere. Exceeding this rating can damage or permanently shift the sensing element.

What is proof pressure?

Proof pressure is the maximum pressure an instrument can withstand without permanent deformation or a change in performance beyond its stated limits. Proof pressure describes a temporary pressure exposure, not a normal operating condition.

What is burst pressure?

Burst pressure is the pressure at which the pressure-containing assembly can no longer be expected to retain the process media. An instrument should never be intentionally operated at or near its burst-pressure rating.

Where are pressure measurement instruments used?

Pressure measurement instruments are used in home equipment, medical systems, OEM products, utilities, and industrial processes. Each application has different requirements for pressure range, pressure reference, accuracy, materials, output, and environmental protection.

The pressures below are representative examples and may vary by system.

Which instrument should you use to measure pressure?

Use a pressure gauge when an operator needs a local visual reading. Use a pressure transducer or transmitter when pressure data must be sent to a monitoring or control system.

When should you use a mechanical pressure gauge?

A mechanical pressure gauge should be used when an operator needs to view pressure directly at the equipment or process. These gauges do not typically require electrical power and are available for a broad range of industrial applications.

When should you use a pressure transducer or transmitter?

A pressure transducer or transmitter should be used when a system needs an electronic signal for remote monitoring, data collection, automation, or process control.

The terms transducer and transmitter are sometimes used interchangeably, but they may describe instruments with different output signals and applications. The required output, such as millivolt, voltage, or current, is an important consideration during selection.

Whether the application involves high or low pressure, the instrument must be selected for the required pressure type, operating range, process media, accuracy, environmental conditions, and output.

Ready to learn more?

If you want more information about measuring pressure in industrial and oem applications, check out the related resources below. For specific questions about your application, contact us and a product expert can assist you. In the meantime, download our guide, 5 Mistakes to Avoid When Choosing a Pressure Gauge.