PSI, PSIA, PSIG: what is compressed air pressure measurement

Pressure is defined as the amount of force applied per square inch of a surface area. It is a critical variable across various industries, particularly when it comes with compressed air systems.

There are several ways to measure the pressure. However, the main types of pressure measurements are:

-          Pascal

-          Bar

-          Standard Atmosphere (ATM)

-          Millimetre of Mercury (mmHg)

-          Pounds per Square Inch (PSI)

To apply to a type of unit or another one depends on the content. When it comes with compressors systems, the main units to measure pressures are:

-          Bar, used in Latin countries mainly

-          PSI, applied in Anglo-Saxons countries

The conversion between is:

1 bar = 14.503774 psi

This article will go in deep of the PSI measurements. Specifically, it will explain the variants of PSI, PSIA and PSIG, going in deep of their usage and relevance. Despite the type of compressed air pressure measurement applied, knowing the correct pressure of your compressed air system is crucial for its correct functioning and to keep it working efficiently.

PSI is pounds per Square Inch and is a fundamental unit used to measure pressure. The PSI metric is applied across numerous industries, from automotive to industrial manufacturing. It helps gauge how much force is being exerted in various contexts, whether it’s monitoring tire pressure or determining the operational conditions of a compressed air system.

In industrial settings, PSI is vital for system’s efficiency as well as for the correct functionality of all equipment. For instance, knowing the PSI of a compressed air system supports you in making an informed decision on maintenance, operational limits, and equipment selection. Ensuring accurate PSI readings minimises risks associated with improper pressure levels, safeguarding both personnel and equipment.

Both PSIA and PSIG provide insight into system pressure, but they employ different baselines, which significantly impacts their readings.

• PSIA (Pounds per Square Inch Absolute): PSIA measures pressure relative to a perfect vacuum (absolute zero). This value includes atmospheric pressure, providing a true representation of the total force exerted. It is particularly important for precise scientific calculations and when absolute pressure measurements are required without external reference influences.
• PSIG (Pounds per Square Inch Gauge): PSIG measures pressure relative to the surrounding atmospheric pressure. Since it does not include atmospheric pressure, PSIG is effectively the gauge pressure, and it is commonly used in everyday applications, such as tire pressure and compressed air systems. PSIG readings provide a practical reference to determinate the pressure exerted by a system compared to ambient conditions.

Atmospheric pressure is a key factor that explains the difference between PSIA and PSIG. At sea level, atmospheric pressure is approximately 14.7 PSI. When measuring pressure in PSIG, this atmospheric component is excluded, whereas PSIA includes it.

For example, if a compressed air system reads 50 PSIG, it means that the pressure inside the system is 50 PSI above the atmospheric pressure. To calculate the absolute pressure (PSIA), atmospheric pressure must be added, resulting in 64.7 PSIA. This distinction is crucial in high-precision industrial applications where even slight pressure variances can significantly affect outcomes.

In practice, the choice between PSIA and PSIG depends largely on the application requirements:

• PSIG is typically used for applications where gauge pressure is sufficient, such as monitoring air compressors or industrial equipment to ensure proper performance. It allows operators to determine how much pressure is being exerted within a system compared to the atmosphere.
• PSIA is necessary for applications that require absolute precision, such as calibration of sensitive scientific instruments, research, and highly controlled processes where even minor differences in pressure matter.

Consider an industrial compressor operating at 100 PSIG. This value represents the pressure above atmospheric pressure. To determine the absolute pressure, you would add 14.7 PSI, making it 114.7 PSIA. Understanding which unit to use in different scenarios helps enhance accuracy and efficiency, minimizing risks associated with incorrect pressure readings.

Converting between PSIA and PSIG is a straightforward process once you understand the influence of atmospheric pressure. The conversion formulas are as follows:

• PSIA = PSIG + Atmospheric Pressure
• PSIG = PSIA - Atmospheric Pressure

For example, if a gauge reads 50 PSIG, adding atmospheric pressure (14.7 PSI) gives an absolute pressure of 64.7 PSIA. Conversely, if you have a PSIA reading of 80, subtracting atmospheric pressure yields a PSIG of 65.3. Accurately converting between these units ensures proper operation and safety across various systems.

A frequent misconception is that PSIG and PSIA can be used interchangeably, which is incorrect. This misunderstanding can lead to serious operational errors, especially in industrial settings.

For instance, assuming a PSIG value is absolute could result in underestimating the real pressure within a system. This mistake could cause the system to operate beyond safe limits, leading to potential mechanical failures or safety hazards. Proper understanding of these pressure metrics is thus critical to avoid such risks, especially in precision-driven environments.

Choosing whether to use PSIA or PSIG depends entirely on your specific application. For most industrial and mechanical uses, PSIG is sufficient since it reflects the pressure differential relevant to the surrounding environment. It is particularly helpful in evaluating system performance and ensuring that components are operating under safe pressure conditions.

Some specific applications prefer PSIA, as it provides an absolute measure of pressure, which is crucial for accuracy. An example of these applications are: laboratory research, calibration of high-precision devices, or controlled experiments.

Accurate pressure measurement is essential for maintaining safety, optimizing performance, and improving the efficiency of compressed air systems. Select the correct type of pressure measurement and you will avoid costly errors and enhance productivity. in this way, your equipment operates at its optimal level.

If you have any questions about compressed air or would like to learn more about our various solutions, feel free to reach out to us. We are committed to helping you make informed decisions, ensuring that your systems are both safe and efficient.

• PSI (Pounds per Square Inch): A unit of pressure that measures the force applied per square inch of an area.
• PSIA (Pounds per Square Inch Absolute): Measures pressure relative to a perfect vacuum, including atmospheric pressure.
• PSIG (Pounds per Square Inch Gauge): Measures pressure relative to the surrounding atmospheric pressure.
• Atmospheric Pressure: The pressure exerted by the weight of the atmosphere, approximately 14.7 PSI at sea level.