PSI, PSIA, and PSIG What’s the Difference?

TL;DR Summary

• PSI (Pounds per Square Inch): A general term for pressure measurement, used for both absolute and gauge pressure depending on context.
• PSIA (Pounds per Square Inch Absolute): Measures pressure relative to a perfect vacuum, including atmospheric pressure. Essential in vacuum systems and scientific calculations.
• PSIG (Pounds per Square Inch Gauge): Measures pressure relative to atmospheric pressure. Commonly used in everyday applications like tire pressure and industrial processes.
• PSID (Pounds per Square Inch Differential): Measures the pressure difference between two points, regardless of absolute pressure. Useful in hydraulic systems and filter performance monitoring.
• Key Relationships and Applications: The blog post explains the mathematical relationships between these units, their use in various engineering fields (pneumatics, vacuum systems, safety valves), and considerations for altitude changes in pressure measurements.

As engineers, we often work with pressure measurements in various applications. Three common units you’ll encounter are PSI, PSID, and PSIG. While they may seem similar at first glance, understanding their differences is crucial for accurate measurements and calculations. Let’s break down these units and explore their applications with some practical examples.

PSI (Pounds per Square Inch)

PSI is the most general term among the three and simply refers to the force applied to a square inch of area. It doesn’t specify a reference point, so it can be used to describe either absolute or gauge pressure, depending on the context.

Example: When you’re checking your car’s tire pressure, you’re typically dealing with PSI. The recommended tire pressure might be 32 PSI, which in this case implicitly means PSIG (we’ll explain why shortly).

PSIA (Pounds per Square Inch Absolute)

PSIA measures absolute pressure, which means it uses a perfect vacuum (zero pressure) as its reference point. This unit includes atmospheric pressure in its measurement.

Example: Let’s say you’re designing a vacuum chamber for a semiconductor manufacturing process. At sea level, even when your chamber is at atmospheric pressure (no vacuum applied yet), the pressure inside would be about 14.7 PSIA. As you pump out the air to create a vacuum, the PSIA reading would decrease, approaching (but never quite reaching) 0 PSIA.

PSIG (Pounds per Square Inch Gauge)

PSIG measures gauge pressure, which uses atmospheric pressure as its reference point. This is the pressure above or below the local atmospheric pressure.

Example: Consider a pressure cooker used in a food processing plant. When the cooker is sealed but not yet heated, the pressure gauge would read 0 PSIG, even though the absolute pressure inside is actually about 14.7 PSIA (assuming you’re at sea level). As the cooker heats up and pressure builds, you might see the gauge rise to 15 PSIG, indicating that the pressure inside is 15 PSI above atmospheric pressure.

PSID (Pounds per Square Inch Differential)

While not as common, PSID is worth mentioning. It measures the difference in pressure between two points, regardless of the absolute pressure at either point.

Example: In a hydraulic system, you might use PSID to measure the pressure drop across a filter. If the pressure before the filter is 100 PSIG and after the filter is 95 PSIG, the pressure differential (PSID) across the filter is 5 PSID.

Key Relationships

Understanding the relationships between these units is crucial:

1. PSIG = PSIA – Atmospheric Pressure
2. PSIA = PSIG + Atmospheric Pressure
3. At sea level, atmospheric pressure is approximately 14.7 PSI

Practical Applications

1. Pneumatic Systems: When designing pneumatic systems, you’ll often work with PSIG. For instance, if you’re selecting a compressor for a manufacturing line, you might specify one that can deliver 90 PSIG. This means it can provide pressure 90 PSI above atmospheric pressure.

2. Vacuum Systems: In vacuum applications, PSIA is more relevant. For example, if you’re designing a vacuum distillation column for a chemical process, you might need to achieve a pressure of 0.1 PSIA at the top of the column.

3. Altitude Considerations: When designing equipment that will operate at high altitudes, remember that atmospheric pressure decreases with elevation. A system specified to operate at 50 PSIG at sea level will actually be operating at a lower absolute pressure when used in a high-altitude location.

4. Safety Valves: When specifying safety relief valves, pay close attention to whether the set pressure is in PSIG or PSIA. A valve set to relieve at 100 PSIG will open at a much lower absolute pressure than one set to 100 PSIA.

Understanding these pressure units and their relationships is fundamental for engineers across various disciplines. Whether you’re working on hydraulic equipment or aerospace applications, knowing when to use PSI, PSIA, PSIG, or PSID will ensure your calculations are accurate and your designs are sound.