Atmospheric Dew Point vs. Pressure Dew Point: Essential Concepts for Dryer Selection

CHANUN Air Compressor

A one-degree difference in dew point can mean a world of difference in drying performance.
In a compressed air system, the choice of drying equipment directly affects the quality of the final compressed air. Understanding the concepts of atmospheric dew point and pressure dew point is crucial — not only for ensuring technical accuracy but also for making the right equipment selection and achieving economic operation.

01 What Is Dew Point? The “Hygrometer” of Compressed Air
To understand the difference between atmospheric and pressure dew points, we must first grasp the basic concept of dew point.
The dew point is the temperature at which air becomes saturated (100% relative humidity) and water vapor begins to condense into liquid form, assuming the water vapor content remains constant.
When compressed air cools to its dew point, moisture begins to condense. In everyday life, morning dew on grass appears because the air temperature drops below its dew point. In compressed air systems, this principle is used to measure and control the moisture content in the air.

02 The Essential Difference Between Atmospheric and Pressure Dew Points
The atmospheric dew point (also called the normal dew point) refers to the dew point temperature measured at standard atmospheric pressure (0.101 MPa). This is the concept commonly used in weather forecasts.
The pressure dew point, on the other hand, refers to the dew point temperature measured under a specific working pressure. Since compressed air is under pressure, its water vapor behavior differs from that at atmospheric conditions.
The key difference: for the same sample of air, the pressure dew point is much higher than the atmospheric dew point.
For example, at 0.7 MPa working pressure, a +2°C pressure dew point corresponds to about a –23°C atmospheric dew point.
This difference arises because compressed air has a higher partial pressure of water vapor, meaning condensation begins at a higher temperature.

03 Why the Pressure Dew Point Matters in Dryer Selection
When selecting a compressed air dryer, the pressure dew point must be used as the basis for evaluation — for several reasons:
• Unified technical standards: International standard ISO 8573-1 and Chinese standard GB/T13277-91 both define compressed air dryness grades based on pressure dew point. This ensures a common technical language between suppliers and users.
• Reflects real working conditions: The pressure dew point shows the actual dryness of compressed air under real operating pressure, while the atmospheric dew point only reflects the air’s condition after expansion.
• Prevents selection errors: Confusing the two can cause serious design issues. For example, a process requiring a –40°C pressure dew point would not be met by a dryer capable of only –40°C atmospheric dew point—the actual pressure dew point would be around –20°C, insufficient to prevent condensation.

04 How to Apply Dew Point Concepts in Dryer Selection
1. Define your process requirements first.
Different applications require different dryness levels:
• General pneumatic tools: +3°C pressure dew point or above (ISO8573-1 Class 6)
• Precision painting or semiconductor production: –40°C or lower (Class 2 or higher)
2. Confirm the measurement units.
Always clarify whether the supplier’s dew point data refers to atmospheric or pressureconditions. If uncertain, request conversion charts or formulas.
3. Consider worst-case conditions.
Factor in high-temperature and high-humidity intake air (summer), lowest working pressure, and maximum air demand. Only then can the chosen dew point value ensure safe and reliable operation year-round.

05 Dew Point Capabilities of Different Dryer Types
Different types of air dryers achieve different pressure dew point ranges:
• Refrigerated dryers: +3°C to +10°C — suitable for general industrial use.
• Desiccant dryers: –20°C to –70°C — ideal for applications requiring very dry air.
• Membrane dryers: intermediate range — suitable for small or special air demands.
It’s worth noting that for every 10°C decrease in dew point, the investment and operating cost of the dryer can increase by 20% or more. Therefore, the goal is not to achieve the lowest dew point possible, but the most appropriate one that meets your process needs efficiently.

06 Practical Tips: Avoid Common Mistakes
• Never mix the two dew point terms. Always specify whether values are at atmospheric or pressure conditions.
• Check measurement conditions. Ensure dew point sensors are suitable for pressure measurement and installed correctly.
• Think system-wide. Dryers are only one part of a compressed air system — filters, air receivers, and piping materials also influence the actual dew point at the point of use.

Correctly understanding and applying the concepts of atmospheric and pressure dew points is fundamental to designing efficient compressed air systems.
Their relationship is much like altitude and air pressure — the same location gives very different readings depending on the reference used.
By mastering this distinction, you can accurately match process requirements with equipment capability, avoiding both over-investment and underperformance.
Next time you choose a dryer, remember to ask:
👉 “Is that dew point measured at atmospheric or pressure conditions?”
That one question could save you from costly technical and economic risks.