Understanding Dew Point: What it is and how to measure it

What is the dew point? 

Let’s start with the most important thing: the general definition of the dew point. Dew point is the temperature at which air becomes saturated with moisture and condensation begins to form. At this temperature, the air can no longer hold additional water vapour.

For instance, if we have a dew point of -40°C, that means that we will get a potential condensation if temperature goes below -40°C.

Three factors influence dew point:

• Humidity: Condensation occurs when humidity reaches 100% saturation.
  
  
• Temperature: While temperature doesn’t directly affect dew point, it does affect relative humidity and absolute humidity.

Here’s an illustration: 

In this graph, the blue line is the saturation line. Condensation begins above this line and becomes more and more important with temperature (from 12°C = we double the amount of water vapour).

• Pressure: Higher pressure raises the dew point because compressed gas holds less water vapour before saturation.

That’s why the dew point in compressed air differs significantly from the atmospheric dew point.

What is dew point in compressed air?

In compressed air systems, dew point is defined at the system’s working pressure, known as the Pressure Dew Point (PDP). It is the temperature at which moisture in compressed air condenses at that specific pressure.

This value is essential because excess moisture can cause corrosion, equipment damage, product contamination, and ice formation (therefore compromising efficiency and safety). 

Different drying technologies achieve different pressure dew points:

• Refrigerated dryers: Typically reach a PDP of 2°C to 10°C, suitable for general industrial use.
  
  
• Desiccant dryers: Achieve much lower PDPs, often -40°C or even -70°C, ideal for applications requiring extremely dry compressed air.

Understanding your dew point requirements is key to selecting the correct drying system and maintaining the desired air quality class.

What is a good dew point for compressed air?

The ideal dew point depends entirely on your application and ambient conditions.

Standards recommend that the dew point of instrument air should be at least 10°C below the lowest temperature the system may be exposed to, and should never exceed 4°C at line pressure.

ISO 8573-1 also links air quality classes to pressure dew point:

• Class 1: ≤ -70°C
• Class 2: ≤ -40°C
• Class 3: ≤ -20°C
• Class 4: ≤ +3°C
• Class 5: ≤ +7°C
• Class 6: ≤ +10°C

Lower PDP = drier air = better protection for your system and end products.

Even when no specific class is required, reducing moisture is always beneficial to prevent corrosion and water buildup.

How to measure dew point in compressed air

To monitor drying performance and ensure consistent compressed air quality, dew-point measurements should be accurate and precise. 

Industries typically use compressed air dew point monitors / sensors (they measure dew point continuously and provide real-time data on moisture levels). 

Important considerations when measuring dew point

• Understand pressure effects: Dew point always changes with pressure. Therefore, expanding compressed air to atmospheric pressure before measurement will alter the dew point reading.
  
  
• Install sensors correctly: Avoid dead-end pipes or stagnant sections.
  
  
• Use the correct measurement range: Instruments must match the expected dew point range (from ambient to as low as -80°C).
  
  
• Monitor regularly: Dew point monitoring helps ensure dryers operate efficiently and prevents moisture-related failures.

Why dew point measurement matters

Checking and measuring dew point regularly benefits process safety (preventing corrosion, pipe blockages, leaks, and icing), product quality (which is essential for pharmaceuticals, metallurgy, and food production), energy savings (preventing dryers from running longer than needed), and regulatory compliance (helping meet industry specific moisture requirements).

In short, dew point monitoring is one of the most effective ways to protect both equipment and end products in any moisture sensitive industrial process.

Conclusion

To sum up, it is essential to have a good understanding of the pressure dew point in compressed air systems in order to effectively control humidity levels and protect equipment in the long term. Thus, by selecting the appropriate drying technology and using accurate dew point sensors and monitors, industries can prevent corrosion, contamination and unnecessary downtime, thereby ensuring reliable air quality and optimal system performance.