By: Brad Taylor

Relative Humidity and Dew Point in a Compressed Air System


The importance of understanding relative humidity to dew point when working with compressed air systems.

How many of you consider the relationship between relative humidity to dew point and the effect it can have on your compressed air systems? We spoke with several customers, and people who work in the industry, and we have discovered that many don’t understand how important it is to know both terms when talking about compressed air dryness. Knowing the relative humidity percentage and dew point temperature helps you ensure your compressed air systems are functioning properly and can help save you from costly mistakes.

Before we dive in more, let’s define each.

Dew Point is the temperature at which air becomes entirely saturated (i.e., when condensation forms). The lower the dew point temperature, the less moisture there is in the air. This is because cold air holds less moisture, so the lower the temperature, the less moisture that the air can hold. If moisture is not present even when you take it to a very cool temperature (i.e., a food refrigerator), this means the air is very dry. Dew point can be measured using a hydrometer, which is better known as a dew point meter.

Relative humidity (RH) is given as a percentage and is defined as the amount of moisture in the air compared to the amount of moisture needed for the air to be 100% saturated (given that the temperature remains the same between the two measurements). The relative humidity percentage is determined by how close the actual air temperature is to the dew point temperature. The closer the air temperature is to the dew point temperature, the closer you are to condensation. Relative humidity alone does not tell you how much moisture is in the air. It simply tells you how close you are to saturation.

To make it even more straightforward, dew point tells you how much moisture is in the air, and relative humidity tells you how close your current temperature takes you to your desired dew point. For extra information, watch this helpful video on the difference between the two.

So why is it important to understand the relationship between the two when you have a compressed air system?

For those just looking for liquid-free compressed air, a higher RH (near 75%) is acceptable. On the other hand, in a food application, it’s important to keep the moisture volume (or dew point) very low to eliminate biological growth in the pipe network. This could be a low as 1% RH.

To help calculate RH and dew point, watch this video.

In nearly all compressed air operations, having clean, dry compressed air will help reduce operating costs and improve overall production. Having water present in your compressed air lines will cause major setbacks as it damages tools, end products, and can cause expensive shutdowns. For example, if an auto body shop gets water in their compressed air lines, it could create fish eyes in the paint finish. In other applications, machines could rust internally, and/or food could be spoiled.

Here are some of the more common issues with water in compressed air lines:

  •       Production equipment will rust and increase wear
  •       Effects the color and finish of paint applied with compressed air
  •       Control lines will freeze in colder weather and hurt operations
  •       Corrosion will occur in both air and gas operated instruments
  •       Can cause false readings on instruments

As you can see, knowing the proper RH percentage and Dew Point temperature makes a big difference in costly operational costs and shutdown, and having a facility that runs smoothly.

Call Fluid-Aire Dynamics today to understand more about Dew Point and RH and why it’s essential to your system! We’re always here to help you with your compressed air needs and questions!


If frequent dew point and RH measurement is necessary, we recommend investing this device.

Devices to lower dew point and RH include: