In HVAC control conversations, one of the most common questions customers face is deceptively simple:
“Do you need Enthalpy or Dry Bulb sensors?”
Before we compare the two, it’s important to understand a few fundamentals about temperature, humidity, and how the human body perceives comfort. This foundation will make it much clearer why these two sensor types behave differently—and why it matters for rooftop units and economizer systems.
Relative Humidity vs. Dew Point — What’s Really Going On?
Most people are familiar with relative humidity, but it’s not actually the best indicator of how humid air feels. Dew point is the better measurement.
Why Relative Humidity Can Be Misleading
Relative humidity (RH) represents how much water vapor the air can hold at a given temperature. Warmer air can store a much higher volume of moisture than cooler air.
Example:
- 50% RH at 55°F contains far less moisture than
- 50% RH at 95°F
So identical RH values can represent dramatically different moisture levels.
Dew Point — The Temperature of Saturation
Dew point is the temperature at which air becomes fully saturated (100% RH) and water begins to condense.
- At 55°F and 50% RH → Dew point is 37°F
- At 95°F and 50% RH → Dew point is 74°F
When the dew point equals the actual air temperature, the air is fully saturated. Fog, dew, or cloud formation begins at this point.
How Dew Point Relates to Human Comfort
Dew point is the best comfort indicator.
Comfort Scale
|
Dew Point |
Comfort Perception |
|
55°F or less |
Dry |