You’re absolutely right that dew point becomes the standard reference once we deal with very low humidity environments, especially in professional drying systems—and yes, chilled mirror hygrometers do literally detect fogging on a surface to determine the dew point. That method is still among the most precise ways to measure ultra-low moisture content.
But here’s where absolute humidity (AH) becomes practical in filament storage and drying:
Example 1: Comparing two dry boxes at different temperatures
Let’s say one dry box shows 10% RH at 40 °C, and another shows 13% RH at 35 °C.
At first glance, 10% RH might sound “drier,” but when converted to absolute humidity, the 13% at 35 °C box is actually dryer in terms of real water vapor content (g/m³).
Without AH (or at least temp-compensated RH), we’d make the wrong decision based on misleading RH alone.
Example 2: Storing filament near dew point thresholds
For moisture-sensitive materials like PA, PVA, or PEI, you want to ensure the air stays well below the critical moisture absorption threshold. Using RH alone won’t help you if the ambient temperature shifts.
AH or dew point tells you how much water vapor is actually available to be absorbed—independently of temperature swings.
Example 3: Designing or calibrating heated drying cycles
If you set a target of “RH < 10%” without accounting for the chamber temperature, you may over- or under-dry the filament. But if you define your goal as “AH < 1.0 g/m³”, it’s unambiguous regardless of temp.
That’s essential when building drying profiles for hygroscopic or advanced materials.
So yes—dew point is key when you’re using desiccant-fed or regenerative dryers with specs like “-40 °C dew point or better.”
But absolute humidity is often more intuitive and more practical for daily filament storage and monitoring, especially when your equipment reports only RH and temperature.
