Definition
The temperature at which water vapor in the air begins to condense when cooled (°Cdp). It is a direct indicator of the absolute moisture content of air and is not affected by temperature changes. It is the most reliable measurement parameter in industrial humidity control. Silica gel rotor dehumidifiers can achieve dew points down to about -50°C (lower with multi-stage systems).
Detailed Explanation
The dew point is the temperature at which air, cooled at constant pressure, reaches saturation (RH = 100%) and water vapor begins to condense. It is expressed in °Cdp and is a single-variable, direct indicator of the absolute moisture content of air. As long as moisture content and pressure do not change, the dew point remains constant even when the dry-bulb temperature changes; this is why dew point measurement is preferred over relative humidity in process control.
Typical dew point targets in industrial humidity control: +8 to +15°Cdp for comfort air conditioning (45–60% RH at 20–24°C); −5 to −15°Cdp for cold storage; −40 to −60°Cdp for lithium battery dry rooms; below −70°Cdp for gas analysis and low-humidity laboratories. Condensation-type dehumidifiers can practically reach down to +5°Cdp; below this limit, silica gel or zeolite rotor desiccant systems are required.
Calculation
Magnus formula (approximate solution):
Tdp = (b × γ(T, RH)) / (a − γ(T, RH)) γ(T, RH) = (a × T) / (b + T) + ln(RH/100)
a = 17.625 b = 243.04 °C T: dry bulb temperature (°C) RH: relative humidity (%) Tdp: dew point (°C)
Accuracy: ±0.4°C between −45°C and +60°C
Practical Example
Consider the air conditions of an ice rink. The ambient air is 14°C, 70% RH. Using the Magnus formula, the dew point is calculated as approximately +8.6°Cdp.
The ice surface temperature is around −5°C. Because the surface temperature is far below the dew point, heavy frost and fog form on the ice and cold structural elements and the ceiling start dripping; this causes a matte layer and loss of skating performance. The solution is not to cool the air further, but to lower its dew point.
A desiccant (silica gel rotor) dehumidifier supplying conditioned air from the ceiling brings the room absolute humidity into the 3.7–4.3 g/kg band, i.e. a dew point between 0 and +2°Cdp. Pulling the dew point below the ice surface (−5°C), for example to −8°Cdp, would stop frost completely, but moisture would then leave the ice surface (sublimation) and thin the ice. The practical design target is therefore the 0 to +2°Cdp band: fog and ceiling drip disappear and surface quality is preserved.
Engineering Note
Key considerations for dew point measurement:
• For low dew point measurements (below −40°C), a chilled mirror dew point meter is the reference technique; it provides ±0.1°C accuracy but has slow response time. • In industrial measurements, polymer capacitive sensors are preferred (humidity and dew-point sensors, low dew-point sensor). They respond quickly, but accuracy decreases below −50°C. • Sample line conditioning is critical: if the line is cold or made of moisture-absorbing material (silicone hose), the measurement becomes misleading. Stainless steel or PTFE tubing is essential. • Pressure difference shifts the dew point: at +1 bar gauge pressure, the dew point rises by approximately 11°C. In compressed air and gas applications, pressure dew point (PDP) must be distinguished from atmospheric dew point.
In pharmaceutical and lithium battery applications, dew point should be monitored in real time, alarm thresholds connected to the BMS, and daily logs maintained.
