Ambient pressure dew point and pressurized dew point correspond to moisture in ppm.

What are the atmospheric pressure dew point and the pressure dew point?

In the field of compressed air treatment, the dew point temperature is the primary metric for assessing the dryness of a gas. Depending on the pressure conditions under which it is measured, the dew point is classified as…Ambient pressure dew pointandPressure dew point.

• Ambient Pressure Dew Point (ADP): refers to the gas under standard atmospheric pressure (0.1013 MPa) is cooled to the temperature at which water vapor begins to condense into liquid water.
• Pressure Dew Point (PDP): The temperature to which a gas must be cooled, at its actual operating pressure, for water vapor to begin condensing.

The conversion relationship between atmospheric pressure dew point and pressure dew point

Changes in gas pressure directly affect the value of the pressure dew point, but do not alter the absolute moisture content of the gas. When the pressure of compressed air increases, the partial pressure of water vapor also rises, resulting inThe pressure dew point is higher than the atmospheric dew point..

For example, at 0. 7 MPa At a working pressure (gauge pressure), if the system’s pressure dew point is 2°C, the corresponding atmospheric‑pressure dew point is approximately −20°C. Therefore, when comparing the dryness of different systems, all dew‑point data must be uniformly converted either to the atmospheric‑pressure dew point or to the pressure dew point at the same reference pressure; otherwise, serious misinterpretations may arise.

The relationship between dew point temperature and moisture concentration in ppm

Moisture ppm(Usually expressed as ppmv by volume) It denotes the volume of water vapor contained in one million volumes of compressed air. There is a strict physical relationship between the dew point temperature and the moisture concentration in ppm, which is primarily converted using a table of saturated vapor pressures.

• Negative correlation trendThe lower the dew point temperature, the smaller the partial pressure of saturated water vapor in the air, and the lower the corresponding moisture concentration in ppm.
• Baseline consistencyThe ppm moisture value is directly correlated with the atmospheric pressure dew point. Regardless of changes in system pressure, as long as the atmospheric pressure dew point remains constant, the corresponding ppm moisture value will also remain unchanged.
• Typical referenceAt standard atmospheric pressure, a pressure‑related dew point of −20°C corresponds to a moisture content of approximately 1,018 ppmv, whereas a dew point of −40°C drops this to about 126 ppmv. For precise values, consult specialized thermodynamic tables of water‑vapor properties or use dedicated conversion tools.

Recommendations for Practical Applications in Compressed Air Systems

Understanding the aforementioned concepts is of guiding significance for humidity control in compressed air systems and equipment selection.

• Clarify technical agreement standardsWhen procuring dryers or establishing air quality standards, it is essential to specify whether the dew point metric refers to atmospheric pressure dew point or pressure dew point at a specified pressure.
• Select drying equipment appropriately.: If the process requires extremely low moisture levels in ppm (e.g., a dew point under atmospheric pressure below – 40℃), an adsorption dryer is typically required; if the only requirement is a pressure dew point between 2°C and 10°C, a refrigerated dryer will suffice.
• Standardized measurement methodWhen using a dew‑point instrument, ensure that the sensor is installed either in a pressurized pipeline (to measure pressure dew point) or in a reduced‑pressure, atmospheric‑pressure environment (to measure atmospheric dew point), and perform appropriate pressure compensation according to the instrument’s settings.