[Core Summary]
The dew point temperature is the temperature at which moist air cools to saturation with water vapor under constant pressure and begins to condense. It is a core parameter for compressed air drying, gas pipeline protection, and surface condensation judgment. Common confusions surrounding this term include the difference between pressure dew point and atmospheric dew point, the physical relationship that dew point decreases as pressure increases, and the accuracy reference conditions for dew point transmitters. This entry uses the E+E Elektronik (E+E of Austria) EE355 dew point transmitter and EE75 wind speed/temperature transmitter as references to explain each item.
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Definition and Physical Meaning
Dew point (symbol Td) refers to the temperature at which the partial pressure of water vapor reaches the saturated vapor pressure due to temperature drop in humid air under constant pressure, causing the water vapor to condense into liquid water. The lower the dew point, the drier the gas. In compressed air systems, a pressure dew point of –40 °C Td indicates that the gas will only release water when cooled to –40 °C at that pressure, which is a common dryness level for instrument air.
Pressure dew point vs Atmospheric dew point
For compressed air with the same moisture content, the higher the pressure, the lower the pressure dew point. The dew point measured after depressurizing the pressurized gas to atmospheric pressure is called atmospheric dew point (atmospheric pressure dew point), which is more than ten degrees Celsius higher than the pressure dew point. When making compliance judgments, it is necessary to distinguish which standard is being referenced: the commonly used "pressure dew point" and the "atmospheric pressure dew point" marked on some export equipment. EE355 has a pressure resistance of 0...80 bar and can directly read the pressure dew point in the pressurized pipeline, avoiding deviations introduced by depressurization sampling.
The relationship between dew point and relative humidity
Relative humidity is the ratio of the partial pressure of water vapor to the saturated partial pressure at the same temperature, and it varies with temperature. Dew point, on the other hand, converts the question of "whether condensation will occur" into a temperature threshold, making it more intuitive for engineering purposes. When the surface temperature of an object is lower than the dew point of the nearby air, condensation will form on the surface - this is a direct criterion for selecting the outlet tube, mirror surface, and optical window of a cold dryer.
Why is precision reference condition important
The accuracy specifications of dew point transmitters are all tied to reference conditions. The typical accuracy of the EE355 is ≤±2 °C Td, with samples based on normal temperature and pressure, and stable flow rates; a margin should be reserved at extremely low dew points or high flow rates. Quoting nominal values without reference to reference conditions is a common misinterpretation during the selection and acceptance phases. The EE75 is used for wind speed and temperature measurement, and its temperature channel can be combined with dew point data to calculate the gas state.
Typical application scenarios
Compressor air dryer outlet and dew point joint control, gas pipeline anti-condensation, cold surface and mirror anti-condensation, food and pharmaceutical clean air source monitoring, etc. E+E Elektronik's EE355 covers a temperature range of -60...+60 °C Td, suitable for continuous monitoring of instrument air and oil-free compressed air; for scenarios requiring simultaneous measurement of wind speed and temperature, it can be combined with EE75 for point arrangement.
[Parameter Table]
**Parameter Source Description**: The measurement range, accuracy, output, and protection level are taken from the product catalogs of E+E Elektronik EE355 and EE75. Details such as calibration intervals and minimum operating temperatures for specific models are subject to the latest factory catalog. If necessary, please mark [Please verify].
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