The dew point depends on how much water vapor is present in the air. If the air is very dry with few water molecules, the dew point is low, and surfaces must be much colder than the air for condensation to occur. If the air is very humid with a high concentration of water molecules, the dew point is high, and condensation can occur on surfaces only slightly cooler than the air.
What is dew point?
The dew point refers to the temperature at which water vapor in the air becomes saturated and begins to condense into liquid water (dew, fog, or frost) under constant pressure.
According to the authoritative definition of the National Physical Laboratory (NPL) in the United Kingdom, when the condensate formed during the cooling process is ice, the temperature is referred to as the frost point. In compressed air systems, dew point is the most direct and reliable indicator for measuring humidity.
We often see condensation in our daily life. For example, in autumn and winter, when the temperature drops, part of the water vapor in the air liquefies, forming dew on the grass and dew or frost on the glass windows. This happens because the decrease in temperature causes the originally unsaturated water vapor in the air to gradually become saturated. The saturated water vapor forms dew in an environment above 0 degrees, and frost forms in an environment below 0 degrees. Dew point is an important meteorological parameter that can predict dew, frost, fog, nighttime low temperatures, and even the formation of rainfall and tornadoes. Provide data support on air humidity for weather enthusiasts.
The dew point depends on how much water vapor is present in the air. If the air is very dry with few water molecules, the dew point is low, and surfaces must be much colder than the air for condensation to occur. If the air is very humid with a high concentration of water molecules, the dew point is high, and condensation can occur on surfaces only slightly cooler than the air.
What are the factors influencing dew point?
1. Water vapor content (humidity)
This is the most direct factor. The dew point is essentially a measure of how much water vapor is actually in the air.
- High water content: If the air contains a large number of water molecules, it doesn’t need to be cooled much before it reaches saturation. This results in a high dew point.
- Low water content: If the air is very dry (processed by a desiccant dryer, for example), it must be cooled to an extremely low temperature before condensation occurs. This results in a low dew point.
| Temperature (°C) | Relative Humidity (%) | Dew Point Temperature (°C) |
|---|---|---|
| 30 | 10 | 1.0 |
| 30 | 20 | 5.4 |
| 30 | 30 | 9.1 |
| 30 | 40 | 12.3 |
| 30 | 50 | 15.0 |
| 30 | 60 | 17.3 |
| 30 | 70 | 19.4 |
| 30 | 80 | 21.3 |
| 30 | 90 | 23.1 |
| 25 | 10 | -3.3 |
| 25 | 20 | 1.2 |
| 25 | 30 | 5.0 |
| 25 | 40 | 8.3 |
| 25 | 50 | 11.0 |
| 25 | 60 | 13.3 |
| 25 | 70 | 15.4 |
| 25 | 80 | 17.3 |
| 25 | 90 | 19.1 |
2. Pressure
During air compression, the air volume decreases while the water vapor content remains unchanged. This causes water molecules to become more concentrated. Increasing air pressure raises the dew point. For example, if air with a dew point of -20 °C at atmospheric pressure is compressed to 0.7 MPa (7 bar), the dew point may rise to +10 °C, resulting in immediate condensation inside the pipeline.
3. Ambient temperature
While the dew point temperature itself is an absolute measure of moisture, the ambient temperature surrounding your pipes determines whether that dew point matters.
If your compressed air has a dew point of +10°C and your factory floor is +25°C, you won’t see liquid water. If that same pipe runs outside where the night temperature drops to +5°C (which is below the +10°C dew point), water will immediately condense inside the pipe, leading to rust and frozen lines.
Why must compressed air systems monitor the dew point?
Moisture is a primary contaminant in industrial processes, and monitoring the dew point offers the following direct benefits:
Protect equipment and assets: It prevents corrosion of pipeline interiors and stops moisture from washing away lubricants inside pneumatic components, thereby extending equipment life.
Ensure product quality: In industries such as pharmaceuticals (preventing powder clumping), food (inhibiting bacterial growth), and precision electronics, dry air is a fundamental production standard.
Significantly reduce energy consumption: By installing a sensor, dryers can operate on a “demand-driven” basis (DDS control), avoiding unnecessary energy use when the air is already sufficiently dry.
Comply with international standards: It ensures the system meets the air quality levels specified in ISO 8573-1.
How is dew point reliably measured?
Dew point meters can measure trace amounts of moisture in various gases, making them suitable for applications with strict moisture control requirements. They offer an economical and versatile solution capable of meeting the control needs of a wide range of industrial processes. Based on different measurement principles, they can be divided into the following four types:
1. Chilled mirror
Air with different moisture contents condenses on a mirror at different temperatures. Using photoelectric detection, the formation of dew is detected and the temperature at which condensation occurs is measured, directly indicating the dew point. Mirror cooling methods include semiconductor cooling, liquid nitrogen cooling, and high-pressure air cooling. Chilled mirror dew point meters use a direct measurement approach and can achieve international precision up to ±0.1 °C (dew point temperature), with general accuracy typically within ±0.5 °C.
2. Thin-film polymer capacitive
This type uses a highly moisture-sensitive polymer thin film as the dielectric to form a capacitor or resistor. When moist air passes over the film, changes in dielectric constant or conductivity occur. By measuring the resulting capacitance or resistance, the moisture content of the gas can be determined. High-precision models internationally typically achieve accuracy better than ±1 °C (dew point temperature), with general accuracy within ±3 °C. This type is suitable for most refrigerated dryers and standard adsorption dryers.
In high precision industrial drying, maintaining a stable pressure dew point is non-negotiable. Leading manufacturers, including Vaisala, Rotronic, and Renke, have developed specialized sensor technologies to address these needs. While Vaisala’s DRYCAP® is well-known for extreme low-humidity stability, Renke’s industrial-grade dew point sensors offer a high-performance, cost-effective alternative for compressed air systems.
3. Electrolytic P2O5
The electrolytic method uses materials such as phosphorus pentoxide, which absorb moisture and decompose into ions that accumulate charge on the electrodes. This principle is mainly applied to measure trace moisture in corrosive or inert gases. It is not commonly used in general industrial compressed air systems due to high maintenance costs and extreme sensitivity to airflow rates.
4. Semiconductor
Each water molecule has its natural vibration frequency. When it enters the voids of a semiconductor lattice, it resonates with the lattice excited by an electric charge. The resonance frequency is proportional to the number of water molecules. The energy from this resonance releases free electrons in the semiconductor junction, increasing lattice conductivity and reducing impedance. Dew point meters designed using this principle can detect trace moisture at dew points as low as -100 °C.
What is the role of dew point meters in compressed air systems?
These sensors are critical for dew point protection in compressed air systems, enabling continuous monitoring and control of moisture content in the air. Compressed air must always remain dry, with free water removed. The presence of any mist or droplets indicates poor air quality. Without effective drying, moisture can cause pipeline corrosion, pneumatic component failure, system degradation, and bacterial growth.
By installing a dew point sensor, the moisture content in compressed air can be accurately measured and monitored, allowing targeted control measures to maintain it within a safe range and prevent system damage.
The most common drying method uses refrigerated dryers, cooling the air close to the freezing point. At this temperature, the dew point of the cold air equals the air temperature because cold air cannot hold additional moisture, and all excess water can be removed through system drainage.
At saturation, cold air has a relative humidity of 100%. After drying, when the air is reheated, relative humidity decreases, as warm air can hold more moisture. Using adsorption dryers can achieve even drier compressed air.
Composed of senior hardware engineers and software architects, the Renke Technical Team stands at the forefront of environmental sensing technology. We specialize in the end-to-end development of temperature and humidity monitoring systems, from underlying hardware to specialized technical support. At Renke, we don't just build devices; we engineer precision. Our mission is simple: Focus on the user and capture every degree of change. By offering tailored, one-stop monitoring solutions worldwide, we empower industries with the robust data needed for superior environmental control.
