What is Dew Point?
Dew point is the lowest temperature (°C or °F) at which water vapour in a gas starts to condensate. Below this temperature, water vapor will turn into a liquid.
Dew point is unaffected by the temperature of the gas, but it is affected by pressure. Increasing gas pressure (compressing a gas) increases water vapour pressure and therefore increases dew point. As a result, water vapour will start to condensate at a higher temperature in compressed gases compared to atmospheric pressure (ambient conditions). Decreasing gas pressure decreases the dew point temperature. In this situation, the gas will become dryer. Very low dew points such as those used in vacuum systems have almost no moisture in the gas.
Dew Point v Pressure Dew Point
Dew Point (DP) or atmospheric dew point (ADP) is a measure of dew point in atmospheric conditions (non-pressurised gases).
Pressure Dew Point (PDP) is a measure of dew point in compressed gas systems. Pressure dew point will differ from dew point as changing the pressure of a gas changes its dew point.
Many people use dew point and pressure dew point interchangeably but its important to understand which dew point they are taking about – atmospheric dew point or pressurised dew point as the measurement results will be different.
Dew Point v Relative Humidity
As described above, dew point is the lowest temperature (°C or °F) at which water vapour in a gas starts to condensate. Higher dew points mean there is more moisture in the gas and the gas feels wetter. Lower dew points mean there is less moisture in the gas and the gas feels dryer.
Relative Humidity (RH) represents how close a gas is to saturation and is expressed as a percentage (%) of the amount of moisture a gas can hold. Relative humidity is affected by temperature and pressure.
Why Measure Dew Point?
Industries using compressed air or gas systems need to keep an eye on the amount of condensation produced because moisture in the gas can clog pipes, break machinery, cause contamination (eg rust, mildew, bacteria), freeze pipes or affect the final product.
Some manufacturers require high purity air, for example where compressed air will touch food or pharmaceutical products. Moisture content is one part of the compressed air purity tests – lower moisture in the air (lower dew point) equates to a higher air purity rating. So, measuring pressure dew point becomes important to maintain compressed air purity standards.
Other reasons to measure pressure dew point include:
- detecting moisture issues before they can cause a problem
- monitoring dryer performance and improving dryer reliability
- improving filter life and performance
- improving equipment life
- improving system reliability
- reducing product contamination risks from bacteria, fungus and yeast
- reducing the risk of rust and corrosion build up
- reducing system maintenance
- reducing operating and energy costs
- demonstrating compliance with industry specific requirements
How do you Measure Dew Point?
The simplest way to measure pressure dew point is by installing a dew point sensor and letting the sensor measure temperature, relative humidity and dew point.
Dew point sensors work through impedance. The sensor is composed of layers; two electrodes on either side of an absorbent layer. As gas passes over the sensor, the absorbent layer soaks up the water vapor. The water molecules change the dielectric constant, which results in a change in the sensor’s impedance
You could also use a dew point mirror to measure pressure dew point. This device uses a mirror and laser to measure dew point. Dew point mirrors are very accurate but also expensive and generally aren’t suitable for manufacturing and industrial settings.
Lastly, if you know the temperature, relative humidity and pressure, you can calculate pressure dew point.
Analysing Dew Point Data
Analysing pressure dew point against ambient temperature will show if your dryer is working correctly or if there are moisture issues with your compressed air or gas system.
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