What Is Dew Point and How to Measure It?

The relationship between dew point and pressure
Industrial Manufacturing and Processes
Industrial Measurements

What is dew point?
Dew point is the temperature to which air must be cooled for water vapor in it to condense into dew or frost. At any temperature there is a maximum amount of water vapor that the air can hold. This maximum amount is called water vapor saturation pressure. Addition of more water vapor results in condensation.

Why is moisture problematic?
Condensation in pressurized air is problematic because it causes blockages in pipes, machinery breakdowns, contamination and freezing.

What is the effect of pressure on dew point?
Air compression increases water vapor pressure and thus dew point. It is important to take this into consideration if you are bleeding the air to atmosphere before taking a measurement. The dew point at the measurement point will be different from the dew point in the process.

What is the typical range of dew point?
Dew point temperatures in compressed air range from ambient even down to -80 °C (-112 °F) in special cases. Compressor systems without air drying capability tend to produce compressed air that is saturated at ambient temperature. Systems with refrigerant dryers pass the compressed air through a cooled heat exchanger, causing water to condense out of the air stream. These systems typically produce air with a dew point no lower than 5 °C (41°F). Desiccant drying systems absorb water vapor from the air stream and can produce air with a dew point of -40 °C (-40 °F) and drier if required.

How is dew point reliably measured?

  1. Select an instrument with the correct measuring range.
  2. Understand the pressure characteristics of the dew point instrument: Vaisala’s instruments are suitable for use at process pressure, but there are instruments on the market that are not. They can be installed to measure compressed air after it is expanded to atmospheric pressure, but the measured dew point value will have to be corrected if pressure dew point is the desired measurement parameter.
  3. Install the sensor correctly: Follow instructions from the manufacturer. Do not install dew point sensors at the end of stubs or other “dead end” pieces of pipe where there is no airflow

Read more about compressed air or scroll down to see Vaisala’s dew point instruments.

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