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

Download Vaisala Compressed Air eGuide or contact us for more information.



Dec 3, 2019
good info

Prasanta Pal

Sep 24, 2021
Need the dew point measurement device for drying tower outlet at Sulphuric acid manufacturing process

Juhani Lehto

Nov 9, 2021
Hello Pal
Thank you for contacting us. Sulphur acid is a harmful chemical (it corrodes the sensor easily) that must be kept away from the dew point transmitter. If the exposure of the Sulphur acid can be minimized, then you can choose the right product depending on the dewpoint measurement range you are looking for. One useful chart to choose the right product can be found here:

Best regards
Juhani, Product Manager


May 31, 2022
What is the equation and/or conversion tables that takes the electric signal from the capacitor to the values of degrees for dew point? Trying to understand how the variables pressure (absolute and relative), along with temperature, and flow impact the dew point reading. Thank you

Juhani Lehto

Jun 7, 2022
Hello Tony
Thanks for contacting us and showing your interest in dew point measurements. The capacitance signal is measured by converting it first to frequency, where the sensor is a variable capacitance component in the oscillator circuit. This frequency is then counted and converted to relative humidity based on the sensor calibration and linearization (this part is based on the sensor model and other proprietary algorithms that are not public, unfortunately). Please note we always measure relative humidity, and therefore, we need also precise temperature reading that is measured on the humidity sensor surface. This is a normal resistive (pt100) measurement.

With a help of this document, you can derive the dew point calculation based on the measured RH and T:
I hope you will also find answers to other humidity-related calculations like absolute humidity, pressure-dependency, and other humidity formulas under the same document.

Best regards


Jul 31, 2022
What is relationship between Dewpoint and RH factor.


Aug 16, 2022
Dear Sunny, More information on RH and calculations you will find here:, or then you
can register for our webinar:
I hope the Humidity Conversion Formulas eBook and our humidity calculator tool will be helpful! Both are free.

Yashwanth kumar

Aug 15, 2022
At what location, should we measure atmospheric and pressure dew point.


Aug 25, 2022
Dear Yashwanth Kumar, Thank you for your question. Would you watch our webinar to get more information on what pressure is and how it is related to humidity?

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