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, browse our compressed air knowledge hub, or contact us for more information.

Webinar: Humidity measurements under pressure

Webinar: Humidity measurements under pressure

Does pressure affect humidity? When should I take dynamic pressure variation into account? Do I need to compensate the humidity values with pressure data? Join this webinar to learn when and how to take pressure into account.

What is pressure and how is it related on humidity?
Application examples
Under pressure: Vacuum drying
Varying atmospheric pressure: Combustion engine test benches
Over pressure: Moisture in Compressed air



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: https://www.vaisala.com/sites/default/files/documents/CEN-G-CompAir-Product-Brochure-B211208EN.pdf.

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: https://www.vaisala.com/system/files/documents/Humidity-conversion-formulas-Technical-eBook-B210973EN-L.pdf
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:
https://www.vaisala.com/en/lp/make-your-job-easier-humidity-conversion-formulas, or then you
can register for our webinar: https://www.vaisala.com/en/events/webinars/lp/real-life-use-cases-humidity-calculations
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 https://www.vaisala.com/en/events/webinars/lp/humidity-measurements-under-pressure to get more information on what pressure is and how it is related to humidity?


Nov 11, 2022
I need to dew point sensor to measure the dew point of compressed air and control the process based on its feedback.
Please guide how to choose


Nov 25, 2022
Dear Vikul, We have an excellent eGuide that helps in choosing the right instrument for your needs:


Feb 7, 2023
What is the ideal dew point required in an industrial application where the working pressure is between 6-7bar and the equipment's connected are mostly pneumatic cylinders, solenoid valves, pneumatic brakes, etc. Ambient temp ranges from 25-50 degC. And the average relative remains between 60-80%

Juhani Lehto

Apr 11, 2023
Dear Ashok Pujari,
To find out the dew point in a certain temperature, pressure, and humidity can be solved relatively easy by using e.g. the Vaisala Humidity Calculator: https://www.vaisala.com/en/measurement/humidity-dew-point-and-moisture. With this free tool we can calculate how much the dew point maximum and minimum are. The maximum dew point happens under higher pressure, higher relative humidity and higher temperature. If we put 7 bar, 80 % RH, and 50 degrees Celsius to the Humidity calculator, we can see the calculated dew point temperature is about 45 degrees C. So, if the gas is cooled down to 45 degrees C, the condensation happens. On the other hand, if our conditions are 6 bar pressure, 60 % RH and 25 degrees C, the dew point temperature would be then about 17 degrees C. Based on this information, the dew point temperature is between 17… 45 degrees Celsius depending on the conditions.


Mar 7, 2023
We tried to measure the dewpoint for Argon and Nitrogen compressed gases which theoretically are at -186degC and -70 degC using the DMP74B dewpoint analyzer, but we are unable to get the accurate reading of the dewpoint.

Wanted a few clarification:
a) If the medium tested for dewpoint have a higher range than the analyzer used, will the analyzer give a lower reading instead as it is beyond the accuracy region?
b) Can dry gases like argon & nitrogen be used to test dewpoint with the Vaisala DMP74B type analyzers?

Juhani Lehto

Apr 11, 2023
Dear Velan, Thank you for your questions.
A) If the real frost point (dew point below zero degrees Celsius) is lower (drier) than e.g. the DMP74B, the instrument typically shows the lowest possible reading (with this product the frost point reading goes somewhere to -74 degrees Celsius because the sensor cannot anymore detect the drier condition.

B) Argon and nitrogen are both inert gases and can be measured with Vaisala capacitive thin-film sensor technology. Please note that the pressure of the measured gas has an influence on the dew/frost point and should be inputted to the MI70 handheld to compensate for the pressure effect (the DMP74B cannot measure the gas pressure itself but you need to tell it to the instrument if the pressure is not ambient, i.e. 1 bar(absolute)).


Apr 26, 2023
I'm trying to use DMT143, but having trouble reading the result.
As far as I know, our ouput is in analog (the value is between 0 to 4095). Is there a conversion chart or formula that will calculate the output into dew point correctly?

Antti Viitanen

Jun 19, 2023
Dear Lin,

Analog output scaling depends on the product you have purchased. Based on the given number you are referring to output figures of analog to digital converter. To get your signal correctly converted you need to first understand what kind of output is selected in DMT143 you are using. If no changes are done to the product it can be done by comparing the configuration code of DMT143 to the order form of DMT143 found at docs.vaisala.com. To be 100% sure the product used can tell it’s scaling through the digital bus. DMT143 scaling could be for example be set such that output is 0 to 5 Volts and range is -70 to +60’C. This means that when the transmitter shows 0 Volts output corresponds to -70’C. If output is 5 volts then it corresponds +60’C dew point. If the transmitter outputs 2.5 Volts then it would correspond -5’C dew point.

After you have determined the scaling of DMT143 you need to make sure that how your analog-to-digital converter is scaled and how analog connections are made. For example, if 12 bit AD converter is able to read the whole 0 to 5 Volts output it means that when reading is decimal 0 output of the transmitter is 0 Volts which is the lower range value. If the reading is 4095 it means a maximum of 12 bit converter it means the transmitter outputting 5 Volts which is the maximum range value. If the output is 2047 it means that the transmitter outputs 2.5Volts which is then the middle of the range.

Issues could be that for example if an analog connection is scaled for 0 to 5 volts but the transmitter scaling is 0 to 1 volts it means that only one-fifth of AD converter capability is used and when the signal shows 819 it means that the transmitter outputs 5 volts which is maximum range value.

Best Regards,
Antti Viitanen
Product Manager


May 24, 2023
Can moisture analyzer measure dew point?

Juhani Lehto

May 25, 2023
Hello Munawer. Thank you for sending your question. It depends on the moisture analyzer if it can output the dew point temperature or not. Most of the Vaisala humidity instruments can also output dew point. We are happy to advise you with all Vaisala related product questions.


Jul 6, 2023
In a compressed air system's pipes are we measuring dew point so that:

1) We keep the compressed air's temperature above the dew point to keep the water vapour in the air
2) We keep the compressed air's temperature at or below the dew point to allow the water vapour in the air to condense out, and if so where does that water go to?


Oct 13, 2023
Dear Paul,
I apologize for the late reply, there have been a few comments gone in our Comments, and come up now. 😯

Compressed air should be dried at all times to e.g. prevent corrosion and bacterial growth in the compressed air system, and to cancel free water in the air itself (any water spray or droplets are a sign of bad quality compressed air). There are different drying methods.

The most common (low-cost) drying method is a refrigerant dryer that cools the air close to the freezing point. The cool air dew point then directly equals the air temperature because the cold air cannot hold water anymore and all excess water can be removed by draining it out of the system.

The cool air is saturated by water meaning the RH is 100 %. After the drying, the air is warmed again, when the relative humidity decreases (hot air can hold more water again). The Vaisala DMT132 is a suitable product for refrigerant driers. With a desiccant drier, it is possible to achieve drier compressed air.

Selvakumar Srinivsaperumal

Jul 22, 2023
I am using DMT 143 in my line at a pressure range from 2 to 34 bar, as it is line mounted am getting PDP, but my customer wants ADP irrespective of the line pressure. I know the line pressure reading from PRT, could you help me with the conversion formula? to convert directly to ADP from PRT and PDP readings?

Juhani Lehto

Oct 13, 2023
Dear Selvakumar Srinivsaperumal,
You can tell the actual process pressure for the DMT143 by using the serial line and XPRES command. The DMT143 can calculate the actual dew point/frost point temperature, converted to atmospheric pressure.
Best regard, Juhani Lehto


Oct 1, 2023
hello ,Thanks for the Information I want to know how I can control the dew point in a compressor reviews with dryer integrate

Juhani Lehto

Oct 13, 2023
Dear Tarek,
Typical solution is that the dryer is controlled by the online dew point measurement. For example, the desiccant dryer regeneration cycle has been traditionally controlled based on time (assumption when the drying agent needs regeneration). A much better way is to switch dryer towers based on the real moisture inside the tower and that can be detected by measuring the outlet of the dryer. As soon as the output gets moisture it is time to switch. This saves energy and money because the tower drying is based on real-time measurement. I recommend reading this article about compressed air drying: https://www.vaisala.com/en/lp/compressed-air-eguide-benefit-industrial-measurements This document also contains a list of suitable dew point transmitter that fits in your dew point range.
Best regards, Juhani Lehto

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