blog

露点と|その計測方法について

露点と圧力の関係
産業製造・プロセス
産業計測

露点とは
露点とは、空気中の水蒸気を冷却したとき、結露または結氷が生じ始める温度のことです。どの温度にも、空気が保持できる水蒸気の最大量が存在します。この最大量を飽和水蒸気量と呼びます。水蒸気量が飽和水蒸気量を上回ると、結露が発生します。

水分が問題になる理由
圧縮空気内の結露が問題になるのは、パイプの閉塞、機械の故障、汚染、凍結の原因になるためです。

圧力が露点に及ぼす影響
空気を圧縮すると水蒸気圧が上昇し、それに伴い露点も上昇します。計測を実施する前に空気を大気中に放出する場合は、この点を考慮しておくことが重要です。計測ポイントの露点は、プロセス中の露点とは異なります。

圧縮空気に見られる一般的な露点温度範囲
圧縮空気の露点温度は周囲温度から、特殊な場合では-80°Cにまで及ぶこともあります。空気乾燥機能のないコンプレッサーシステムは、周囲温度で飽和する圧縮空気を生成する傾向があります。冷凍式ドライヤーのあるシステムでは、圧縮空気を冷却式熱交換器に通す際にフローから結露が生じます。通常、これらのシステムでは、露点が+5°C以上の空気を生成します。乾燥剤を使用した乾燥システムは水蒸気を吸着し、露点が-40°Cの空気、必要に応じてさらに乾燥した空気を生成できます。

信頼性の高い露点計測をするには

  1. 適切な計測範囲の計測機器を選びましょう。
  2. 露点計測機器の圧力特性を理解しましょう。ヴァイサラの計測機器はプロセス圧力下での使用に適していますが、市販の計測機器の中にはそうした用途に適さないものもあります。それらは大気圧に戻した後の圧縮空気を計測する目的で取り付けることが可能ですが、希望する計測パラメータが圧力下露点の場合は、計測された露点の値を修正する必要があります。
  3. センサを適切に取り付けましょう。メーカーの指示に従い、パイプの「末端」部など、フローのないところに露点センサを取り付けないでください。

圧縮空気に関する詳細はこちらから。下にスクロールすると、ヴァイサラの露点計測機器を確認できます。

Comment

nikhil

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

Tony

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
Juhani

sunny

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

Vaisala

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.

Vaisala

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?

vikul

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

Vaisala

Nov 25, 2022
Dear Vikul, We have an excellent eGuide that helps in choosing the right instrument for your needs:
https://www.vaisala.com/en/lp/download-vaisala-compressed-air-eguide

ASHOK PUJARI

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.

Velan

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)).

Lin

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
Vaisala

Munawer

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.

新規コメントを送信