blog 露点と|その計測方法について 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 適切な計測範囲の計測機器を選びましょう。 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。 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。
DMT143 小型露点変換器/DMT143L 露点変換器 DMT143およびDMT143L 露点変換器は正確な露点計測を提供し、小型圧縮空気ドライヤー、プラスチックドライヤー、積層造形、その他OEM用途にも最適です。
DMT152 露点変換器 -80°Cまでの計測に対応する低露点計測用として設計されているDMT152は、半導体業界をはじめ、純ガス、ドライヤー、グローブボックス、乾燥チャンバー、積層造形などの産業用途や、微量の水分管理が必要とされる用途に最適です。
Prasanta Pal Sep 24, 2021 Need the dew point measurement device for drying tower outlet at Sulphuric acid manufacturing process Reply
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 Reply
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 Reply
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 Reply
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. Reply
Yashwanth kumar Aug 15, 2022 At what location, should we measure atmospheric and pressure dew point. Reply
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? Reply
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 Reply
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 Reply
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% Reply
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. Reply
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? Reply
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)). Reply
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. Reply