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Mikä on kastepiste, ja miten sitä mitataan?

Kastepisteen ja paineen välinen suhde
Teollisuustuotanto ja -prosessit
Teollisuuden mittaukset

Mikä on kastepiste?
Kastepiste on lämpötila, johon ilma on jäähdytettävä, jotta siinä oleva höyry tiivistyy kasteeksi tai huurteeksi. Jokaisessa lämpötilassa on tietty enimmäisraja sille, kuinka paljon vesihöyryä ilmassa voi olla. Tätä enimmäismäärää kutsutaan vesihöyryn saturaatiopaineeksi. Vesihöyryn lisäämisestä vielä seuraa kondensaatiota.

Miksi kosteus on ongelmallista?
Kondensaatio paineistetussa ilmassa on ongelmallista, koska se aiheuttaa putkitukoksia, koneiden hajoamista, kontaminaatiota ja jäätymistä.

Mikä on paineen vaikutus kastepisteeseen?
Ilman paineistaminen lisää vesihöyryn painetta ja kasvattaa näin kastepistettä. On tärkeää ottaa tämä huomioon, jos ilma päästetään ulos prosessista ennen mittauksen suorittamista. Mittauskohdan kastepiste eroaa tällöin prosessin kastepisteestä.

Millainen on kastepisteen tyypillinen vaihteluväli?
Paineistetun ilman kastepiste voi poiketa ympäristön kastepisteestä joissakin tilanteissa jopa -80 °C:een asti. Kompressorijärjestelmät, joissa ei ole ilmankuivausominaisuutta, tuottavat yleensä paineilmaa, joka on saturoitunut ympäristön lämpötilassa. Jäähdytyskuivaimella varustetut järjestelmät siirtävät paineistetun ilman jäähdytetyn lämmönvaihtimen läpi, jolloin vesi tiivistyy pois ilmavirrasta. Nämä järjestelmät tuottavat tavallisesti ilmaa, jonka kastepiste on alimmillaan 5 °C. Kuivausainetta hyödyntävät kuivausjärjestelmät imevät vesihöyryä ilmavirrasta ja voivat tuottaa ilmaa, jonka kastepiste on -40 °C, ja tarvittaessa kuivempaakin.

Miten kastepistettä voi mitata luotettavasti?

  1. Valitse mittalaite, jonka mittausalue on oikea.
  2. Ota selvää kastepistemittalaitteen paineominaisuuksista: Vaisalan mittalaitteet soveltuvat käyttöön prosessin paineessa, mutta markkinoilla on sellaisiakin laitteita, jotka eivät siihen sovellu. Ne voidaan asentaa paineilman mittaamiseksi sitten, kun se on laajennettu ympäristön ilmanpaineeseen, mutta kastepisteen mittaustulos on korjattava, jos haluttu mittausparametri on paineistetun ilman kastepiste.
  3. Asenna anturi oikein: Noudata valmistajan ohjeita. Älä asenna kastepisteantureita putkien umpinaisiin päihin, joissa ei ole ilmavirtausta.

Lue lisää paineistetusta ilmasta tai tutustu alla oleviin Vaisalan kastepistemittalaitteisiin.

Comment

nikhil

03. Joulu 2019
good info

Prasanta Pal

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

Juhani Lehto

09. Marras 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

31. Touko 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

07. Kesä 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

31. Heinä 2022
What is relationship between Dewpoint and RH factor.

Vaisala

16. Elo 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

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

Vaisala

25. Elo 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

11. Marras 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

25. Marras 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

07. Helmi 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

11. Huhti 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

07. Maalis 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

11. Huhti 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

26. Huhti 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

19. Kesä 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

24. Touko 2023
Can moisture analyzer measure dew point?

Juhani Lehto

25. Touko 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.

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