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How can I measure still air in 90-100% RH range with constant 25°C temperature?

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    \$\begingroup\$ I've no idea. All psychrometer concepts I know about need a sufficient amount of air flow. If you are not willing to supply that, then I'm left without knowing a method. It will be interesting if someone can suggest a still-air concept. Better still, one that has useful accuracy (not an easy thing to achieve) for the narrow range you are discussing. Do you know of any commercial devices that work without air flow? \$\endgroup\$
    – jonk
    Sep 17 '21 at 18:15
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    \$\begingroup\$ This sounds physically hard – maybe investigate the properties of a microwave cavity oscillator, where the moisture changes \$\varepsilon_r\$, maybe? \$\endgroup\$ Sep 17 '21 at 18:59
  • \$\begingroup\$ Can we use a tiny fan to blow air slowly to the sensor? \$\endgroup\$
    – tlfong01
    Sep 18 '21 at 0:41
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I may be missing something here but I have used humidity sensors in the past that can measure ambient humidity and will go up to 100% RH, with 2-4% accuracy when over 90% humidity.

Could you use one of these sensors? There are several companies building them, such as Sensirion, Honeywell, TE among others.

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    \$\begingroup\$ The problem is: all sensors which datasheets I've read clearly state that they drift upward when measuring high (80+%) humidity for longer time, requiring periods of lower humidity to reset the drift. This, in my case, is not an option. I theoretically could compensate for that, but from the datasheets I understood that the drift is not limited and won't reach a steady value after a finite amount of time. \$\endgroup\$ Sep 18 '21 at 8:44
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So I think I've solved my own problem. There is a dew sensor called SY-DS-1, which starts changing it's resistance way before condensation occurs - in fact, its operational range starts below 70% RH and is specified to work in high humidity for extended time. Datasheet specifies testing 1000 hours in 95% humidity. I expect it to be designed for prolonged contact with liquid water - it's a dew sensor after all (duh). Information in the datasheet is scarce, but I've already ordered two of them - one for torturing, other as reference for later.

http://samyoungsnc.com/wp-content/uploads/2019/03/Datasheet-_-SY-DS-1-Series-Ver.1.8-Standard.pdf The plot doesn't come from the datasheet, I've found it in a dew sensor project based on the same device. SY-DS-1 humidity-resistance plot

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  • \$\begingroup\$ Well... I will definitely continue testing it, but it exhibits some strange behaviour - it definitely changes value when loaded with DC with a long (1+ minute) settling time. After turning on my multimeter it shows 2 Meg, but it ramps up to 2,6 Meg after some time. The testing conditions imply that DC can be used as long as it doesn't exceed 0,8 V (which my multimeter doesn't reach in resistor measuring mode) - but I guess it should be used with AC just like regular RH sensitive resistors, even though it's built differently. \$\endgroup\$ Sep 24 '21 at 19:26

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