Relative Humidity more than 100%

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Discussion Overview

The discussion revolves around the phenomenon of relative humidity exceeding 100%, particularly in the context of calculations related to a membrane humidifier. Participants explore the implications of such calculations, including the presence of liquid water and the behavior of water vapor under varying conditions.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes that relative humidity over 100% suggests the presence of liquid water alongside saturated vapor and questions how to calculate liquid water content.
  • Another participant mentions that exceeding 100% relative humidity is possible in simulations but unlikely in real-world conditions, citing that condensation typically requires foreign particles.
  • A participant describes their specific scenario involving a membrane humidifier, where dry air and humid air are present, leading to calculations showing >100% RH due to partial pressure exceeding saturation pressure.
  • Another response indicates that saturation vapor pressures are based on still air, suggesting that movement of air could lead to partial pressures exceeding saturation pressure, and questions the accuracy of the RH calculation or measurement.

Areas of Agreement / Disagreement

Participants express differing views on the likelihood and implications of relative humidity exceeding 100%. There is no consensus on the conditions under which this occurs or the accuracy of calculations versus measurements.

Contextual Notes

Participants mention potential measurement errors and the influence of air movement on saturation pressures, indicating that assumptions about still air may not hold in all scenarios.

nishith17
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In a calculation I am getting relative humidity more than 100%. What does it suggests?
I know this means liquid water is also present in addition to saturated vapour. How should I calculate liquid water content then? Do I have to calculate activity first? Also what happens when I am considering diffusion of water vapour to a dry side (through a membrane)? Does liquid water present will evaporate to compensate drop in partial pressure of water vapour (considering total pressure remains constant)?
 
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nigarbageh17 said:
In a calculation I am getting relative humidity more than 100%. What does it suggests?
I know this means liquid water is also present in addition to saturated vapour. How should I calculate liquid water content then? Do I have to calculate activity first? Also what happens when I am considering diffusion of water vapour to a dry side (through a membrane)? Does liquid water present will evaporate to compensate drop in partial pressure of water vapour (considering total pressure remains constant)?

You haven’t said what your calculations were based on. If they were based on a simulation, it is entirely possible for the relative humidity to exceed 100%. If they were based on actual real world air conditions, then it is still possible, but very unlikely. Water droplets condense on foreign particles, such as dust, that are suspended in the air. Without such particles, condensation becomes difficult, and relative humidity’s as high as 800 % have been recorded in extremely clean air under laboratory conditions. In real life conditions, with all the combustion products and pollution that is present in the air, it is very unlikely that relative humidity will ever exceed 100%.
 
I did calculations for a membrane humidifier. I am having dry air on one side and on other inlet I have humid air. The problem is that I am getting >100% RH in humid air. Partial pressure of water vapour is more than saturation pressure at that temperature
 
nigarbageh17 said:
I did calculations for a membrane humidifier. I am having dry air on one side and on other inlet I have humid air. The problem is that I am getting >100% RH in humid air. Partial pressure of water vapour is more than saturation pressure at that temperature

Saturation vapor pressures are compiled for still air. If your air is in movement, it is entirely possible that your partial pressures should exceed the saturation pressure by a small amount. What is the magnitude of your excess vapor pressure? Is the RH being calculated or is it being measured? Keep in mind that all measurements are subject to measurement errors.
 

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