Converting humidity ratio to specific humidity

In summary, the conversation discusses the difference between humidity ratio and specific humidity in psychrometry. The conversation also mentions a disagreement between the speaker and their professor regarding the conversion between the two quantities. The speaker concludes that their calculation was correct based on research, but their professor did not agree. The conversation ends with the speaker expressing their gratitude for the input and acknowledging that the disagreement will not affect their learning.
  • #1
MexChemE
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Hello PF! I have a very simple question on psychrometry. I had an argument with one of my professors regarding these two quantities, humidity ratio and specfic humidity. Humidity ratio is defined as mass of water vapor per unit mass of dry air. It's the one you can read from a psychrometric chart. Specific humidity is mass of water vapor per unit mass of moist air. Suppose you read a value from the chart of:
[tex]y' = \frac{0.005 \ \textrm{lb water}}{1 \ \textrm{lb dry air}}[/tex]
Then I figured one can go from humidity ratio to specific humidity with the following conversion:
[tex]y = \frac{0.005 \ \textrm{lb water}}{1 \ \textrm{lb dry air} + 0.005 \ \textrm{lb water}} = \frac{0.00498 \ \textrm{lb water}}{1 \ \textrm{lb moist air}}[/tex]
However, my professor said this was incorrect and gave me a bad grade in an exam, even though he once performed the following calculation in class:
[tex]x' = \frac{0.5 \ \textrm{lb water}}{1 \ \textrm{lb dry pulp}}[/tex]
[tex]x = \frac{0.5 \ \textrm{lb water}}{1 \ \textrm{lb dry pulp} + 0.5 \ \textrm{lb water}} = \frac{0.33 \ \textrm{lb water}}{1 \ \textrm{lb moist pulp}}[/tex]
However, he said this type of calculation did not apply with the air-vapor system, therefore my calculation is incorrect. I don't find that logical at all, what do you think?

Thanks in advance for any input!
P.S. I did not post this in order to keep arguing with my professor, it's just for the sake of my knowledge.
 
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  • #2
From this link, it looks like you are right: http://www.daytonashrae.org/psychrometrics_hum.html. However, I have seen other links where specific humidity is defined as the same thing as humidity ratio.

Chet
 
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  • #3
I knew there was nothing wrong with my calculation. Anyway, the grade doesn't reflect how much I've learnt. The class was Material and Energy Balance, btw. Thanks, Chet!
 

1. How do you convert humidity ratio to specific humidity?

To convert humidity ratio to specific humidity, you need to use the formula: specific humidity = humidity ratio / (1 + humidity ratio).

2. Why is it important to convert humidity ratio to specific humidity?

Converting humidity ratio to specific humidity is important because it allows for a more accurate representation of the amount of water vapor in the air. Humidity ratio measures the mass of water vapor per unit mass of dry air, while specific humidity measures the mass of water vapor per unit mass of moist air. This is especially important in meteorology and air conditioning, where specific humidity is used to determine the dew point and relative humidity.

3. What is the difference between humidity ratio and specific humidity?

The main difference between humidity ratio and specific humidity is the unit of measurement. Humidity ratio is measured in mass of water vapor per unit mass of dry air, while specific humidity is measured in mass of water vapor per unit mass of moist air. This means that specific humidity takes into account the moisture in the air, while humidity ratio does not.

4. Can specific humidity be greater than 100%?

No, specific humidity cannot be greater than 100%. This is because specific humidity is a ratio of the mass of water vapor to the total mass of moist air, and it is not possible for the mass of water vapor to exceed the total mass of moist air.

5. How does humidity ratio and specific humidity affect the weather?

Humidity ratio and specific humidity are important factors in determining the weather. Higher humidity ratio and specific humidity levels indicate a greater amount of water vapor in the air, which can lead to increased chances of precipitation and higher humidity levels. In contrast, lower humidity ratio and specific humidity levels can lead to drier conditions and lower humidity levels.

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