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Relative Humidity Exercise
Practice Sheet

1.    The air holds 6 grains of water vapor and can hold 10 grains.  What is the relative humidity (H_R)?

 In this example, the capacity is 10 grains and the content is 6 grains.

Therefore

    

2.    At the given temperature, the air can hold 15 grams of water vapor.  It only holds 7.5 grams.  What is the
        H_R?

In this example, the capacity is 15 grams and the content is 7.5 grams. 

Therefore

            H_R =  =  = 50%

3.    Once cubic meter of air can hold 15 grams of water vapor at 30^OC.  You find that 2 cubic meters holds
        16 grams.  What is the H_R?

Remember that all of the measurements are related to a given volume of air.  As long as the volume is one of anything, the math is simple.  In this case the volume of the capacity is in one cubic meter.  The volume of the content is in two cubic meters.  You have to equate the two, either by doubling the capacity or taking one/half the content.  It doesn’t matter as the math works for either one.

 

Therefore:

 

H_R  =    =    =  53%

4.    The content of the air is 8 grains and the capacity is 24 grains,  what is the H_R?

In this example the content is 8 grains and the capacity is 24 grains.

 Therefore:

 H_R  =    =   =  33%

 

5.    The air temperature is 35^OC and the air can hold 35 grams of water vapor.  The H_R is 45% so what is the
        content?

In this example we are asked to rearrange the formulae.

 H_R  =    therefore content =  (H_R ) (Capacity)

 Content = (45%) ( 35 grams)  =  (0.45) ( 35 grams)  =  15.8 grams

        Using the H_R graph found at the end of this practice sheet, answer the following questions.

6.    The temperature is 70^O F and the air holds 6 grains of water vapor per cubic foot.  What is the  H_R?

At 70^OF the capacity is 8 grains per ft³

H_R    =    =    =  75%

7.    The temperature is 10^O C and the air holds 5 grams of water vapor per cubic meter.  What is the  H_R?

At   10^O C the capacity is 10 grams

 Therefore: 

 H_R    =    =      =   50%

8.    The air holds 2 grains of water vapor per cubic foot.  The H_R is 50%.  What is the temperature?

Once again we are asked to rearrange the formulae.

H_R    =      

Therefore Capacity  =   =     =   4 grains

 a Capacity of 4 grains occurs at 50^O F.

    To use this graph you must first determine if you are in the English or Metric measurement system.  Lets assume you are in the English system of grains, cubic feet, and degrees Fahrenheit.  Enter at the degrees Fahrenheit, going vertically until you come to the curved line separating the two measurement systems.  Then proceed to the right to read the capacity in grains per cubic foot.  For the metric system the degrees Celsius are located along the top margin and the capacity in grams per cubic meter are located along the left hand side.