Manual

to inside a building. Because relative humidity is dependent on temperature, the relative humidity will vary in a
small area even though the specific humidity is not varying. A few examples will help illustrate this point.
In a building, we measure the temperature and relative humidity in one location to be 75 degrees F and 50% RH,
respectively. Referring to the psychrometric charts, we locate 75 on the bottom scale. We then follow the line
vertically up to the intersection with the 50% RH curve. From this point, we move horizontally to the right and
read the specific humidity as approximately 64 gr./lb. (grains of moisture per pound of natural air). We then
measure the temperature in another location within the building and find it to be 70 degrees. Now, assuming the
specific humidity to be constant, we can find the relative humidity at the new location. On the chart, we find the
intersection of the 70-degree line and the 64 gr./lb. line. This point is on the 60% RH curve. So, if we were to
place a hygrometer in the new location it would read 60% RH. An indoor temperature variation of 5 degrees is
not uncommon. In many buildings we have measured 20-degree variations from one area to another. This shows
that within a building, 10% RH variations are common and 50% RH variations are possible.
Let us examine some indoor to outdoor variations. The local weather service reports that the temperature is 40
degrees F and the humidity is 60% RH. Using the chart, we find specific humidity to be 25 gr./lb. If the
temperature inside is 68 degrees F then, from the chart, the indoor relative humidity will be about 25%. This
example illustrates why it feels so dry inside during the cold winter months.
Next, let us assume that the weatherman says it is 90 degrees F and 50% RH outside. And because your home is
in the shade, your indoor temperature is 80 degrees F. Referring to the chart, we find the specific humidity to be
106 gr./lb. which, at the indoor temperature of 80 degrees, will yield an indoor humidity of 70% RH.
The previous examples show that if the specific humidity remains constant, then as the air temperature increases,
the relative humidity decreases. Conversely, as the air temperature decreases, the relative humidity increases.
This phenomenon explains why cellars tend to be wetter and attics tend to be drier than the rest of a building.
In modern buildings there are many factors besides air temperature that can influence humidity readings. One
factor is the material used in the construction of the building. Many common building materials (i.e. wood,
plaster, concrete) absorb and then release water vapor as the humidity varies. This causes the specific humidity to
vary, which in turn causes the relative humidity to vary. Probably the single biggest factor affecting the humidity
is heating and air conditioning systems. A heating system can increase or decrease the specific humidity. Air
conditioning systems cause a decrease in the specific humidity. This decrease is because the cooling element in
an air conditioner is very cold. From the chart, you can see that as air passes by the very cold element, its relative
humidity increases rapidly. When the relative humidity reaches 100%, the water vapor condenses to liquid water.
The removal of vapor by condensation causes a drop in the specific humidity.
Another factor affecting the indoor humidity is that many newer buildings incorporate humidifiers and/or
dehumidifiers into the heating and air conditioning systems. We have had a number of customers who question
why their hygrometers never change more than 10% RH. When we check the instrument calibration, we find it
accurate. We then contact the customer and find out that their home has climate control. So, as it turns out, their
hygrometer is simply indicating that their control system is controlling the relative humidity very well. The
hygrometer, then, is a good indicator that all is well with their climate control systems.
With all of the aforementioned variables affecting humidity measurement, one might wonder, “How will I ever
know if my hygrometer is reading correctly?” Well, there is an acceptable method of checking hygrometer
accuracy that applies to all hygrometers.
First, you should place the instrument on a non-absorbent surface (i.e. glass or plastic, not wood) in the center of a
small room. You should leave the unit there at least two hours before proceeding. Next, you must obtain an
accurate instrument to compare against. The only commercially available instruments accurate enough to
compare against are:
Instrument Type Accuracy Approximate Cost
Sling Psychrometer +2.3% RH $40-80
Aspirated Psychrometer +1-2% RH $100-200
Chilled Mirror Dewpoint +0.5% RH $1,000-5,000