When air blows over liquid water, it causes liquid water to change state to water vapor. This produces what is called transpirational cooling.
As the wet-bulb thermometer goes through the air, the water in the stocking evaporates. Energy is used to change the water from liquid to vapor. This causes the wet-bulb temperature to go down.
Two things influence how much the wet-bulb temperature goes down. The first is temperature. The hotter the air, the more evaporation takes place, and the more the temperature will go down. The second factor is the humidity. If there is already a lot of water in the air, not as much liquid water on the stocking will be able to enter the air in the vapor state.
When you sling the psychrometer, what happens to the thermometers? The dry-bulb thermometer records the air temperature. 2. The wet-bulb thermometer goes down in temperature until no more water will evaporate. This frame has an animation. Wait a few seconds to see the psychrometer swing around!
The difference between the wet- and dry-bulb temperatures is used to determine the absolute humidity, which is the actual amount of water vapor in the air, and the saturation value, which is the maximum amount of water vapor which can enter the air at a particular temperature. Scientists have made tables which allow us to easily look up these values.
Once we have the numbers for absolute humidity and saturation value, we can easily calculate the relative humidity, which is what we are looking for.
This chart shows how relative humidity decreases as the difference between wet-and dry-bulb temperatures increase.
The sling psychrometer we use has a sliding scale on the back, which allows us to directly read relative humidity without having to do all the calculations ourselves.
A Sling PsychrometerA Sling Psychrometer
is used to measureis used to measure
relative humidity.relative humidity.
Get yours today!Get yours today!
Psychrometer Parts:Psychrometer Parts:
•Dry Bulb ThermometerDry Bulb Thermometer
•Wet Bulb ThermometerWet Bulb Thermometer
•Reservoir and StockingReservoir and Stocking
But why do we sling itBut why do we sling it
What happens to your skin when youWhat happens to your skin when you
sweat and then stand in a breeze?sweat and then stand in a breeze?
As the wet-bulb themometer goes through the airAs the wet-bulb themometer goes through the air
1. Water1. Water
2. Energy is2. Energy is
used; waterused; water
from liquid tofrom liquid to
3. Wet-bulb temperature goes down3. Wet-bulb temperature goes down
What influences howWhat influences how
much the wet-bulbmuch the wet-bulb
temperature goestemperature goes
Using our wet- andUsing our wet- and
dry-bulb temperatures,dry-bulb temperatures,
we can consult tableswe can consult tables
made by scientists…made by scientists…
……to determineto determine humidityhumidity andand saturation valuesaturation value..
Now we can calculate relative humidity!Now we can calculate relative humidity!
Partial Relative Humidity Chart for 30° CPartial Relative Humidity Chart for 30° C
Dry Bulb and Wet Bulb
Fortunately, we don’t have to do all this every time…Fortunately, we don’t have to do all this every time…
On the side of the psychrometer isOn the side of the psychrometer is
a sliding scale which calculatesa sliding scale which calculates
relative humidity for us.relative humidity for us.
Align the dry-bulb reading with theAlign the dry-bulb reading with the
wet-bulb reading. The arrow will pointwet-bulb reading. The arrow will point
to the relative humidity reading on theto the relative humidity reading on the
center scale. It’s easy!center scale. It’s easy!