Using Sling Psychrometer
Also click HERE to view Psychrometric calculator information
and HERE for information on recording
temperature and dew point
Do not read the Dry Bulb temperature until you have 2 consecutive low Wet Bulb readings.
The sling psychrometer is a type of hygrometer, or device to measure moisture in
The sling psychrometer consists of two mercury thermometers mounted together with a
handle attached on a chain.
One thermometer measures ambient air temperature (the temperature of the
air around you).
The other thermometer has a muslin wick over its bulb. This is called
the wet-bulb thermometer.
When ready to take a reading, the observer first dips the "wet bulb" into
clean water. Then the observer grasps the handle and whirls the psychrometer around, causing the air to
quickly dry the wet bulb. The evaporation of the water on the wick causes cooling to occur. The cooling
is measured on the wet-bulb thermometer. When the wet-bulb temperature has reached its lowest point, the
observer reads both wet-bulb and dry-bulb thermometers.
The wet-bulb temperature is subtracted from the dry-bulb temperature to determine
the depression. This information is used to determine the Dew Point or Relative Humidity. If the
ambient air is dry, the thermometer will cool to a lower temperature than if the air is most, so the larger the
difference between the two temperatures, the dryer the air.
You can make your own Sling Psychrometer by finding two (non-mercury)
thermometers; a sturdy support (ruler, wooden dowel, piece of vinyl, wood, or similar); thick rubber bands or
strong duct tape; a piece of cotton shoestring; thread; and strong string or cord.
Drill a hole through the top of your support and securely attach the cord through
the hole. The cord is what you will hold in your hand when slinging to psychrometer.
Using duct tape or rubber bands, attach the two thermometers to your support, one
on each side of the support. It is critical that you make sure the thermometers are SECURELY
attached. You do not want them to fly off the psychrometer when you whirl it around. The wet-bulb
thermometer bulb should extend lower than the dry-bulb thermometer.
If you want to get fancy, you can attach the cord to a handle made from a piece of
dowel with an eye-screw on the end.
Now that you have your sling psychrometer made, cover the wet-bulb with the muslin
shoe string and tie it to the bulb with thread.
Dip the web-bulb thermometer into the clean water, hold the sling psychrometer away
from your body at arms length and whirl it around for about 10 seconds. This whirling aspirates the
thermometer, encouraging evaporation in the wick.
Note the wet-bulb temperature and whirl again for another 10 seconds or so.
Check the wet-bulb temperature again. Continue to do this until you are certain you have reached the lowest
When you have reached the lowest wet-bulb temperature, read the dry-bulb temperature
Subtract the wet-bulb reading from the dry-bulb reading to find the
Use this information to determine relative humidity. If you have a dew point
table you can use this information to find the dew point.
Also click HERE to view Psychrometric calculator
information and HERE for information on
recording temperature and dew point
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