The sling psychrometer is used to measure relative humidity. This is one of the simpler designs of psychrometers available on the market. It consists of two thermometers, a wet bulb, and a dry bulb. These are attached to a frame that has a string or light chain in the middle. When in use, you must swing around the instrument in the air. It enables you to predict weather patterns and is also useful if you keep indoor plants. Below are steps on how to use the instrument.
Step 1 - Assemble the Psychrometer
Attach both the wet bulb and dry bulbs onto a frame using tape. Maintain enough distance between the thermometers so that no collisions occur when swinging the frame. Make sure the bulbs hang at the bottom. The numbers on the thermometers should be visible to allow reading with ease. Attach a string around the middle of the frame.
Step 2 - Take Readings
Note the temperature indicated on both thermometers on your chart. You should obtain the same figure for both. If they register different temperatures it will give you inaccurate results when computing humidity.
Step 3 - Prepare the Wet Bulb
Thoroughly saturate the piece of fabric with distilled water. Wrap it around the bulb of one of the thermometers. Use rubber bands to secure both ends of the fabric. Be sure the other bulb remains absolutely dry.
Step 4 - Swing the Psychrometer
Subject the instrument to a circular motion using your hand. Keep up the activity for a minute then stop the swinging. Note down the wet-bulb temperature first, followed by the dry bulb. Begin the swinging activity again and after another minute, stop to take both readings. Always take the reading from the wet-bulb first before the temperature begins to rise. Do not swing the instrument for more than 3 minutes for any period as the wet fabric will dry out. This will cause the wet bulb temperature to rise considerably and give you inaccurate humidity readings.
Step 5 - Take Final Readings
No more swinging is necessary once you register the same reading from the wet-bulb for two to three consecutive periods. Record final temperatures obtained to the nearest 0.1 degrees. The temperature will have reached its lowest end because the air is saturated with moisture. This state of equilibrium prevents any more evaporation from the wet-bulb from taking place. Relative humidity can then be computed from the psychometric chart using all the readings obtained.
Be sure your hands are clean when handling the instrument. Dirt or grease will interfere with accuracy.
Body heat and moisture can affect readings. It is best to hold the instrument at arm’s length when you begin to swing it.
Avoid touching the bulb ends with your fingers when taking readings. Body warmth will cause false rises in temperature. It is also vital that you take readings quickly once the swinging stops. If you allow too much time, the temperatures will begin to rise. This will give you inaccurate humidity when computed.