The sling psychrometer is used to measure relative humidity. This is one of the simplest models of psychrometers available on the market. It consists of two thermometers, a wet bulb and dry bulb. These are set on a frame that has a chain or light in the middle. When using, you must swing around the instrument in the air. It allows you to predict the weather patterns and is also useful if you keep the houseplants. Here are the steps on how to use the instrument.
Step 1 – Fit the Psychrometer
Attach both the wet bulb and the dry bulbs to a frame using tape. Maintain a sufficient distance between the thermometers so that no collision occurs by swinging the frame. Make sure the bulbs hang down. The numbers on the thermometers should be visible to allow reading with ease. Attach a chain around the middle of the frame.
Step 2 – Take the readings
Write down the temperature shown on both thermometers on your graph. You should get the same number for both. If they record different temperatures, it will give you inaccurate results when calculating the humidity.
Step 3 – Prepare the Wet Bulb
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. Make sure the other bulb stays dry.
Step 4 – Rotate the Psychrometer
Subject the instrument to a circular motion using your hand. Continue the activity for one minute then stop swinging. Record the wet bulb temperature first, followed by the dry thermometer. Start the swinging activity again and after one minute, stop taking both readings. Always take the first wet reading before the temperature starts to rise. Do not swing the appliance for more than 3 minutes for a period until the damp tissue dries. This will cause the wet temperature to increase dramatically and give you inaccurate moisture readings.
Step 5 – Final Readings
No more swinging is necessary once you register the same reading of the wet bulb for 2 to 3 consecutive periods. Final document temperatures obtained within 0.1 degrees. The temperature will have reached its lowest end because the air is saturated with moisture. This state of equilibrium prevents any evaporation of the wettest to take place. The relative humidity can then be calculated from the psychometric map using all the readings obtained.
Be sure your hands are clean when handling the instrument. Dirt or grease will interfere with precision.
Body heat and humidity can affect readings. It is best to hold the instrument at arm’s length when starting to swing.
Avoid touching the bulb ends with your fingers when taking readings. The heat of the body will cause false temperature rises. It is also vital that you take readings quickly once the failover stops. If you allow too much time, temperatures will begin to rise. This will give you inaccurate humidity when calculated.