Constructing a sling psychrometer is a straightforward process. Take a 1-foot length of industrial-strength string and tie each end securely around two identical thermometers -- tie the string around the part where the bulb of the thermometer meets the thin part. Then dampen a thick piece of cloth and wrap it around the bulb of one of the thermometers to ensure it remains damp during the experiment. Find a clear open space where you will not collide with anything and, holding the middle part of the string, whirl the psychrometer around your head for at least 30 seconds. After this time, record the discrepancy between the temperature shown on the two thermometers and, with a psychrometer chart, determine the relative humidity.
With your homemade psychrometer, you can test the relative humidity of different rooms and environments, including the science lab itself. Use at least four different environments for testing, swinging the psychrometer for 30 seconds in a clear space before comparing the temperatures of the wet and dry thermometers. Compare the results for each room or environment you test and think about the reasons why some environments may have a significantly lower or higher relative humidity than others.
For an alternative experiment that you can carry out in the science lab, use your sling psychrometer to test the relative humidity of the lab once each day for a set period of time. Swing the psychrometer in a clear space and note the results, making sure you list the date each time. Look at your results across different days and consider the reasons that relative humidity may have changed. After you have collected results for at least two weeks, search for patterns in the humidity and try to predict the coming week's humidity on a daily basis.
As you will find out during your experiments, a sling psychrometer can be a hazardous piece of equipment to operate. When carrying out testing, always ensure that you have a clear environment and that other people who may enter the lab are aware of what you are doing by leaving a sign on any entrances. Furthermore, you may want to consider using non-mercury thermometers, given the risk of shattering and causing a spill.