How can a sling psychrometer can measure relative humidity and dew point?

Answer 1

A sling psychrometer an instrument that measures relative humidity (a hygrometer). The sling has two thermometers, a dry bulb and a wet bulb thermometer, mounted together on a chain. The wet bulb thermometer is wrapped in muslin that is moistened with distilled water. The user then swings the sling until the temperatures stabilize, then quickly notes the two temperatures. To interpret the readings and determine the humidity, a chart is then used for the specific altitude at which the reading was taken. Basically, the drier the air is, the more moisture evaporates from the moistened muslin-covered thermometer, and the greater the difference between the two thermometers. If the relative humidity is 100%, then the two temperatures will be the same.

Slings have been around for years, and are becoming less and less used for many reasons. With the advent of digital hygrometers, it's just easier to pull out a device and read the screen. Additionally, besides being cumbersome and time consuming, slings contain Mercury, a hazardous material, and must be handled with care. A sling will typically read 5-10% high due to user error. Some of the mistakes that can be made while using a sling are:

1. If the sock on the wet bulb thermometer is not clean, the humidity measured from a sling will err on the high side.

2. If the water used for the wet bulb thermometer is not distilled water, the humidity measured from a sling will err on the high side.

3. If the sling is not swung around long enough, the humidity measured from a sling will err on the high side.

4. If the thermometer measurements are not read quickly enough after swinging the sling, the humidity measured from a sling will err on the high side.

5. If the charts used to interpret the results are not for the same altitude where the readings were taken, the humidity will be incorrect. Additionally, the charts are only available in 100' increments, and it is often difficult to determine the exact altitude of the user.