How to Read a Sling Psychrometer

A psychrometer's chart.
••• Creative Commons: Arthur Ogawa

A sling psychrometer is an instrument that measures the relative humidity and dew point in an area. A sling psychrometer has two thermometers: a wet bulb and a dry bulb. The wet bulb has a cotton wick over the bulb of the thermometer, which is moistened with room temperature water. The dry bulb is simply a thermometer. Both are attached to a dowel with a screw so that they may be spun through the air. A sling psychrometer works on the premise that evaporation is a cooling process. The drier the air, the more evaporation takes place off of the wet bulb, dropping the temperature on the thermometer.

    Wet the cotton wick of the wet bulb thermometer water at room temperature.

    Make sure that both thermometers are secured on the dowel and swing them around for one minute.

    After you are finished swinging the psychrometer, record the temperature of the dry bulb and of the wet bulb. Note: The wet bulb temperature is never warmer than the dry bulb temperature. If the wet bulb temperature reads warmer than the dry bulb, then the water was too warm or the psychrometer is broken.

    Find the difference between the wet bulb temperature and the dry bulb temperature. Record. For example, if the dry bulb temperature is 22°C and the wet bulb is 18°C, the difference between the two is 4°C.

    Find the dew point temperature by using the top chart from the Resource listed below. Use the dry bulb temperature on the y-axis and the difference between the wet and dry bulb on the x-axis. The temperature where the two meet is the dew point temperature in °C. The dew point temperature is the air temperature that dew begins to form.

    Find the relative humidity using the bottom chart from the Resource listed below. Use the dry bulb temperature on the y-axis and the difference between the wet and dry bulb on the x-axis. The number where the two meet in the chart is the relative humidity expressed as a percent.

    Things You'll Need

    • Sling psychrometer (or two alcohol thermometers, one with a cotton wick)
    • Water at room temperature
    • Relative humidity chart
    • Dew point chart

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