Final answer:
Using a perfectly dry wick on the wet-bulb thermometer of a sling psychrometer will underestimate the relative humidity, as no cooling through evaporation will take place to provide an accurate reading.
Step-by-step explanation:
If a technician uses a sling psychrometer that has a perfectly dry wick on the wet-bulb thermometer, the result will be a underestimate of humidity. The wet-bulb thermometer is designed to have a dampened wick which allows it to measure the temperature after evaporation has occurred. Since evaporation is a cooling process, the wet-bulb temperature is usually lower than the dry-bulb (current air) temperature.
The level of cooling depends on the moisture of the air; more moisture means less evaporation and a higher wet-bulb temperature. Without water in the wick, no evaporation can occur, therefore no cooling effect will be observed, resulting in no difference between the wet-bulb and dry-bulb temperature readings, falsely indicating that the air is fully saturated with water vapor when it may not be. This misreading will lead to an underestimate of the actual humidity level.