How does a psychrometer measure relative humidity

Answer 1

A psychrometer is a device used to measure relative humidity using two thermometers: a wet-bulb thermometer and a dry-bulb thermometer.

1. Dry-Bulb Thermometer: This thermometer measures the air temperature directly. It is simply exposed to the air and provides a standard temperature reading.

2. Wet-Bulb Thermometer: The bulb of this thermometer is wrapped in a wet wick or cloth. The wet bulb also measures temperature, but it is influenced by the evaporation of water from the wick. As water evaporates, it cools the bulb, resulting in a lower temperature reading compared to the dry bulb.

The basic principle behind the psychrometer is that the rate of evaporation of water from the wet bulb is affected by the humidity of the surrounding air. When the air is dry (low humidity), evaporation is more efficient, leading to more cooling and a lower wet-bulb temperature. Conversely, when the air is humid (high humidity), evaporation is less effective, and the wet bulb temperature will be closer to the dry bulb temperature.

Calculation of Relative Humidity

To determine relative humidity (RH), the dry-bulb temperature and the wet-bulb temperature are used in conjunction with psychrometric charts or formulas. The difference between the two temperatures (known as the wet-bulb depression) can be used to determine the relative humidity percentage.

1. Calculate the Temperature Difference: Measure the temperature difference between the dry bulb (T_db) and the wet bulb (T_wb).

2. Use a Psychrometric Chart or Formula:

   • With the temperatures measured, you can use a psychrometric chart to find the relative humidity based on the dry bulb temperature and the wet bulb temperature.
   • Alternatively, there are formulas and online calculators that can provide relative humidity when given these two temperature readings.

Summary

In summary, a psychrometer measures relative humidity by comparing the temperature of a dry bulb and a wet bulb, with the difference reflecting the humidity level of the air. The greater the difference, the lower the humidity, while a smaller difference indicates higher humidity.