March 26, 2008

Make Your Own Psychrometer

Objective

The goal of this project is to build and use a psychrometer, a simple instrument for measuring the relative humidity of the air.

Introduction

Changes in humidity are an important part of daily weather. In this project you will build an instrument to measure the humidity level in the air. The instrument is called a psychrometer.

A psychrometer is a pretty simple device. It is basically just two securely mounted thermometers and a piece of gauze. The gauze is wrapped around the bulb of one thermometer, and wetted with water. We will refer to this thermometer as the wet bulb thermometer. The other thermometer is simply left dry. The instrument is then swung through the air (which is why you will often see these instruments referred to as sling psychrometers.

As the psychrometer moves through the air, water evaporates from the gauze. This evaporation cools the wet bulb thermometer. The dry bulb thermometer is unaffected. After 2–3 minutes, the temperature reading of the wet bulb thermometer will stop changing. This indicates that the thermometer has reached a new equilibrium state, where the heat of the surrounding air and the evaporative cooling from the wet gauze are in balance.

The wet bulb and dry bulb thermometers will now have different readings. The amount of the difference will depend on three factors:

1. the air temperature,
2. the air pressure (i.e., barometric pressure),
3. the amount of water vapor already in the air.

You can read the air temperature from the dry bulb thermometer, and you can look up the current barometric pressure on the National Weather Service webpage for your local area. With these numbers and the wet bulb reading, you can calculate the relative humidity.

This project shows you how to build a simple psychrometer. Instead of swinging it through the air, you can use a fan to move the air past the psychrometer. (The Variations section has a suggestion for securely mounting the thermometers if you want to make your own sling psychrometer.) You can use your psychrometer to measure humidity changes inside your home, or outside to help you predict the weather.

Experimental Procedure and Additional Information

Used with permission. Copyright © 2002-2008 Kenneth Lafferty Hess Family Charitable Foundation. All rights reserved.

For more science project ideas, go to www.sciencebuddies.org/mentoring/areas_of_science.shtml.

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