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Table of Contents

1. Introduction
2. Background: Dry-Bulb Wet-Bulb and the Psychrometric Table
   • Dry-Bulb Wet-Bulb Theory
   • Relevant Equations
3. Standard Dry-Bulb Wet-Bulb Sling Procedure
4. Using a Single Thermometer Approach
5. Software Interface Guide
   • 4-Step Guide for Single Thermometer Dry-Bulb Wet-Bulb Measurement
   • Dry-Bulb Wet-Bulb Calculator
   • Humidity Table Creation
6. Experiment Naming and Data Management

1. Introduction

The "Calibration of Hygrometers" section in CHEMIX School allows users to calibrate hygrometers through a simplified (and probably better) Dry-Bulb Wet-Bulb procedure than the classic Sling Psychrometer (using two thermometers), which estimates relative humidity and calculates dew point temperature. The software provides tools calculate and manage calibration data.

2. Background: Dry-Bulb Wet-Bulb and the Psychrometric Table

Dry-Bulb Wet-Bulb Theory

The Dry-Bulb Wet-Bulb method is a traditional technique for measuring atmospheric humidity. It involves two temperature readings:

• Dry-Bulb Temperature: The air temperature measured without any additional factors.
• Wet-Bulb Temperature: The temperature measured by covering a thermometer with a moist cloth and allowing air to pass over it, cooling it through evaporation.

As water evaporates from the cloth, the wet-bulb temperature decreases until equilibrium, at which point the cooling effect reflects the humidity level of the air.

The Psychrometric Table relates these temperatures to relative humidity, where:

Key psychrometric equations include:

where $P$ is atmospheric pressure and $P_{v}$ is the vapor pressure.

Relevant Equations

Relative Humidity $RH$ can also be derived by approximating the saturation point:

where $T_{db}$ is dry-bulb temperature, $T_{wb}$ is wet-bulb temperature, $e_w$ is saturation vapor pressure, and $A$ is a psychrometric constant.

3. Standard Dry-Bulb Wet-Bulb Sling Procedure

In the traditional sling method, two thermometers are attached to a rotating sling. One thermometer measures the dry-bulb temperature, while the other is wrapped in a wet cloth and rotated to encourage evaporation, providing the wet-bulb reading. This approach has been the standard for measuring humidity for many years.

4. Using a Single Thermometer Approach

Practical Reasoning

Using a single thermometer simplifies the setup and can be done with commonly available tools. Most users already have a single thermometer, making this method highly accessible.

Theoretical Reasoning

The approach assumes that during the short experiment time, humidity and ambient temperature remain constant. The wet-bulb temperature generally stabilizes within five minutes, which is not enough time for significant environmental changes to occur.

Note on Calibration for Home Hygrometers: Most low-cost home hygrometers lack a direct adjustment feature for calibration. As these devices do not allow manual value adjustments, users can instead make a note on the hygrometer showing the difference between the displayed reading and the calibrated value obtained using this software. This correction note can be useful across all humidity levels, especially if the hygrometer’s readings are linear, meaning the difference remains consistent across a range of humidity levels.

Fig. CHEMIX School Dry Bulb / Wet Bub step by step approach using a calibrated baking thermometer to find the Relative Humidity

5. Software Interface Guide

4-Step Guide for Single Thermometer Dry-Bulb Wet-Bulb Measurement

Follow these steps to measure humidity with a single thermometer.

• Step 1: Measure and record the ambient temperature (Dry-Bulb) using the thermometer.

  • Figure: Thermometer recording ambient temperature.

• Step 2: Wrap a cloth around the thermometer sensor.

  • Figure: Thermometer with cloth covering around 70% of the metal sensor.

• Step 3: Moisten the cloth with distilled water at ambient temperature.

  • Figure: Thermometer with a wet cloth.

• Step 4: Blow air over the damp cloth using a fan. Observe and record the stabilized wet-bulb temperature.

  • Figure: Fan blowing air over the thermometer.

Dry-Bulb Wet-Bulb Calculator

The calculator on the left side of the screen enables users to enter and compute Dry-Bulb, Wet-Bulb, and Humidity values.

• Dry Bulb: Input the recorded ambient temperature.
• Wet Bulb: Input the stabilized wet-bulb temperature after blowing air.
• Humidity: The software will calculate relative humidity based on the Dry-Bulb and Wet-Bulb values.

The calculator also displays the Dew Point based on your inputs.

Radio Button Functionality

• Dry Bulb, Wet Bulb, Humidity: Select the field to calculate by enabling the radio button next to it. The selected field will be grayed out, indicating that its value is computed based on the other two inputs.

6. Experiment Naming and Data Management

The upper right side of the interface allows users to name and save different experiment settings and notes.

• Name Field: Input a name for the current experiment or notes.
• Big Edit Field: Manage and record/store experiments here.
  
• Edit List Box Frame: Use the four buttons to manage saved entries.
  • Save: Stores current settings as a "Before" or "After" entry.
  • Replace: Updates the selected entry with the current settings.
  • Delete: Removes the selected entry from the list.

List Box: Displays all saved experiment names. To load an experiment, double-click an item in the list.

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