Understanding Wet and Dry Bulb Thermometers & Relative Humidity Calculation

🌡️ Wet & Dry Bulb Thermometer Explained | Psychrometric Table, Humidity Calculation & Examples 🌍

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Understanding Wet and Dry Bulb Thermometers

Introduction

Wet bulb and dry bulb thermometers also called psychrometers are essential tools in HVAC, weather monitoring, industrial humidity control, and agriculture. These instruments help determine Relative Humidity (RH) using the temperature difference between two thermometers.

This guide explains how wet and dry bulb thermometers work, how to read the tables, and how to calculate humidity accurately with examples. Perfect for HVAC technicians, engineers, and students worldwide.

A wet and dry bulb thermometer, also known as a psychrometer or whirling hygrometer, is a device used to measure relative humidity in the air. It consists of two thermometers:

🌡️ Dry Bulb Thermometer

Measures ambient air temperature.
Exposed directly to air.
Represents the standard temperature.

Wet & Dry Bulb Thermometer

🌡️ Wet Bulb Thermometer

Bulb is wrapped with a moistened cloth (wick).
Evaporation cools the bulb (temperature drops).
More evaporation = lower wet-bulb temperature.
As water evaporates from the cloth, it cools the Thermometer, causing it to show a lower temperature than the dry bulb thermometer.
The rate of evaporation depends on the humidity of the air.

How It Works

When the air is dry, evaporation from the wet bulb occurs more quickly, causing a larger temperature difference between the wet and dry bulb thermometers.
When the air is humid, evaporation slows, resulting in a smaller temperature difference.
The relative humidity is determined by comparing the two temperatures using a psychrometric chart or formula.

Concepts

Relative Humidity (RH)🌡️ The amount of water vapor in the air compared to the maximum it can hold at that temperature, expressed as a percentage.
Dew Point ❄️ The temperature at which air becomes saturated with water vapor, leading to condensation.
Evaporative Cooling 💧 The process by which the wet bulb thermometer cools due to water evaporation.

Steps to Measure Relative Humidity

Record the dry bulb temperature (ambient air temperature).
Record the wet bulb temperature (after evaporation cools the thermometer).
Use a psychrometric chart or formula to find the relative humidity based on the two temperatures.

USE OF TABLES:

Table of WET & DRY Bulb Thermometer Readings with Corresponding Percentages of Humidity.

Table-1

Table-2

Table-3

The dry bulb temperature from -1°C to + 60°C are given down the left-hand side of each table and the depression reading of the wet bulb from 0.5° to 7.0 °, 7.5° to 16.0° and 17.0° to 30 respectively across the top.
It should be noted that the depression value increases by 0.5° from 0° to 10° and by 1° from -1° to +30°.
Similarly, the dry bulb readings increases by 1 from -1 to +30 and by 2 from 30 to 60.
Locate the reading of the dry bulb and depression value given by the hygrometer and at the intersection of the two columns, read off the percentage humidity.
In the case of temperatures falling between those given on the tables, Interpolation is necessary.
Interpolation should never be made across the thick zig-zag line but the required value should be obtained by extra polation downwards or upwards according as to whether wet bulb is water covered or ice covered. The values of relative humidity above the thick line are valid.
Only when the wet bulb is coated with ice. The following examples illustrate the method of using the tables.

📘 Example: 1

→ Dry bulb 30°C

→ Wet bulb 25 °C

Hence the depression of the web bulb = 30°C - 25°C = 5°C

Reading directly from the tables, the relative humidity is 65%

Example-1

📘 Example: 2

→ Dry bulb 28.5°C

→ Wet bulb 17 °C

The depression of the wet bulb,

Wet bulb = 28.5°C - 17 °C = 11.5°C,

In order to find the relative humidity, it is necessary to interpolate between the dry bulb columns 28°C and 29°C and the depression of wet bulb columns 11°C and 12°C,

→ The result is 26%

Example-2

📘 Example: 3

→ Dry bulb 5.5°C

→ Wet bulb -0.25 °C

The depression of the Wet bulb = 5.5°C – (-0.25) °C = 5.75°C,

✔️ The obtain the relative humidity, it is necessary to interpolate between the dry bulb columns 5°C and 6°C and the depression of wet bulb columns 5.5°C and 6.0°C = 0.5°C.

As the wet bulb is below 0°C,

✔️ It is assumed that it is covered with ice, hence extra polation should be numerically upwards from the figures below the line.

✔️ The values for dry bulb 5°C and depression 5.5°C and 6.0°C are respectively 21°C and 15°C looking at the run of figures outwards from the line and extra polating numerically upwards, the corresponding value for dry bulb 6°C would be 24 and 19.

✔️ Hence the value required is found by interpolating midway between:

21 ➡️ 15

24 ➡️ 19

→ This gives a relative humidity of 19.75%

Example-3

⚙️Applications

Weather forecasting,
HVAC (heating, ventilation, and air conditioning) systems,
BMS Systems,
Agriculture (monitoring conditions for crops),
Industrial processes requiring humidity control,
Textile Industry,
Cold storage and warehousing,

📚 Why Wet & Dry Bulb Measurement Is Important

Helps design proper HVAC systems,
Ensures indoor comfort,
Controls industrial humidity,
Supports energy-efficient cooling,
Essential for environmental monitoring,

📌 Conclusion

Wet and dry bulb thermometers provide a simple and effective way to measure humidity using evaporation principles. By comparing temperatures and using psychrometric tables, one can accurately calculate relative humidity, which is essential for HVAC, agriculture, weather, and industrial processes.

Mastering these calculations helps technicians and engineers work more accurately and efficiently.

I hope you learnt something from this article.

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