How to Calculate Dew Point Temperature from Wet and Dry Bulb Thermometer Readings

Measuring environmental conditions accurately is essential for various applications in meteorology, engineering, and HVAC. One such measurement is the dew point temperature which indicates the temperature at which air becomes saturated with water vapor. This article explains how to calculate the dew point temperature using the readings from a wet and dry bulb thermometer. Let's delve into the step-by-step process with practical examples.

1. Measure the Temperatures

To begin with, you need to take two essential temperature measurements:

Dry bulb temperature (T_d): This is the regular air temperature indicated by the dry bulb thermometer. Wet bulb temperature (T_w): This is the temperature indicated by the wet bulb thermometer, which also measures the air's moisture content.

2. Calculate the Humidity Ratio or Mixing Ratio

Using the wet bulb temperature, you can calculate the humidity ratio (W) using the following formula:

W 0.62197 × (E_sT_w / (P - E_sT_w))

where:

E_sT_w: Saturation vapor pressure at the wet bulb temperature (T_w) P: Atmospheric pressure in hPa or mb

To find E_sT_w, use the formula:

E_sT 6.112 × e^{(17.67 × T / (T - 243.5))}

3. Calculate the Dew Point Temperature

Once you have the humidity ratio (W), you can calculate the dew point temperature (T_{dp}) using the following formula:

T_{dp} (243.5 × ln((W × P) / (0.62197 - W))) / (17.67 - ln((W × P) / (0.62197 - W)))

4. Example Calculation

Let's assume the following measurements:

- Dry bulb temperature (T_d) 30°C

- Wet bulb temperature (T_w) 25°C

- Atmospheric pressure (P) 1013 hPa

First, calculate the saturation vapor pressure at the wet bulb temperature (E_sT_w):

E_s25 6.112 × e^(17.67 × 25 / (25 - 243.5)) ≈ 6.112 × 2.96 ≈ 18.1 hPa

Next, calculate the humidity ratio (W):

W 0.62197 × (18.1 / (1013 - 18.1)) ≈ 0.62197 × (18.1 / 994.9) ≈ 0.62197 × 0.0182 ≈ 0.0113

Finally, calculate the dew point temperature (T_{dp}):

T_{dp} (243.5 × ln((0.0113 × 1013) / (0.62197 - 0.0113))) / (17.67 - ln((0.0113 × 1013) / (0.62197 - 0.0113)))

This calculation will yield a dew point temperature. Keep in mind that the accuracy of the dew point temperature depends on the precision of your measurements and the atmospheric pressure used in the calculations.

Notes:

These calculations use logarithmic and exponential functions; a calculator or programming software is recommended for accuracy. The precision of your measurements is critical to ensuring the reliability of the calculated dew point temperature.

Understanding and utilizing wet and dry bulb thermometers with these calculations can help you measure and predict important environmental parameters effectively, making it an indispensable tool in various scientific and practical applications.