Relative humidity Table (Whirling Psychrometer)

1: Dry bulb is ambient temperature.
2: Minus (subtract) wet bulb temperature from dry bulb temperature. Dry-bulb temp (20) - Wet-bulb temp (15) =  20 °C - 15 °C = 5  °C (value will always be positive +)

Result = RH 58 %

Dew Point Table (Whirling Psychrometer)

1: Dry bulb is ambient temperature.
2: Minus (subtract) wet bulb temperature from dry bulb temperature. Dry-bulb temp (20) - Wet-bulb temp (15) =  20 °C - 15 °C = 5  °C (value will always be positive +)

Result = DP 12