WPS 264 Dew point & psychrometric charts

Dewpoint temperature

• T_dew can be used to calc H

• Each set of T_d, T_w has its own associated T_dew

Determination of humidity !!

• condensation temp is detected visually or with photo detector

• p = p₀ (T_dew)

• RH = p₀ (T_dew) / p₀ (T_d) * 100

Psychrometric charts

• Relates T_d, T_w, T_dew, H, specific volume (V/m), enthalpy & moisture content

• Moisture content is horizontal line

• T_d on vertical line

• T_w lines approx 30 degrees to x-axis

• specific volume approx 45 degrees to x-axis

• H follows curved lines

Reading psychrometric charts

• Find point for T_w & T_d

• H is given by curved lines bottom left to top right

• MC is determined by horizontal line to right from point

• MCs determined by vertical line throught point to 100% H line & then horizontal line to right

• Specific volume is determined by line parallel to sV lines through point

• T_dew determined by horizontal line through point to left & vertical line down

Controlling & limits RH

• Controlling T_d with heating/cooling devices

• Regulation T_w or dew point:

1. Spray water from water bath with controlled temp into chamber → evaporative cooling

2. Reheat air to desired T_d

• Min humidity is given by vapour pressure of water at freezing point

• Low humidity may be obtained with refrigerator coils (T_d < T_dew) through which moist air is passed → condensation

• Upper limit about 98%