PYCHOMETRIC CHART
Moisture content
This is also known as the humidity ratio and is usually designated as W. It is the proportion of the mass of water vapor per unit mass of dry air. Humidity ratio is dimensionless, but in the US it is usually expressed as pounds of moisture per pound of dry air; elsewhere it may be expressed as grams of water per kilogram of dry air or as a percentage.
The moisture content is the vertical axis of
the chart.
Enthalpy
(total heat)
Enthalpy (usually
designated as h) is the total amount of heat energy of the moist air and
therefore includes the amount of heat of the dry air and the water vapor in the
air. In the approximation of ideal gases, lines of constant enthalpy are
parallel to lines of constant WB temperature. Thus the enthalpy is indicated by
diagonal lines on the chart.2
In the US, enthalpy is measured in BTU per pound of dry air; elsewhere it is measured in Joules per kilogram of air.
Dry
bulb (DB) temperature
We measure the temperature of the air with a thermometer. Traditional thermometers have a bulb that contains a liquid that expands, and a tube indicating the temperature on a scale. As the liquid expands, it rises up the scale. In the HVAC business, we use a thermocouple and electronic meter or an infrared thermometer because these are faster and more rugged. Whichever method is used, this measurement is called the dry bulb temperature because the end of the thermometer that is making the measurement has no moisture on it.
The temperature of the air is measured in °F in the USA and in °C
everywhere else.
This temperature is shown as the horizontal axis of the chart.
