Description Of The Psychrometric Chart - Honeywell AUTOMATIC CONTROL Engineering Manual

WATER-SOAKED WICK
DRY-BULB THERMOMETER
Although commonly used, sling psychrometers can
cause inaccurate readings, especially at low relative
humidities, because of factors such as inadequate air
flow past the wet-bulb wick, too much wick wetting
from a continuous water feed, thermometer calibration
error, and human error. To take more accurate readings,
especially in low relative humidity conditions,
motorized psychrometers or hand held electronic
humidity sensors are recommended.
Specific volume: The volume of air per unit of mass. Specific
volume can be expressed in cubic feet per pound of
dry air. The reciprocal of density.
Total heat (also termed enthalpy): The sum of sensible and
latent heat expressed in Btu or calories per unit of mass
of the air. Total heat, or enthalpy, is usually measured
from zero degrees Fahrenheit for air. These values are
shown on the ASHRAE Psychrometric Charts in
Figures 33 and 34.

DESCRIPTION OF THE PSYCHROMETRIC CHART

The ASHRAE Psychrometric Chart is a graphical represen-
tation of the thermodynamic properties of air. There are five
different psychrometric charts available and in use today:
Chart No. 1 — Normal temperatures, 32 to 100F
Chart No. 2 — Low temperatures, –40 to 50F
Chart No. 3 — High temperatures, 50 to 250F
Chart No. 4 — Normal temperature at 5,000 feet above
sea level, 32 to 120F
Chart No. 5 — Normal temperature at 7,500 feet above
sea level, 32 to 120F
WET-BULB THERMOMETER
PIVOT
Fig. 1. Sling Psychrometer.
Wet-bulb temperature: The temperature read on a thermom-
Chart No. 1 can be used alone when no freezing temperatures
are encountered. Chart No. 2 is very useful, especially in
locations with colder temperatures. To apply the lower range
chart to an HVAC system, part of the values are plotted on
Chart No. 2 and the resulting information transferred to Chart
No. 1. This is discussed in the EXAMPLES OF AIR MIXING
PROCESS section. These two charts allow working within the
comfort range of most systems. Copies are provided in the
ASHRAE PSYCHROMETRIC CHARTS section.
39
PSYCHROMETRIC CHART FUNDAMENTALS
HANDLE
RELATIVE HUMIDITY SCALE
eter with the sensing element encased in a wet wick
(stocking or sock) and with an air flow of 900 feet
per minute across the wick. Water evaporation causes
the temperature reading to be lower than the ambient
dry-bulb temperature by an amount proportional to
the moisture content of the air. The temperature re-
duction is sometimes called the evaporative effect.
When the reading stops falling, the value read is the
wet-bulb temperature.
The wet-bulb and dry-bulb temperatures are the easiest
air properties to measure. When they are known, they
can be used to determine other air properties on a
psychrometric chart.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
C1828