Psychrometry is the study of air and water vapor mixtures, specifically their physical and thermal properties. It describes seven key characteristics: dry bulb temperature, humidity ratio, relative humidity, enthalpy, dew point temperature, wet bulb temperature, and specific volume. Evaporative cooling works by maximizing contact between added moisture and air, causing the air temperature to decrease as water evaporates. This principle applies even in humid conditions since hotter air can hold more water vapor. Knowing two of the seven psychrometric variables allows determining the others using a psychrometric chart.

1. Talking Points: Psychrometry

2. Definition and Key Elements
• Psychrometry is the study of the physical
and thermal properties of air and water
vapor mixtures.
• The seven key physical and thermal
characteristics used to describe
psychrometry are: dry bulb temperature,
humidity ratio, relative humidity, enthalpy,
dew-point temperature, wet bulb
temperature, and specific volume.

3. Common Examples
• When the body sweats to cool itself, it is
using psychrometry.

4. Definitions
• Dry Bulb Thermometer-regular
thermometer.
• Humidity ratio. Saturated air is air holding
100% of the water possible.
– For every 20 degrees Fahrenheit temperature
increase, the holding capacity of the air
doubles. (The inside of a hot car is often 20
degrees more than the outside air.)

5. Definitions
• Relative humidity is a percentage of the
saturated air content.
• Enthalpy: Heat energy content of the
air/water vapor mixture.
– The air’s energy content changes if either or
both the dry bulb and humidity ratio changes.
• Dew Point: The temperature at which
moisture starts to condense from the air.

6. Definitions
• Wet-bulb temperature: measured using a
thermometer whose bulb or sensor is covered
with a moistened wick in a moving air stream.
– The difference between wet and dry bulb
temperatures is important in evaporative cooling.
• Specific volume: the volume in cubic feet
occupied by a pound of dry ear at a specific dry
bulb temperature and pressure.

7. Keys to defined variables
• Knowing only two of the seven outlined
definitions will lead to knowing all seven
variables using the psychrometric chart.
– A typical local weather forecast usually
includes the dry bulb temperature in degrees
Fahrenheit, dew point in degrees Fahrenheit,
and the humidity measured in inches of
mercury.

8. Common Sense Answers
• Evaporative cooling works even in humid areas,
particularly in the afternoon.
– As the temperature rises, so too does the amount of
water that can be carried.
• Water temperature has little effect on
evaporative cooling.
– Placing a gallon of 50 degree water on 90 degree
cement produces 9000 Btu’s of cooling
– Placing a gallon of 90 degree water on 90 degree
cement produces 8700 Btu’s of cooling
• A difference of only three percent.

9. Common Sense Answers
• The key to getting the most out of any
evaporative cooling system is to maximize
the amount of air that comes into contact
with the moisture added.

10. Psychrometry
• Psychrometry is the study of the physical
and thermal properties of air and water
vapor mixtures.
– “The key to getting the most out of any
evaporative cooling system is to maximize the
amount of air that comes into contact with the
moisture added.”
– “Evaporative cooling works even in humid
areas, particularly in the afternoon.”
• As the temperature rises, so too does the amount of water that
can be carried.
Source: University of Kentucky