Contents
I. What is a Psychrometric Chart
O. Pressure
O. Temperature
O. Moisture
O. Other
IV. How to read a Psychrometric Chart
V. Examples
What is a Psychrometric Chart?
A Psychrometric Chart is a tool. It is used to visually see the thermal and moisture properties of air. The key principle of a psychrometric chart is, moisture levels of air are dependent on temperature. There really isn’t much more to it. The number of variables that we are going to cover and the overwhelming amount of lines on this chart are all different measurable variables that come from measuring air at different altitudes, temperature, and moisture content. It can be used to understand how warmer air can hold more moisture and how cooling the air can result in condensation (Family Guy reference).
It is also important to note that the Psychrometric Charts very from creator to creator. Note whether they are in S.I. or English units, and what variables are present.
For simplicities sake, let’s call the Psychrometric Chart a PC for short. There are a number of applications and industries a PC can be used in. I first learned about the Psychrometric Chart in a Meteorology class while learning how to determine at what temperature it will rain!
It is also used in greenhouse and livestock applications to determine environmental quality.
Energy and HVAC Engineers use it to determine occupancy comfort levels and equipment troubleshooting.
Once you understand the principles, it’ll not only become simple but also quite useful.
What is the importance of moisture in the air?
Before we get into the variables and lines of the chart, I first want to talk about water and energy.
Why?
Because, when analyzing the atmosphere, we are actually looking at a mixture of multiple components that have different phase changing properties: air and water. Though air is also a mixture of more components (oxygen, carbon dioxide, nitrogen, etc.), for simplicity sake, let’s just say air.
From experience, we have all seen water in all three of its forms, ice (solid), water (liquid), and vapor (gas), without seeing air be anything but a gas. That’s because air doesn’t liquify until −194.35 °C (−317.83 °F).
This matters for a couple of different reasons:
1. During a phase change, temperature remains the same while energy continues to increase. See the graph below.
2. Molecule distance and speed at different temperatures. Whether considering a solid, liquid, or gas, higher temperatures results in the substance molecules being further apart and moving at higher speeds, rather than at lower temperatures molecules are slower and closer together.
Psychrometric Variables
Variables with numbers make up the basics of a Psychrometric Chart. The other variables are present depending on who has made the chart and the application.
Pressure
1. Barometric Pressure (Altitude) – First! Take note of the altitude of the chart. It is usually labeled in the upper left hand corner. As you move up in the atmosphere the pressure decreases. Changing the pressure of a liquid or gas also changes its properties, giving you different values or results. Each psychrometric chart is based on a certain altitude in the atmosphere.
We aren’t going to talk about pressure much further than this section, so just make sure the altitude of the graph makes sense for the application you are using it for.
O. Vapor Pressure – If you have a closed container with a liquid inside, the liquid will evaporate into the empty space above until the substance reaches a point of equilibrium between its own vapor and liquid phases. The pressure of the vapor onto the liquid is the vapor pressure. The vapor pressure is dependent on temperature.
Temperature
2. Dry Bulb Temperature ( $ T_{DB} $ ) – [Fahrenheit (F) or Celsius (C)] This is what people refer to just as “temperature”. It is the temperature of air without taking into consideration the moisture content.
O. Sensible Heat ($Q_{s}$) – Sensible Heat is the amount of energy it takes to change the dry bulb temperature.
O. Sensible Heat Ratio ($SHR$) – Ratio between Sensible Heat energy and total energy.
$SHR = Q_{s} / Q_{t}$
Moisture
3. Wet Bulb Temperature ( $ T_{WB} $ ) – Conversely to Dry Bulb Temperature, Wet Bulb Temperature considers how moist or dry the air is. If you take a regular thermometer and wrap it in a wet towel, you will get the Wet Bulb Temperature.
How is this different from regular temperature?
Well, this is where the properties of water come into play. If it is not raining outside and you compare the temperature of your dry bulb and wet bulb thermostats, your wet bulb will be lower than the dry bulb temperature.
Why?
Because the water is evaporating into the air. Energy is first being absorbed by the water for its phase change instead of warming up the thermostat. For comparison, if the air was saturated, that is, it cannot absorb any more water, the wet bulb and dry bulb thermostats would read the same temperature.
This same affect can be felt on your skin. Use room temperature water to wet your arm and allow the water to evaporate. The water is at room temperature, why does that spot feels cooler than the rest of your body? Because the water is absorbing heat from your skin to evaporate.
4. Relative Humidity ($ RH $) – RH is the ratio between how much water vapor is in the air versus the maximum possible amount.
O. Humidity Ratio (Specific Humidity) – This compares moisture levels to the dry air. Humidity Ratio is the weight of water vapor per unit weight of dry air (lb/lb or kg/kg).
O. Dew Point Temperature – Temperature at which water vapor will begin to condensate. On the Psychrometric Chart, you will see I labeled the “Saturation Line”. This is where the Wet Bulb, Dry Bulb, and Dew Point Temperatures are equal.
Other
5. Specific Volume ($v$) – [$ ft^{3}/lb \: of \: dry \: air $] Specific Volume is the inverse of density, it is a substance’s volume divided by its mass.
6. Enthalpy ( $ H $) – Enthalpy is equal to the total heat content of a substance. If you’ve read through the rest of the variables, you’ve read about temperature, moisture, volume, and pressure. Enthalpy considers all of these variables into one total heat content number called Enthalpy.
- $ E = Internal Energy $
- $ P = Pressure $
- $ V = Volume $
$ H = E + PV $
How to Read a Psychrometric Chart
Psychrometric Chart Examples
Example 1 – Given a room temperature of 77°F (25°C) and a Relative Humidity of 40%, what is the Wet Bulb Temperature?
Wet Bulb Temperature = 61°F (16°C)
Example 2 – Given a room temperature of 77°F (25°C) and a Relative Humidity of 40%, what is the Dew Point Temperature?
Dew Point Temperature = 51°F (10.5°C)
Whats the difference between Wet Bulb and Dew Point Temperature?
Great question! When preparing this article, I was struggling myself to come up with a good explanation. By their definitions they seem like they could be the same thing; however, through the two examples I gave above, you can see they are not.
The Dew Point Temperature is found by only decreasing the temperature of the air (going horizontal to the left). The Dew Point Temperature is read at the saturation line. This tells you the temperature the air needs to reach at the first sign of condensation.
On the other hand, Wet Bulb is measured during evaporation.
Remember two things: 1. Wet Bulb is found by wrapping a wet cloth around a thermostat; 2. Enthalpy considers the total system energy (water & air).
During the Wet Bulb measuring process, energy is being absorbed by the water, increasing the internal energy of the water. Well, where is that energy coming from? The surrounding air. The surrounding air’s internal energy is decreasing at the same rate the water’s energy is increase. This results in an Enthalpy change of zero.
The Dew Point Temperature will always be less than the Wet Bulb Temperature.
Leave a comment if you found this helpful…or not!
Do you still have more questions? I’d love to hear from you.