Psychrometric Chart
You know from your own personal experience of the weather that some days are just PERFECT! You may even wish you could construct an object to replicate those perfect days again and again.
Click HERE to take the orientation tour.
On those rare perfect days it is not too hot or cold, not too humid or dry, not too windy or still – you just feel very comfortable.
Well, that’s what AC engineers, passive solar and climate-responsive building designers aim to construct when they engage with buildings – an object that can replicate perfect days and nights and make you comfortable 365 days a year.
How do they manage such a feat? They use the Psychrometric Chart.
Click on ANSWER below, to watch me resolve some everyday situations using the Psychrometric Chart.
Q1: I just got home from work and the house is superhot! How can I avoid this without ending up with a massive AC electricity bill?
Q2: It’s winter and mornings are rather cold. Should I turn on my heater or are there other ways to heat my house?
Q3: My house smells musty despite me using a heater to keep it dry, why?
What more videos?
Scroll down to have more questions answered.
Please note this post contains simplified explanations of the psychrometric chart adequate for general knowledge rather than working knowledge. If you are interested in more information I recommend Dr Andrew Marsh’s website http://andrewmarsh.com/software/psychro-chart-web/ specifically https://drajmarsh.bitbucket.io/psychro-chart2d.html
What Does the Psychrometric Chart Show?
It shows the atmosphere’s behaviour in indoor and outdoor spaces at any hour of the day or night as affected by the seasons.
This is why the Psychrometric Chart is an essential tool for designing buildings that are comfortable to the largest number of people throughout the largest number of days in any year and why it is essential that engineers, architects, and related building design practitioners understand how to read it and use it to inform their designs.
What Am I Looking At?
The Psychrometric Chart is actually 8 different graphs laid onto one another which it’s why it looks so complicated.
Each graph shows the behaviour of one of the parameters (building blocks) of the atmosphere. The reason why the graphs are laid onto one another is because, just like in the atmosphere, each parameter has an effect on all the others.
Break It Down For Me:
The 8 overlaid charts are:
1) Dry Bulb Temperature: temperature of the air as you would read it off a typical wall thermometer
Good for: giving you a general idea of the amount of clothing you should wear and blankets you should have on your bed. See also my post on the CLO.
(2) Absolute Humidity or Moisture Content or Humidity Ratio: The actual amount of water gas (moisture) present in any one kilogram of air no matter what the temperature. For any one kilogram of air at any temperature, moisture content is the weight of the water gas only vs the weight of all other air gasses.
Good for: meteorologists to predict rain, where, how often, and how severely it will fall.
(3) Relative Humidity: amount of moisture in the air at present (right now) as a % of how much more moisture the air could contain assuming the temperature at present stays the same.
Good for: maintaining comfortable and healthy conditions inside buildings which avoids problems like mould growth on one end, and dry skin and stinging eyes on the other.
(4) Saturation Line for Dew Point Temperature: is a way to find out at what temperature would the moisture in the air right now, turn into a liquid or rain.
Good for: working out at what temperature your windows will fog up and eventually drip with water, and working out if dampness in the place you rent is being caused by waterproofing problems or the way you use your heater.
(5) Wet Bulb Temperature: how easy it is for water to evaporate into the air depending on temperature, amount of moisture in the air, and how much more moisture the air could contain.
Good for: figuring out how easy it is for you feel cooled by sweating. In a normal room where the wet bulb temperature is on the low side you will sweat and feel cooler because of it, whilst in a sauna where the wet bulb temperature is on the higher side, you will sweat but not feel cooled.
(6) Enthalpy or Total Heat: total heat in the atmosphere based on temperature and the heat energy required for some water to stay in the air as moisture.
Good for: scientist mostly, for example meteorologists use it to predict when a storm is likely to turn into a cyclone or hurricane, and to predict when the overall effect of various weather parameters will put the lives of outdoor workers in danger (known as outdoor occupational heat stress).
(7) Specific Volume or the Inverse of Density: how much volume (space in three dimensions) is occupied by one kilogram of air without any moisture.
Good for: scientists mostly, but it explains why generally air floats above water rather than the other way around, or why bubbles of air from a scuba-diver always rise to the top rather than sink to the bottom of the ocean.
(08) Vapour Pressure: whilst not always shown, measures the likelihood of water to change into water gas (moisture); the likelihood changes depending on temperature.
Good for: meteorologists to predict rise and fall in humidity. Additionally, because moisture in the air absorbs radiation from the sun, vapour pressure can be used to interpret how powerful (intense) people will feel sunshine to be. Vapour pressure in humid equatorial countries is higher which will result in less intense sunshine, while vapour pressure in dry deserts is lower, which will result in more intense sunshine.
Benefits of Using the Psychrometric Chart:
Economic, Social and Ecological benefits can be achieved by using the Psychrometric Chart to inform design.
Building and AC designers use the Psychrometric Chart to answer such questions as:
Q4: During the daytime it’s 33°C and 40% humidity, will most people just feel hot or hot and sticky? What should a building provide in the above hot situation? Shade from the sun with openable windows OR use the AC system?
Q5: The temperature right now is 8°C and the relative humidity is 40%, will people feel it’s cold and dry or cold and damp (the kind of weather that makes the elderly complain about their joints)? What should a building provide in the above situation? Allow sunshine inside various rooms OR decorate interiors with real plants OR use a conventional heater?
Economic Benefits:
Are generally in the form of lowered energy bills due to lowered dependence on AC use.
Social Benefits:
Are achieved by designing buildings that a better calibrated to achieve comfort for their occupants.
Ecological Benefits:
Come in the form of lowered energy consumption which directly lowers the amount of carbon dioxide (a green-house gas) released into the atmosphere.
References – websites accessed and correct at time of publishing.
Moisture Content or Humidity Ratio is http://ocp.ldeo.columbia.edu/res/div/ocp/WestCLIM/PDFS/LeporeEtAl2016.pdf
Psychrometric chart in colour for video presentations is https://www.researchgate.net/figure/Psychrometric-chart-Givoni-1992_fig1_327768540
Psychrometric chart in blog is https://www.tecquipment.com/assets/documents/downloads/EC-Psychrometric-Chart-poster-A3-0617.pdf
Question 1 and Question 2 effects of too low and too high relative humidity on human health link is https://www.ncbi.nlm.nih.gov/books/NBK215639/
Question 5 Video Answer Podcast link is https://www.abc.net.au/radio/programs/little-green-pod/hot-house-cold-house-green-house-your-house/11966098
Other resources you may find useful:
Australian Bureau of Meteorology glossary of terms http://www.bom.gov.au/climate/glossary/
Baruch Givoni’s pivotal 1992 journal article on the Psychrometric Chart and Building Design Guidelines https://www.sciencedirect.com/science/article/abs/pii/037877889290047K
Dr Andrew Marsh’s website http://andrewmarsh.com/software/psychro-chart-web/
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