The Psychrometry Panel sits within the main application window and controls the display of the psychrometric chart and the various psychrometric and passive design analysis functions.

You can open and/or close this panel by clicking in its title area. If you click on the status icon on the right-hand side, the panel will open/close without affecting any of the other panels around it.

The Psychrometric Chart provides a graphic representation of the state or condition of the air at any particular time. The chart relates temperature along the horizontal scale to moisture content along the vertical scale. At first glance they can look quite complex, like something only an engineer could love. However, they play an important role in bio-climatic design and comfort analysis so it is worthwhile becoming reasonably familiar.

Graphical Display

The amount of moisture vapour in the air varies quite significantly under different conditions. When the air is hot it can contain a large amount of moisture vapour, sometimes as much as 5% by volume. When it is cold, its capacity to hold the moisture as a vapour is reduced. When the temperature of warm air begins to fall, the vapour also cools and, if cooling continues, will eventually condense into tiny moisture droplets. In the atmosphere this results in the formation of clouds and eventually rain, whilst it is more commonly manifest as condensation running down the outside of a glass of iced water.

When condensation is begining to occur, the air is said to be saturated and cannot hold any more moisture, the point of 100% relative humidity. For a given amount of humidity, the temperature along the X-axis at which this occurs is called the dew point. For a given temperature, the humidity along the Y-axis at which this occurs is called the saturation point. The dew point for each temperature in the Psychrometric Chart above is therefore represented by the inner curved boundary. This is because the air simply cannot exist at a state above and to the left of this line. If the air is cooled beyond its dew point, excess vapour is lost as condensation.

Panel Controls

The following is a description of the items available in this control panel.

Chart Overlay

Highlight Chart Elements

The items in this sub-menu can be used to highlight in yellow the various components of the psychrometric chart and the many values that can be read from it.

Psychrometric Processes

At any specific time, the condition of the air can be represented as a single point somewhere within the chart. If conditions change, then the point will move around the graph. The direction the point moves depends upon what properties of the air are changing. The items in this sub-menu can be used to display the direction of move resulting from the various psychrometric processes represented.

Passive Design Analysis...

This item invokes the Passive Design Analysis dialog.

Active Cooling Strategies

Divides the chart into a series of regions within which different active cooling strategies are likely to be most effective.

Climate Classification

Divides the chart into regions characteristic of different climate types. The classification is based on Average Monthly Maximum temperatures, which are shown in the chart as a thick line between 12 points, each marked with a character representing the first lettor of each month name.

Passive Solar Heating
Thermal Mass Effect
Mass + Night Ventilation
Natural Ventilation
Direct Evaporative Cooling
Indirect Evaporative Cooling

These items all refer to bio-climatic design elements and show the base comfort zones in yellow and any increased comfort area due to the design element in red.

The methodology for determining the effect of various passive design strategies involves plotting hourly conditions on the Psychrometric Chart. Overlaid on the graph is a comfort band derived from the monthly thermal neutrality temperature for that climate (shown in yellow). The percentage of plotted points inside this band is the percentage of time the climate itself provides conditions that would be considered comfortable.

Multiple Techniques

Allows you to select any number of the above bio-climatic design elements for simultaneous display on the chart.

Clear All Overlays

As it says, clears all current chart overlays to display the basic information only.

Chart Settings...

This item invokes the Psychrometric Chart Settings dialog.

Point Colours

Displays a point on the chart for each hour of the selected period. The colour of each point is determined by which sub-menu item is selected. This allows you to colour by:

• Month,
• Time of Day,
• Wind Speed,
• Solar Radiation,
• Cloudiness, and
• Rainfall.

Display

The items in this sub-menu determine what is displayed within the diagram, currently limited to the Title, and Axis markers.

Overlay Type

Use this selector to choose the way hourly weather data is overlaid on the chart.

• Cumulative Frequency
  Displays the frequency of occurrence of different hourly conditions as a coloured block within the chart. The more solid the colour, themore frequently the conditions represented by a block occured over the selected period.
• Hourly Data Points
  Displays each hourly weather condition as a small cross within the chart.
• Monthly Ranges
  Displays a single line for each month between the average monthly maximum and the average monthly minimum.
• Average Monthly Maximums
  Displays average monthly maximum temperatures in the chart as a thick line between 12 points, each marked with a character representing the first lettor of each month name.

Date

Use this item to select the period over which hourly data values are overlayed on the chart. The seasonal settings depend upon the currently set latitude to determine which hemisphere the location is in.

Overlay Parameters

The number of sliders at the bottom of this panel will vary depending on the bio-climatic design element currently selected. By default, only the level of activity of a typical person is displayed as it will affect the location of the comfort zone. Other values appropriate to each bio-climatic design element will appear and disappear as the selection changes, such as the following:

Activity Slider

Use this slider to interactively set the activity rate of the people for whom the comfort band is displayed. This will affect both the base comfort zone as well as the passive design extensions. As you drag, you should see the comfort overlay interactively update. In terms of activity rates, the following is a guide:

• Low activity refers to anyone seated and relaxed or simply reclining.
• Sedentary activity refers to office workers, houses, schools and laboratories. Anywhere people undertake only minor physical tasks and only occasionally move around.
• Light activity refers to people moving around, washing, dressing, shopping or working in light industry.
• Medium activity refers to tasks such as ironing, brick-laying and other aerobic workouts.
• Heavy activity means running, ice skating, shovelling or working with a sledge hammer.

Glazing/Air Speed Slider

The effect of this slider changes with different overlays. In terms of passive heating it refers to the area of glazing on the equator-facing facade relative to its overall area. Obviously the greater the area of glass, the more solar radiation is captured. Note that this is true also in summer. In terms of natural ventilation, this slider controls air speeds. It is usually not desirable in an office to go beyond 'Noticeable' as this can cause other undesirable effects (papers blown off desks, etc). Whereas anything too far below pleasant can lead to a feeling of stuffiness.

Insulation Slider

This slider affects passive heating and sets the level of insulation within the building envelope. A poorly insulated building will loose as much heat through conduction as it gains through solar radiation, making the passive heating system relatively ineffective. A very well insulated building on the other hand will retain almost all the solar heat it collects.

Efficiency Slider

To work well, a passive solar heating system needs to be designed correctly. In a direct gain system, the shading coefficients of glazing panels, location of air vents and the colour/nature of incident surfaces all play a part. A system with high-level windows coated with a reflective foil and with white plush-pile carpet on the floor would probably rate quite lowly on the efficiency scale. On the other hand, a system using full height glazing with dark slate tiles on the floor, well grouted to a 100mm dense concrete insulated slab will probably collect and release the heat far more effectively and rate quite highly.