Overview

The aim of this web app is to create an interactive psychrometric chart on which you can project a range of comfort metrics as well as mapping weather data or room air conditions calculated using EnergyPlus. You can add or remove lines for a range of different metrics or highlight them individually to assist with dynamic explanations or presentations.

The Psychrometric Chart

The terms psychrometry and psychrometrics refer to the study of moist air and its thermodynamic properties. Whilst obviously important in the design of air-conditioning systems, these concepts are also fundamental to the understanding of many aspects of thermal comfort in buildings and the principles of climate-responsive design.

The Relative Humidity Chart

When developing this tool, I really wanted to be able to morph seamlessly between the standard psychrometic chart and the much simpler relative humidity chart. Having taught psychrometrics to architects in the past, I found that a major hurdle was trying to convincingly explain why the psychrometric chart uses absolute humidity in the vertical axis instead of the more familiar metric of relative humidity, and why its characteristic curved layout is so important. Now, by simply animating back and forth between the two chart types, most of that explanation becomes pretty obvious as you get to see very clearly the majority of psychrometric processes change from straight lines under the absolute humidity axis to complex curvilinear lines under relative humidity.

Interesting Features

The following are some of the features of this app:

• You can position the current indicator by pressing and dragging anywhere within the chart to see the relevant psychrometric values at that position, all listed in the top-left corner of the psychrometric chart. To display the Heat Index, Predicted Mean Vote (PMV), ASHRAE Standard 55 comfort class or EU Standard 15251 comfort category, simple select the appropriate comfort indicator in the COMFORT OVERLAY panel.
• When overlaying the Givoni Bioclimatic Chart, you can adjust the average mean outdoor air temperature used to derive the comfort zone in order to investigate the effects of adaption. You can also choose to automatically track average mean outdoor air temperature based on loaded hourly weather data and the current date range. This is calculated as a 30-day running mean using the methodology described in ASHRAE Standard 55.
• In addition to annual hourly weather data, you can also load and display the results of EnergyPlus calculations. Either select the Load CSV... button or simply drag-and-drop the EnergyPlus generated .CSV file into the browser window containing this application. If you include the external outdoor dry-bulb temperature and relative humidity as output variables, they will be automatically selected as the mapping axis, though you can select any other temperature or humidity based output using the Data Axis tab in the Select Display Metric dialog box.
• The default data overlay shows the frequency that different dry-bulb and humidity values occur within the chart. However, you can select any other loaded metric to map over the chart. For grid data, you can choose to display the average, minimum or maximum value, or even the number of hours above, below or between a given threshold range. For hourly data points, the actual recorded value is mapped.
• If you check the Show Date Range Selector item within the DATA MAPPING panel, a small slider bar will appear beneath the main chart. This slider allows you to interactively change the date range over which the currently displayed data overlay will be generated. This selector changes the same Date Range values as in the Select Display Metric dialog box. When displayed, the three buttons that also appear under the checkbox can be used to quickly (re)select the entire year, a single month or the current day. Pressing multiple times on the month or day buttons will increment the date range each time. Holding the SHIFT key while pressing will decrement date range.
• You can add your own lines to the chart using the PROCESS LINES panel. To do this, first position the indicator by either dragging it or using the Dry-Bulb Temperature and Relative Humidity sliders. Then click the Add Point button. Once you have added points, you can use the EDIT PROCESS POINTS popover to add titles, re-order them or make changes to their position.
• Using the Export Psychrometric Data dialog box, you can view and save pychrometric data as either JSON or CSV files. Whilst the current indicator data is pretty obvious, the nature and type of the Current Data Overlay depends on what metric is being mapped and the type of selected display.

© Dr. Andrew J. Marsh, 2018.