Introduction to Psychrometry Science and Calculations

The psychrometric chart looks intimidating the first time you unroll one on a job site. It is covered in curved lines going in different directions, numbers that do not seem to line up the way you expect, and axes that take a moment to get your bearings on. But once you understand what it is actually showing you, it becomes the most useful single piece of paper you will carry. The chart maps the relationship between air temperature, moisture content, and energy. Every drying decision you make, where to set your dehumidifier, whether to run the air conditioning, whether to open windows or seal the building, can be traced back to a point on that chart.

Start with the two axes. The horizontal axis is dry bulb temperature in degrees Celsius. That is the number your thermometer gives you, the everyday air temperature reading. The vertical axis is humidity ratio, expressed as grams of water vapour per kilogram of dry air. This is not the same as relative humidity, and that distinction matters enormously in the field. Relative humidity tells you how full the air is relative to its current capacity. Humidity ratio tells you the actual mass of water vapour sitting in that air, regardless of temperature. The curved upper boundary of the chart is the saturation curve. When your air state plots on that line, the air is holding every gram of moisture it possibly can at that temperature. Above the line, condensation is occurring. The curved lines running parallel to that boundary are your relative humidity lines, each one representing a constant percentage of maximum moisture capacity. Your target in structural drying is to keep sustained relative humidity below 55 percent. That is not an arbitrary number. Above that level, conditions begin to support secondary damage including mould growth and ongoing absorption into building materials.

Two other values you will read from the chart constantly are dew point and wet bulb temperature. Dew point is the temperature at which your air parcel would hit saturation and condensation would begin. You find it by tracking horizontally from your plotted air state across to the saturation curve and reading the temperature there. This matters in practice because if any surface in the drying environment is at or below the dew point, moisture will condense on it, and you will be creating new damage while you are trying to fix existing damage. Wet bulb temperature is read by a thermometer with a wet wick over its bulb. The closer the wet bulb reading is to the dry bulb reading, the more humid the air. On the chart, wet bulb lines run diagonally and also connect to the saturation curve. The enthalpy lines, which represent total heat content in the air, run roughly parallel to the wet bulb lines and are used when you need to calculate the energy your equipment is actually moving, not just the moisture.

Here is how this plays out on a real job. Say you walk into a water-damaged lounge room and take your readings: dry bulb 24 degrees Celsius, relative humidity 72 percent. You plot that point on the chart. You can immediately see the air state is well above your 55 percent target line. You trace across to the saturation curve and read a dew point of around 18 degrees Celsius. You look at the concrete slab near the external wall and it feels noticeably cooler to the touch. A quick non-contact thermometer reading shows 17 degrees at the surface. That surface is below dew point. Moisture is condensing on it right now. You need to address that before you can claim the environment is drying. The assessor walks in and asks, "How do you know the drying is actually working?" You point to the chart, show where the air state was at setup, where it is now after 24 hours of equipment running, and explain the trajectory toward the target condition. That is a conversation you can have with confidence when you have been reading the chart from day one of the job, not just at the final inspection.