A psychrometric chart looks dense at first glance, but once you know what a straight line drawn across it means, it becomes one of the fastest ways to reason about an HVAC process. This page walks through the chart's basic anatomy and the process-line directions worth memorizing — the same chart this calculator draws live on the homepage follows exactly this layout.
The axes and curves
Dry-bulb temperature runs along the bottom (x-axis); humidity ratio runs up the side (y-axis). The heavy curve along the top is the saturation line (100% RH) — air physically cannot exist above it at a given pressure without water condensing out. Lighter curves below it mark constant-RH lines (this tool draws 75%, 50% and 25%); every point between the x-axis and the saturation curve represents one unique, valid air condition.
Process lines and what direction they move
- Sensible heating — moves straight right (humidity ratio constant, temperature rises). A duct heater or a furnace coil draws this line.
- Sensible cooling — straight left, same logic in reverse, as long as the air doesn't cross the saturation curve.
- Cooling + dehumidification — a diagonal line down and to the left, bending toward the saturation curve. This is what a real cooling coil draws once its surface temperature is cold enough to pull moisture out as well as sensible heat — see the apparatus dew point page for exactly how that line's endpoint is determined.
- Humidification — straight up (temperature roughly constant, humidity ratio rises), the line a steam humidifier draws.
- Evaporative cooling — down and to the left, but following an almost constant-enthalpy path rather than constant temperature, moving toward (and sometimes very close to) the saturation curve. See the evaporative cooling page for a full worked example.
- Mixing two airstreams — a straight line connecting the two source points, with the mixed result landing on that line at a position weighted by each stream's mass flow rate (more of one stream pulls the mixed point proportionally closer to it).
Reading the chart this calculator draws
The homepage chart computes every curve live from the same Tetens formula behind every other number on this site — it isn't a static picture. Your calculated point is marked in red: how close it sits to the saturation curve is a quick visual read on how humid the air actually is, and its height above the x-axis is its humidity ratio directly, without needing to compute anything further.
Why this beats reading numbers in isolation
The real value of chart literacy is being able to picture an entire sequence of equipment — mixed outside/return air, then a cooling coil, then reheat — as one connected path, and immediately see which stage is doing the humidity work and which is doing the temperature work, rather than tracking five separate numbers through five separate calculations.
FAQ
- Why does cooling sometimes move straight left and sometimes diagonally?
- It depends on whether the cooling surface drops below the air's dew point. Above dew point, no moisture condenses and the line stays horizontal (sensible-only). Below dew point, moisture starts condensing out and the line bends down toward the saturation curve.
- Does mixing two airstreams always land exactly on the line between them?
- Yes, assuming no heat or moisture is added or removed during mixing — it's a straight linear interpolation weighted by each airstream's mass flow rate.
- Is the chart just a lookup convenience, or does it show something a table can't?
- Both are reading the same underlying math, but the chart makes an entire multi-stage HVAC process visible as a single connected path, which is genuinely harder to picture from a table of numbers alone.
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