A psychrometric chart plots dry-bulb temperature against humidity ratio, overlaid with curves for relative humidity, wet-bulb temperature, enthalpy, and specific volume. It is the fundamental tool for HVAC system design and air conditioning process analysis. The chart reveals the thermodynamic properties of moist air at a glance, enabling engineers to trace heating, cooling, humidification, and dehumidification processes as paths on the diagram.
# anyplot.ai
# psychrometric-basic: Psychrometric Chart for HVAC
# Library: makie 0.22.10 | Julia 1.11.9
# Quality: 88/100 | Created: 2026-06-16
using CairoMakie
using Colors
# --- Theme tokens (see prompts/default-style-guide.md) -----------------------
const THEME = get(ENV, "ANYPLOT_THEME", "light")
const PAGE_BG = THEME == "light" ? colorant"#FAF8F1" : colorant"#1A1A17"
const ELEVATED_BG = THEME == "light" ? colorant"#FFFDF6" : colorant"#242420"
const INK = THEME == "light" ? colorant"#1A1A17" : colorant"#F0EFE8"
const INK_SOFT = THEME == "light" ? colorant"#4A4A44" : colorant"#B8B7B0"
const GRID = RGBAf(INK.r, INK.g, INK.b, 0.12)
# Imprint palette — one hue per psychrometric property family
const RH_GREEN = colorant"#009E73" # 1 — relative humidity / saturation curve
const COMF_LAV = colorant"#C475FD" # 2 — comfort zone fill
const WB_BLUE = colorant"#4467A3" # 3 — wet-bulb temperature
const ENT_OCHRE = colorant"#BD8233" # 4 — enthalpy (energy)
const VOL_CYAN = colorant"#2ABCCD" # 6 — specific volume
const PROC_ROSE = colorant"#954477" # 7 — HVAC process path
# --- Psychrometric model at sea-level pressure (101.325 kPa) ------------------
# Standard atmosphere moist-air properties from the ASHRAE relations.
const P = 101325.0 # Pa
dry_bulb = collect(range(-10.0, 50.0, length = 400)) # °C, x-axis primary
# Magnus saturation vapour pressure over water (Pa)
p_ws = 610.94 .* exp.(17.625 .* dry_bulb ./ (dry_bulb .+ 243.04))
# Saturation humidity ratio (g water / kg dry air)
W_sat = 1000 .* 0.621945 .* p_ws ./ (P .- p_ws)
# --- Figure ------------------------------------------------------------------
fig = Figure(
resolution = (1600, 900),
fontsize = 14,
backgroundcolor = PAGE_BG,
)
ax = Axis(
fig[1, 1];
title = "psychrometric-basic · julia · makie · anyplot.ai",
titlesize = 20,
titlecolor = INK,
xlabel = "Dry-Bulb Temperature (°C)",
ylabel = "Humidity Ratio (g water / kg dry air)",
xlabelsize = 15,
ylabelsize = 15,
xlabelcolor = INK,
ylabelcolor = INK,
xticklabelsize = 12,
yticklabelsize = 12,
xticklabelcolor = INK_SOFT,
yticklabelcolor = INK_SOFT,
xtickcolor = INK_SOFT,
ytickcolor = INK_SOFT,
xticks = -10:5:50,
yticks = 0:5:30,
yaxisposition = :right, # humidity ratio reads on the right, as on a real chart
backgroundcolor = PAGE_BG,
leftspinevisible = false,
topspinevisible = false,
rightspinecolor = INK_SOFT,
bottomspinecolor = INK_SOFT,
xgridcolor = GRID,
ygridcolor = GRID,
xminorgridvisible = false,
yminorgridvisible = false,
limits = (-10, 50, 0, 30),
)
# --- Comfort zone (≈ 20–26 °C, 30–60 % RH) -----------------------------------
comfort_T = range(20.0, 26.0, length = 40)
comf_bottom = [Point2f(t, 1000 * 0.621945 * (0.30 * pw) / (P - 0.30 * pw))
for (t, pw) in zip(comfort_T,
610.94 .* exp.(17.625 .* comfort_T ./ (comfort_T .+ 243.04)))]
comf_top = [Point2f(t, 1000 * 0.621945 * (0.60 * pw) / (P - 0.60 * pw))
for (t, pw) in zip(reverse(comfort_T),
610.94 .* exp.(17.625 .* reverse(comfort_T) ./ (reverse(comfort_T) .+ 243.04)))]
poly!(ax, vcat(comf_bottom, comf_top); color = (COMF_LAV, 0.20),
strokecolor = (COMF_LAV, 0.85), strokewidth = 1.5, label = "Comfort zone")
text!(ax, 23.0, 8.0; text = "Comfort\nzone", color = INK_SOFT, fontsize = 12,
align = (:center, :center), font = :bold)
# --- Lines of constant specific volume (m³/kg) -------------------------------
# Thinned family + lower alpha so the RH/saturation hierarchy reads cleanly upper-right.
for (k, v) in enumerate(0.80:0.03:0.95)
Wv = 1000 .* ((v .* 101.325 ./ (0.287042 .* (dry_bulb .+ 273.15))) .- 1) ./ 1.607858
Wc = [(0.0 <= w <= ws) ? w : NaN for (w, ws) in zip(Wv, W_sat)]
lines!(ax, dry_bulb, Wc; color = (VOL_CYAN, 0.45), linewidth = 1.0,
linestyle = :dashdot, label = k == 1 ? "Specific volume (m³/kg)" : nothing)
end
# --- Lines of constant enthalpy (kJ/kg) --------------------------------------
for (k, h) in enumerate(0.0:10.0:120.0)
Wh = 1000 .* (h .- 1.006 .* dry_bulb) ./ (2501 .+ 1.86 .* dry_bulb)
Wc = [(0.0 <= w <= ws) ? w : NaN for (w, ws) in zip(Wh, W_sat)]
lines!(ax, dry_bulb, Wc; color = (ENT_OCHRE, 0.6), linewidth = 1.0,
linestyle = :dot, label = k == 1 ? "Enthalpy (kJ/kg)" : nothing)
end
# --- Lines of constant wet-bulb temperature (°C) -----------------------------
for (k, twb) in enumerate(-5.0:5.0:30.0)
pws_wb = 610.94 * exp(17.625 * twb / (twb + 243.04))
Ws_wb = 0.621945 * pws_wb / (P - pws_wb)
Ww = 1000 .* ((2501 .- 2.326 * twb) .* Ws_wb .- 1.006 .* (dry_bulb .- twb)) ./
(2501 .+ 1.86 .* dry_bulb .- 4.186 * twb)
Wc = [(0.0 <= w <= ws) ? w : NaN for (w, ws) in zip(Ww, W_sat)]
lines!(ax, dry_bulb, Wc; color = (WB_BLUE, 0.5), linewidth = 1.0,
linestyle = :dash, label = k == 1 ? "Wet-bulb temp (°C)" : nothing)
end
# --- Relative-humidity curves (10 %–100 %), saturation prominent -------------
for rh in 0.1:0.1:1.0
Wr = 1000 .* 0.621945 .* (rh .* p_ws) ./ (P .- rh .* p_ws)
saturated = rh == 1.0
lines!(ax, dry_bulb, Wr;
color = saturated ? RH_GREEN : (RH_GREEN, 0.7),
linewidth = saturated ? 3.4 : 1.4,
label = saturated ? "Relative humidity" : nothing)
end
# --- Direct property labels --------------------------------------------------
# Relative humidity: along the W ≈ 16 g/kg reading line; curves that never reach it
# (the drier 20 % line) are anchored a few °C inside the right edge so the label
# clears the right spine.
for rh in (0.2, 0.4, 0.6, 0.8, 1.0)
Wr = 1000 .* 0.621945 .* (rh .* p_ws) ./ (P .- rh .* p_ws)
i = findfirst(>=(16.0), Wr)
if i === nothing
i = findlast(<=(47.0), dry_bulb)
text!(ax, dry_bulb[i], Wr[i]; text = "$(round(Int, rh * 100))%",
color = RH_GREEN, fontsize = 12, align = (:right, :bottom), font = :bold)
else
text!(ax, dry_bulb[i], Wr[i]; text = "$(round(Int, rh * 100))%",
color = RH_GREEN, fontsize = 12, align = (:center, :bottom), font = :bold)
end
end
# Enthalpy: in the empty triangle just outside the saturation curve
for h in (20.0, 40.0, 60.0, 80.0, 100.0)
Wh = 1000 .* (h .- 1.006 .* dry_bulb) ./ (2501 .+ 1.86 .* dry_bulb)
Wc = [(0.0 <= w <= ws) ? w : -Inf for (w, ws) in zip(Wh, W_sat)]
j = argmax(Wc)
text!(ax, dry_bulb[j] - 0.7, Wc[j] + 0.5; text = "$(round(Int, h))",
color = ENT_OCHRE, fontsize = 11, align = (:right, :bottom), font = :bold)
end
# Wet-bulb: at the dry end (lower right) of each line
for twb in (5.0, 15.0, 25.0)
pws_wb = 610.94 * exp(17.625 * twb / (twb + 243.04))
Ws_wb = 0.621945 * pws_wb / (P - pws_wb)
Ww = 1000 .* ((2501 .- 2.326 * twb) .* Ws_wb .- 1.006 .* (dry_bulb .- twb)) ./
(2501 .+ 1.86 .* dry_bulb .- 4.186 * twb)
Wc = [(0.3 <= w <= ws) ? w : Inf for (w, ws) in zip(Ww, W_sat)]
j = argmin(Wc)
text!(ax, dry_bulb[j] + 0.4, Wc[j]; text = "$(round(Int, twb))",
color = WB_BLUE, fontsize = 11, align = (:left, :center), font = :bold)
end
# Specific volume: at the mid-point of each valid segment
for v in (0.80, 0.86, 0.92)
Wv = 1000 .* ((v .* 101.325 ./ (0.287042 .* (dry_bulb .+ 273.15))) .- 1) ./ 1.607858
valid = findall(i -> 0.5 <= Wv[i] <= W_sat[i], eachindex(dry_bulb))
isempty(valid) && continue
m = valid[cld(length(valid), 2)]
text!(ax, dry_bulb[m], Wv[m]; text = "$(v)", color = VOL_CYAN, fontsize = 11,
align = (:center, :bottom), font = :bold)
end
# --- Example HVAC process: cooling + dehumidification ------------------------
# State 1: warm humid return air (30 °C, 50 % RH) → State 2: cool supply air (14 °C, 90 % RH)
pw1 = 0.50 * 610.94 * exp(17.625 * 30 / (30 + 243.04))
pw2 = 0.90 * 610.94 * exp(17.625 * 14 / (14 + 243.04))
W1 = 1000 * 0.621945 * pw1 / (P - pw1)
W2 = 1000 * 0.621945 * pw2 / (P - pw2)
lines!(ax, [30.0, 14.0], [W1, W2]; color = PROC_ROSE, linewidth = 3.4,
label = "Cooling + dehumidification")
arrows!(ax, [30.0 + 0.8 * (14.0 - 30.0)], [W1 + 0.8 * (W2 - W1)],
[0.2 * (14.0 - 30.0)], [0.2 * (W2 - W1)];
color = PROC_ROSE, linewidth = 3.4, arrowsize = 18)
scatter!(ax, [30.0, 14.0], [W1, W2]; color = PROC_ROSE, markersize = 13,
strokecolor = PAGE_BG, strokewidth = 2)
text!(ax, 30.5, W1; text = "① return air", color = INK, fontsize = 13,
align = (:left, :center), font = :bold)
text!(ax, 13.0, W2 - 1.7; text = "② supply air", color = INK, fontsize = 13,
align = (:right, :top), font = :bold)
# --- Legend ------------------------------------------------------------------
axislegend(ax; position = :rb, labelsize = 12, labelcolor = INK,
backgroundcolor = ELEVATED_BG, framecolor = INK_SOFT,
framevisible = true, padding = (10, 10, 8, 8), rowgap = 3)
# --- Save --------------------------------------------------------------------
save("plot-$(THEME).png", fig; px_per_unit = 2)
Part of Psychrometric Chart for HVAC on anyplot.ai.