Psychrometric Chart May 2026

She measured the dry bulb: 94°F, straight up from the bottom axis. She measured the wet bulb from a sling psychrometer she’d spun outside: 72°F, following that diagonal down. Where the two lines crossed, she placed a dot.

Her grandfather had called it "the wizard’s abacus." He had been a millwright here in 1957, back when this floor hummed with a thousand looms and the air was a thick, wet rope of heat and cotton dust. He taught her when she was nine: how dry-bulb temperature (the straight vertical lines) was just the obvious lie the thermometer told. Wet-bulb temperature (the diagonal lines sloping down to the right) was the truth of how much the air could weep. And relative humidity—those swooping parabolas—was the air’s current mood, its level of satisfied thirst. psychrometric chart

But the chart told her more. The enthalpy lines—running diagonally from upper left to lower right, marked in BTUs per pound—gave her the total heat hiding in the air, sensible and latent together. She traced along the constant enthalpy line to the saturation curve (the leftmost boundary, where relative humidity hits 100%, the edge of fog and rain). The number there told her how much energy she’d need to wring the water out. She measured the dry bulb: 94°F, straight up