GrainPoint Psychrometric Calc

It is 7am, the building is wet, and the only question is whether to open the windows. GrainPoint answers that one.

Type the inside reading and the outside reading. GrainPoint compares them on GRAINS — the actual weight of water each holds — and tells you whether ventilating would dry the place out or make the job longer.

THE MISTAKE IT EXISTS TO PREVENT

Outside at 85°F and 50% has a lower relative humidity than inside at 75°F and 60%. It feels drier. It is not drier: it holds about twelve more grains per pound. Open the windows on that and you have just pumped water into a building you are trying to dry.

GrainPoint names that case out loud when it happens, because every instinct says the opposite.

It also refuses to decide when the two readings are within a few grains of each other. A field hygrometer is commonly ±3% RH, and pretending a two-grain difference is a decision would be false precision.

READ ACROSS, BOTH STATES AT ONCE

Dry bulb, wet bulb, dew point, dew point depression, relative humidity, humidity ratio, enthalpy, specific volume and vapour pressure — for inside and outside side by side, which is how you actually use them.

Everything comes from the ASHRAE Fundamentals formulation, the same correlations the printed psychrometric chart is a plot of, computed directly rather than interpolated off a digitised chart. Separate saturation correlations over ice and over water, so a February job is right too.

CORRECTED FOR THE JOB

Humidity ratio is water per pound of DRY air, and thin air weighs less. At the same temperature and humidity, a job a mile up carries more grains than one at sea level. Set the altitude and every number accounts for it — barometric pressure, not an assumption.

DRYING TARGETS

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