Psychrometric & Will This Air Dry Your Grain

Heating air does not add or remove water — it only raises the temperature, so the humidity ratio stays constant while the relative humidity falls. As RH falls, the air's equilibrium moisture content (the moisture it holds grain at) drops too. If that EMC is below your grain's current moisture, the air will dry the grain; if it is above, the same air would re-wet it. This tool plots exactly that on a psychrometric chart and gives the EMC the heated air reaches.

EMC is the moisture content a grain settles at when left in air of a given temperature and relative humidity — neither gaining nor losing water. It is calculated here from the ASABE D245.5 Modified Henderson isotherm, 1 − ERH = exp(−A·(T+C)·M^B), with species coefficients for wheat, maize, rice, barley, sorghum and oilseeds. The air can only dry grain down toward its EMC, never below it.

A lot. Heating 20°C, 80% RH air to 40°C at constant humidity ratio drops its RH to about 25%, because the air's saturation pressure roughly triples over that range while the actual vapour is unchanged. That is why even moderately humid morning air becomes a strong drying air once warmed — the calculator shows the new RH for your exact temperature rise.

Each grain has its own EMC isotherm. Oilseeds like canola and sunflower hold far less water at a given RH than starchy grains like wheat or maize, so air that comfortably dries maize to 14% may already be too humid to dry canola to its 8% target. The tool uses the matched species coefficients so the EMC line reflects the grain you actually have.

It is the boundary, drawn for your grain at its current moisture, between air that dries and air that re-wets. Any air point sitting to the lower-right of the line (hotter or drier) has an EMC below your grain and will dry it; air to the upper-left will re-wet it. Watching the heated point cross that line is the whole decision in one picture.

Yes — shelled maize stores safely long-term at about 14% wet basis, wheat around 13.5%, rice around 13%, and oilseeds lower (sunflower ≈ 9.5%, canola ≈ 8%) because their oil makes them spoil at lower moisture. The calculator flags an over-drying risk when the air's EMC sits well below the grain's safe-storage target, so you don't burn fuel shrinking saleable weight.

Over-drying means taking the grain below the moisture you can legally sell at — you lose weight you were paid for and spend extra fuel doing it. If the heated air's EMC is more than about 1.5% below the safe-storage target, the tool warns you and suggests cutting the burner temperature so the air lands near the target instead.

Often yes — set the heated-air temperature equal to the ambient temperature and the chart uses the raw air. Whether it dries depends entirely on the ambient RH versus your grain's EMC: cool, dry autumn air can finish grain without any burner, while warm humid air cannot. The tool tells you immediately which case you are in.

Humidity ratio is the mass of water vapour per kilogram of dry air (g/kg here). Sensible heating with a burner or in-bin heater adds heat but no water, so the humidity ratio is unchanged — the process moves horizontally on the chart. Only adding moisture (or condensing it out) changes the humidity ratio; heating alone does not.

The saturation vapour pressure uses the Hyland-Wexler equation from the ASHRAE Handbook of Fundamentals, which matches published chart values to within about 0.1%, and the EMC uses the ASABE D245.5 Modified Henderson coefficients used throughout the grain-drying literature. Treat the EMC as a solid planning figure; real drying also depends on airflow, depth and grain condition.

Stop when the grain reaches its safe-storage moisture — about 14% for maize, 13.5% for wheat. Running past that point on air whose EMC is below the target only over-dries the crop. If conditions turn humid and the air's EMC rises above the grain, switch off entirely, because that air would add moisture back.

Dew point tells you how much the air can be cooled before it condenses, and it does not change on heating — it depends only on the humidity ratio. A low dew point means genuinely dry air with strong drying potential once warmed; a high dew point means humid air that no amount of moderate heating will turn into good drying air. The tool reports the dew point alongside the heated RH.