Equilibrium relative humidity (ERH) is the RH that the headspace of a closed container settles at when the product inside neither gains nor loses water. At that point, the air's RH reflects the product's water activity: ERH (%) = aw × 100. ERH depends on temperature, so readings are compared at a stable room temperature.
In practice
ERH is what a hygrometer in a curing jar actually tells you, but only after equilibration. Right after you seal or open a jar, the reading is the room's air, not the bud's. Wait several hours, ideally overnight, at stable temperature before trusting it.
House position: jar RH is a proxy. The target is the bud's water activity (about 0.55-0.65) and moisture content, and equilibrated jar RH is a practical stand-in for aw. A jar that settles above about 65% needs more air; one under about 55% is over-dried.
Calibrate cheap hygrometers before trusting them; errors of 3-5 points are common. A salt test (a sealed bag with damp table salt gives about 75% RH) is the usual home check.
Units and formula
aw = ERH / 100 (at equilibrium, same temperature).
Don't confuse with
- Room RH: the air in the drying or storage room.
- Moisture content: water by weight, not humidity.
- Humidity pack setting: what a sachet tries to impose.
