Grain Depression
Grain depression is the difference in absolute humidity, measured in grains per pound, between the air entering a dehumidifier and the air leaving it, used to verify that the machine is actively removing moisture.
Understanding Grain Depression
Grain depression is a diagnostic measurement used to confirm that a dehumidifier is working effectively. It is the difference between the amount of moisture in the air entering the dehumidifier and the amount in the air leaving it, expressed in grains per pound (GPP) of air. A grain is a small unit of weight (7,000 grains equal one pound), and grains per pound measures the absolute amount of water vapor the air is carrying.
If a dehumidifier is pulling moisture out of the air, the exhaust air will contain fewer grains per pound than the intake air. That difference is the grain depression. A meaningful positive number confirms the machine is actively condensing and removing water; a number near zero signals that the unit is not accomplishing much, whether due to a mechanical issue or because the air is already quite dry.
How It Is Measured
Measuring grain depression is an application of psychrometry. Technicians use a thermo-hygrometer to record the temperature and relative humidity of the air at two points: the intake and the exhaust of the dehumidifier. Those two values are then converted to grains per pound using a psychrometric chart or calculator, and the intake GPP minus the exhaust GPP gives the grain depression.
For example, if intake air measures 90 grains per pound and exhaust air measures 60 grains per pound, the grain depression is 30, meaning the machine removed 30 grains of moisture from each pound of air passing through it. Recording this on the daily moisture log documents equipment performance objectively rather than assuming the unit is doing its job.
Why It Matters on a Job
Grain depression is one of the most useful real-world checks a restorer performs, because it distinguishes a dehumidifier that is genuinely drying the structure from one that is merely running. A healthy grain depression confirms the equipment is contributing to the dry-out, which supports both efficient drying and a defensible insurance claim.
It also helps technicians tune the drying system. If grain depression is low despite wet materials remaining, it may indicate the wrong type or capacity of dehumidifier for the conditions, prompting a switch, for example, from a refrigerant to a desiccant unit in a cold or very low-humidity environment. Tracked alongside moisture meter readings, grain depression is a key indicator that structural drying is proceeding as planned toward the dry standard.