Grain depression is how many grains per pound the dehumidifier pulls out of the air passing through it: inlet GPP minus outlet GPP. Take both readings at the unit, at the same time.
What grain depression means
Grain depression is the drop in grains per pound between the air entering a dehumidifier and the air leaving it. It shows how much moisture the unit strips from each pound of air. Multiply it by the airflow and you have the water removed.
Worked example
Inlet 78°F at 65% RH is 93.4 GPP. Outlet 90°F at 22% RH is 46.0 GPP. Grain depression is 47.4 GPP. At 250 CFM that is roughly 173 pints of water a day.
Depression is not the whole story
The 2021 S500 judges drying performance by controlling the environment and by material moisture readings falling over time, not by grain depression alone. A unit can show a big depression in a small sealed room that is already nearly dry. Log room GPP and material readings every day alongside it.
Frequently asked
What is a good grain depression for an LGR dehumidifier?
On a wet job, LGR units commonly show 20–40 GPP or more. The number shrinks as the room dries, so a falling depression late in a job is normal. A low depression on day one, with high room GPP, points to an undersized or faulty unit.
Where do I take the readings?
Measure the air entering the dehumidifier and the exhaust air leaving it, at the same time, with the same meter. Readings taken across the room or minutes apart will not compare cleanly.
Why is my grain depression negative?
The exhaust holds as much or more water than the inlet, so the unit is not dehumidifying. Check that the compressor is running, the coil is not iced, the filter is clean, and the room is warm enough for the unit to work.
More free tools
This knowledge, built into the software
RestoredOS runs the whole job on the same field knowledge behind these tools — drying logs, moisture mapping, scope, and billing.