--- canonical_url: "https://buildwithwave.com/blog/grain-depression-dehumidifier-water-damage" title: "Grain Depression: How to Calculate It for a Dehumidifier on a Water Loss" description: "How to calculate dehumidifier grain depression from temperature and RH readings, what low or negative numbers mean, and how the readings affect WTR DHM billing." publishedAt: "2026-09-15" author: "Wave Team" category: "Estimating" tags: ["water damage", "grain depression", "psychrometrics", "dehumidifiers", "xactimate", "drying log"] ogImage: "/og-default.png" status: "published" --- # Grain Depression: How to Calculate It for a Dehumidifier on a Water Loss Grain depression is the affected area's grains per pound (GPP) minus the GPP of the air leaving the dehumidifier. If the room reads 92 GPP and the dehumidifier outlet reads 63 GPP, the grain depression is 29, meaning each pound of air passing through the unit leaves with 29 fewer grains of water vapor. The number shows whether a dehumidifier is removing moisture at the moment of the reading, but it is only one of several readings a carrier reviewer looks at before paying WTR DHM dehumidifier days. ## Why GPP instead of relative humidity Relative humidity changes with temperature even when the amount of water in the air stays the same. A room at 80°F and 60% RH holds about 92 GPP. A room at 70°F and 50% RH holds about 55 GPP. Warm the second room up and the RH drops while the water content stays at 55 GPP. Dehumidifier exhaust is hot, so its RH will always look low. Converting every reading to GPP (7,000 grains equal one pound of water) puts the room and the outlet on the same scale so they can be subtracted. ## How to calculate grain depression 1. **Read the affected area.** Take temperature and RH with a thermo-hygrometer in the middle of the drying chamber, away from the direct discharge of air movers and dehumidifiers. 2. **Convert to GPP.** Use a psychrometric chart, a psychrometric calculator app, or a meter that calculates GPP on screen. Record the GPP, not just the temperature and RH. 3. **Read the dehumidifier outlet.** Hold the sensor in the exhaust airstream within a few inches of the grille and let the reading settle for a minute or two. Readings taken a few feet away pick up mixed room air and understate the depression. 4. **Convert the outlet reading to GPP.** 5. **Subtract.** Affected area GPP minus outlet GPP equals grain depression. 6. **Log it with the date, time, and unit.** When a job has three dehumidifiers, record each unit separately so one weak unit is visible in the log. ### Worked example over three days | Day | Affected area | Room GPP | Outlet | Outlet GPP | Grain depression | | --- | --- | --- | --- | --- | --- | | 1 | 80°F, 60% RH | 92 | 100°F, 22% RH | 63 | 29 | | 2 | 76°F, 50% RH | 67 | 92°F, 25% RH | 56 | 11 | | 3 | 72°F, 40% RH | 47 | 84°F, 23% RH | 40 | 7 | The depression fell from 29 to 7, and the unit is working correctly. As the room dries, there is less vapor for the coil to condense. On Day 3 the room air has a dew point near 47°F, so a refrigerant coil has to run colder than that before it pulls any water at all. A shrinking grain depression alongside a falling room GPP and falling material moisture readings is the normal pattern on a successful dry out. ## What low or negative grain depression means Negative grain depression means the air leaving the unit carries more water than the air entering it, so the reading was taken wrong or the unit is not dehumidifying. Low depression early in a job, while the room is still humid, deserves a check. Common causes: - **Clogged or missing filter** restricting airflow across the coil. - **Iced coil**, usually from a cold drying chamber. Refrigerant units lose capacity as inlet air temperature drops. - **Full reservoir or kinked condensate hose**, which stops the unit or causes it to cycle off. - **Humidistat set too high**, so the compressor is off when the reading is taken. - **Bad reading location**, such as measuring room air that is being pulled back into the intake. - **Room air too dry for the technology.** Once room GPP approaches the practical floor of a refrigerant unit, removal slows toward zero. Switching to desiccant equipment (WTRDHMD) or adding heat is the fix, not adding more refrigerant units. Cotality's Mitigate documentation illustrates the calculation with affected air at 46 GPP and HVAC supply air at 115 GPP, a result of negative 69 from a system adding moisture to the space. The same subtraction works for an HVAC system, a dehumidifier, or outside air. ## Grain depression and getting paid for WTR DHM days The term grain depression does not appear in ANSI/IICRC S500. A C&R Magazine article on the topic reports contractors being denied payment when a dehumidifier did not show 5 grains of depression within 24 hours, even though the unit was working. The same article cites the S500 language that the primary indicators of dehumidifier performance are ongoing control of the drying environment and the resulting reduction in material moisture content. That gives three readings to log together every day: 1. **Material moisture content** at each numbered monitoring point, compared against the dry standard. This is the reading that ends the job. 2. **Room GPP compared with unaffected area and outside GPP.** A drying chamber holding well below the unaffected area shows the environment is under control. 3. **Grain depression at each dehumidifier outlet.** This shows each unit was contributing. WTRDHM, WTRDHM>, and WTRDHM>> all bill per 24 hour period, so each day needs its own entry. If a reviewer cuts a dehumidifier day because depression was 4 grains, the response is the material readings for that same day showing continued drying above the dry standard. If the materials had already reached goal, that day is hard to defend with any number, and the unit should have come off the job. ## Frequently Asked Questions ### What is a good grain depression for an LGR dehumidifier? The S500 does not set a target number, and depression depends on the inlet conditions. In the worked example above, the same unit showed 29 grains in a humid room on Day 1 and 7 grains in a nearly dry room on Day 3. The trend alongside falling material moisture readings matters more than any single day's number. ### What does negative grain depression mean? Negative grain depression means the outlet air holds more moisture than the affected area air. Check the reading location first, then the filter, coil, condensate line, and humidistat. ### Is grain depression required by the IICRC S500? No. The S500 requires monitoring of the drying environment and material moisture content, and drying log software such as Cotality Mitigate calculates grain depression from outlet readings. Recording grain depression daily makes the dehumidifier's performance visible to a reviewer. ### How do you calculate GPP without a psychrometric chart? Most current thermo-hygrometers and moisture meter apps calculate GPP directly from temperature and RH. The manual formula converts the humidity ratio (pounds of water per pound of dry air) to grains by multiplying by 7,000. Wave automates the documentation behind every WTR DHM day by turning voice notes into a full Xactimate estimate in minutes. Learn more at buildwithwave.com.