What is a good Grain Depression (GPP delta) coming out of an LGR dehumidifier?

On Days 1–2 of a wet loss, an LGR dehumidifier exhaust stream typically shows a 15 to 35+ GPP reduction compared to intake air. On Days 3–5 as bound water releases more slowly from framing and subfloors, a 6 to 15 GPP depression confirms the machine is still actively removing moisture.

Why can Relative Humidity (RH%) go down while GPP actually goes up?

Because warm air holds exponentially more moisture than cool air! For example, air at 70°F and 50% RH contains 55 GPP, whereas air heated to 85°F at 40% RH contains 73 GPP. That is why IICRC S500 requires converting Temp/RH into GPP to prove true moisture removal.

Psychrometric GPP (Grains Per Pound), Vapor Pressure & Dew Point Drying Calculator

Engineering Formula (ANSI/IICRC S500 §10.6.4 Psychrometric Drying Science)

GPP = 7000 × 0.62198 × Pv ÷ (Patm - Pv), where Pv = RH% × Es(T). Grain Depression = Affected Intake GPP − Dehumidifier Exhaust GPP (Positive delta proves active moisture extraction).

Grains Per Pound (GPP) measures the exact weight of water vapor (7,000 grains = 1 pound of water) suspended in one pound of dry air, derived from dry-bulb Temperature (°F) and Relative Humidity (RH%) using the Magnus-Tetens saturation vapor pressure equation. When insurance desk adjusters attempt to cut Xactimate WTR DHM and WTR DRY runtime after Day 3, showing a positive Grain Depression (Intake GPP minus Dehumidifier Exhaust GPP > 8–15 GPP) alongside elevated structural Wood Moisture Equivalent (WME%) proves that the dehumidifier is actively pulling moisture out of wet building assemblies. Enter your daily thermo-hygrometer Temperature (°F) and RH% below to compute exact GPP, Dew Point (°F), and Vapor Pressure (inHg) for your drying log.

Reference Table

Psychrometric Condition (Sea Level)	Calculated Humidity Ratio (GPP)	IICRC S500 Drying Status & Xactimate Action
80°F @ 70% RH (Day-1 Wet Room Ambient)	108.6 GPP (Dew Point: 69.3°F)	High vapor pressure — immediate LGR dehumidification required to prevent secondary mold
75°F @ 45% RH (Day-3 Conditioned Chamber)	58.4 GPP (Dew Point: 52.3°F)	Active drying window — continue WTR DHM if structural framing remains >4% above Dry Standard
92°F @ 24% RH (LGR Dehumidifier Exhaust Stream)	53.8 GPP (Dew Point: 49.8°F)	Proves +12 to +25 GPP grain depression across coil; justifies ongoing WTR DHM billing
72°F @ 36% RH (Final Dry Standard Equilibrium)	42.1 GPP (Dew Point: 43.5°F)	Target psychrometric environment achieved; verify material WME% for equipment takedown

Copy-Paste Xactimate F9 Note

F9 NOTE (PSYCHROMETRIC GPP VERIFICATION — WTR DHM / WTR MON): Daily thermo-hygrometer readings were recorded at outdoor ambient, unaffected control, affected chamber intake, and dehumidifier exhaust ports per ANSI/IICRC S500 §10.6.4. Positive grain depression (Intake GPP > Exhaust GPP) and elevated structural moisture content confirmed active moisture evaporation requiring continuous dehumidifier operation.