GPP and grain depression calculator
Turn thermo-hygrometer readings into grains per pound (GPP) and dew point, compare the affected area with a dry one, and check the grain depression across a dehumidifier.
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How GPP and grain depression are worked out
Relative humidity on its own doesn't say how much water is in the air, because warm air holds more than cool air: 60% at 80 °F is about 92 grains per pound, while 60% at 65 °F is about 55. Grains per pound (GPP), the humidity ratio, does: it is the weight of water vapour in a pound of dry air, in grains (7,000 grains make a pound). That makes readings taken at different temperatures comparable.
From the temperature and relative humidity, the calculator works out the saturation vapour pressure (the Hyland-Wexler formula in the ASHRAE Handbook, Fundamentals), the actual vapour pressure (that times the relative humidity), and the humidity ratio: 0.621945 × vapour pressure ÷ (air pressure − vapour pressure). Times 7,000 gives GPP. The dew point is the temperature at which the same vapour pressure would saturate the air.
Air pressure falls with elevation, so the same temperature and humidity hold more grains per pound higher up. Printed psychrometric charts are usually for sea level; enter your elevation and the calculator uses the standard atmosphere for it. At 70 °F and 50%, the air is about 54.5 GPP at sea level and about 65.6 GPP at 5,000 feet.
Grain depression is the GPP of the air going into a dehumidifier (the affected area) minus the GPP at its outlet: the grains each pound of air loses on the way through. It is a quick check that the unit is pulling water out. Comparing the affected area with an unaffected room or the outside air shows whether the drying area is drier than the rest of the building, which restorers usually want once drying is under way.
The maths is exact for the readings you type; the answer is only as good as the meter, where it was held and how long it settled. This is a calculator, not a drying log: keep the readings and their dates in your job's drying records.
Tips
- Take readings in the same places on every visit: the affected area, an unaffected room, outside, and each dehumidifier's outlet.
- Let the meter settle for a minute or two at each spot, and hold it in the dehumidifier's exhaust, not off to one side.
- A low grain depression in wet air can point to a dehumidifier that needs attention, such as a clogged filter or dirty coils. In air that is already dry, a smaller depression is normal.
- Watch the dew point near cold surfaces: walls, windows or ducts colder than the dew point collect condensation.
Document the water loss with a scan
vPlan AR doesn't take temperature or humidity readings. It documents the rest of the job: scan the wet rooms with a LiDAR iPhone or iPad, or draw them on the web, tag the damage and pin photos room by room, and on Pro the Water mitigation pack in Scope adds extraction, flood cuts, antimicrobial, drying equipment days and daily moisture monitoring to each room from its measurements, ready for the estimate and the scope report.
Frequently asked questions
How do you calculate grains per pound?
From the temperature and relative humidity: work out the vapour pressure in the air, then the humidity ratio, 0.621945 × vapour pressure ÷ (air pressure − vapour pressure), and multiply by 7,000. At 80 °F and 60% relative humidity at sea level that is about 92 GPP.
What is grain depression?
The difference between the GPP of the air going into a dehumidifier and the air coming out of it. With 80 °F and 60% going in (about 92.1 GPP) and 95 °F and 20% coming out (about 48.9 GPP), the grain depression is about 43 grains.
How do I use GPP to check drying?
Compare the affected area's GPP with an unaffected room or the outside air on each visit. Once drying is under way the affected area is usually the driest air in the building. The air only tells part of the story: the materials' moisture readings, falling to the dry standard, show when the job is done.
Does elevation change GPP?
Yes. Air pressure is lower higher up, so the same temperature and relative humidity hold more grains per pound. At 70 °F and 50% the air is about 54.5 GPP at sea level and about 65.6 GPP at 5,000 feet. Enter your elevation for readings off a sea-level chart.
What is the dew point?
The temperature at which the air would be saturated, with the water already in it. At 70 °F and 50% relative humidity it is about 50.5 °F, so any surface colder than that collects condensation.
Guides
- Water Damage Categories and Classes Explained: Category 1–3, Class 1–4Every water loss gets a category (how contaminated the water is) and a class (how much is wet and how hard it is to dry). Here is what each one means, how to work out the class from a room's measurements, and how both change the scope.
- How to Create a Water Damage Scope-of-Work Report from a Phone ScanScan the affected rooms, tag the damage, and export a professional scope-of-work report — with floor plan, room-by-room condition schedule, and photos — straight from your iPhone.
- Mold Remediation Documentation Checklist for ContractorsMold jobs get questioned more than most. This checklist covers what to record before, during and after remediation, and how to keep it organized room by room on a measured floor plan.