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As we discussed last time, relative humidity (%RH) is a way to measure moisture in your grow room’s air. Today, we’ll talk about “grains per pound” (gpp), which is a different way to measure the amount of moisture in the air.

What’s the difference between %RH and gpp?

While %RH measures the percentage of water in the air at a particular temperature, gpp (as in grains of moisture per pound of dry air) measures the weight of water in the air. “Relative” humidity is a “relative” measure denoted by the percentage of moisture in the air (which changes with air temperature). By contrast, gpp is an absolute measure. Instead of reporting the percentage of saturation as %RH does, gpp tells us the actual weight of the water in a pound of air.

How the grain came to be as a unit of moisture measurement

The original weight was not exact, since it was based on the approximate mass of a single cereal grain. This measure was used as a sort of rough standard from the Bronze Age to the Renaissance.

In 1959, the International Yard and Pound Agreement standardized a grain to equal precisely 64.79891 mg (or about 65 mg for our purposes), with 7000 grains equal to a pound.

Why would you want to measure gpp in your grow room?

Measuring gpp is useful when you want to know how much actual moisture is in the air. %RH measures how close the air is to being saturated with water at a particular temperature, but gpp measures the exact weight of moisture in air being measured. Here’s an example:

A pound of warm air is roughly 14 cubic feet by measure. That’s a volume about the size of a cube with dimensions a bit less than 2.5 feet on each side. If that air measures 100 gpp, then there is about 6500 mg of moisture in that pound of air, which equals about 0.22 ounces, or a little less than a quarter of an ounce of moisture.

Calculations like this can help you figure out how much water needs to be removed with a dehumidifier (usually at lights-off, or in sealed rooms) or added (sometimes needed as the temperature climbs, or with very dry ventilation air) to your grow room’s air to provide your plants with optimal conditions for growth.

In the next article: Learn about dew point as a measure of moisture in your grow room.