A water activity test seals a small sample in a chamber and measures the equilibrium relative humidity of the air above it, with a chilled-mirror dew point or capacitance sensor. Water activity (aw) is that equilibrium RH divided by 100. It tells you how freely water is available to microbes and chemical reactions, which total moisture alone does not.
In practice
Regulated markets often set an upper limit around 0.65 aw for flower; check local rules. A COA might read aw 0.58 with moisture 10.8%: well-cured and stable. Readings above about 0.65-0.70 raise the risk of mould growth during storage. Home growers approximate the test by sealing buds in a jar with an accurate hygrometer and waiting for it to equilibrate; the jar RH then roughly equals aw × 100. Jar readings before equilibrium are only a proxy.
aw = equilibrium RH (%) / 100. Dimensionless, 0-1. Pure water = 1.0.
Don't confuse with
- Moisture content: the mass of water (% w/w); two samples with equal moisture can have different aw.
- Jar RH before equilibrium: tells you about the air, not yet the bud.
Myth check
House position: humidity packs rob terpenes and are a marketing gimmick; curing is a slow moisture decline, not holding it constant. See curing.
The relative humidity of the air in a sealed container once it has stopped exchanging moisture with the material inside; ERH / 100 equals the material's water activity.Dew point is the temperature at which air becomes saturated and water starts condensing; when leaves or buds cool to it, free water forms and mould risk jumps.A 0-1 measure of how freely available the water in a material is; for dried cannabis flower the accepted range is about 0.55-0.65 aw.A COA (certificate of analysis) is a lab's signed report of what it measured in a specific sample, such as cannabinoids, terpenes, moisture and contaminants, with methods and limits.The amount of water in a material as a percentage of its weight; cured cannabis flower usually sits around 10-12% w/w, though measurement method matters.Relative humidity is the water vapour in air as a percentage of what it could hold at that temperature, so the same RH dries plants differently at different temperatures.A salt-solution sachet sold to hold a sealed container at a fixed relative humidity; GenetixBay regards them as a marketing gimmick that robs terpenes.Terpenes are volatile hydrocarbon compounds built from five-carbon isoprene units; in cannabis they are made in the trichome heads and are a major part of aroma and flavour.Moisture correction converts a lab result from as-received to dry-weight basis using the sample's measured water content, usually from a loss-on-drying test.Dry-weight basis reports a result per gram of sample with its water removed, while as-received basis reports it per gram as delivered, water included; dry-weight figures are always higher.Microbial testing counts or detects bacteria and fungi in a cannabis sample, including total aerobic count (TAMC), total yeast and mould count (TYMC) and specific pathogens.Mycotoxins are toxic compounds made by certain moulds, such as aflatoxins and ochratoxin A from Aspergillus and Penicillium species; cannabis labs test for them at parts-per-billion levels.
