Resembling coarse sand or fine gravel, it is generally white but is also available with an indicator blue dye that changes to pink when saturated with moisture. To save money, we mix some blue with a larger quantity of white gel. We have heard that beans and rice, if baked dry, will also work as desiccants. Relative humidity can be measured with a hygrometer. We have found that dial hygrometers occasionally require calibration, so it may be safer to have two.
Electronic versions are available.
When choosing any hygrometer, it is important to know how accurate it is, especially at low humidities where many perform poorly. In practice, complete impermeability is difficult if not impossible to achieve. Although expensive sealers that deliver the correct temperature either constantly or in short bursts are available, a humble home clothes-iron will work at the correct temperature setting. After seeds have been cleaned, counted, dried and packaged, they are ready for long-term cold storage. Like the controversy over how dry seeds should optimally be, there are two schools of thought about storage temperature.