They can be expensive depending on the model. Mettler toledo is a huge brand and so is rototronic and aqua labs. You want your water activity to be less than 0.65 for safety reasons but for shelf life purposes (to help with this sweating issue) its best around 0.55 or lower. whoever you buy from can give you best practices for using the aW meter and it will likely be in the instructions that come with it - but basically you want to get your sample into a sample cup and put the lid on right away. Then, if needed, make sure your samples are left out long enough to reach room temp before taking a reading (take the lid off of the sample cup before reading). water activity is widely used in food to make sure shelf stable products will be safe/not spoil throughout their shelf life.
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