A.J. Rawls wrote "Does anyone use Metabisulfite powder for small quantities of wine? If so, how do you measure it out for 1, 3, 5 and 6 US gallon batches??"
A.J. - You need a small gram scale that will make it possible to weigh parts of a gram. I usually have at least 5 gallons of wine so the quantities of K metabisulfite are easily weighed;
25ppm SO2 ~ (0.16 gram K meta/gallon wine)...0.8 grams for 5 gallons
50ppm SO2 ~ (0.33 gram K meta/gallon wine)...1.7 grams for 5 gallons
75ppm SO2 ~ (0.49 gram K meta/gallon wine)...2.5 grams for 5 gallons
K metabisulfite is pretty cheap so you can prepare a standard solution and use a portion for your wine. Example; 25ppmSO2 needed for 1 gallon wine. Dissolve 1.6grams K meta in 100ml water. Pipet 10ml into your gallon of wine.
Some time ago I weighed up multiple samples of K metabisulfite. The average of these weights worked out to 1.5grams per 1/4 teaspoon. If 1/4 teaspoon is added to 5 gallons of wine you are adding about 45ppm SO2.
A note about Campden tablets. While it's probably not worth worrying about Campden tablets do vary in weight from batch to batch. They are convenient to use but probably are no more accurate in providing SO2 than weighing small quantities of K metabisulfite with a proper scale.
Bill Frazier Olathe, Kansas