Pino wrote:
> How does one calculate how to add 20-40ppm of SO2?

Potassium metabisulphite salt contains 57% free SO2 (by
weight). For x liters, to get y ppm (parts per million) = y mg/l (y
l)/ (0.57) = grams of potassium metabisulphite salt.

For 23 liters, to get 40 ppm (parts per million) =
(40 mg/l x 23 l)/(0.57) = 0.99 grams of potassium
metabisulphite salt.
HTH,
Guy