Workings of Potassium metabisulphate addition calculator

Example
Before we start, it should be noted that winemakers tend to use parts per million (ppm) interchangeably with mg/l.

Let's assume a Pinot Noir wine (red wine) has a pH of 3.65, there is currently 12 mg/l free sulphur dioxide in the wine and the total sulphur dioxide (TS02) is 35 mg/l, how much KMS is needed to provide an effective antiseptic dose?

1. Firstly it is necessary to calculate the free sulphur dioxide level (FS02) required to sufficiently inhibit microbial growth. From charts (not given here) the amount required is 36 mg/l (this equates to a molecular sulphur dioxide level of 0.5 mg/l for red wine - 0.8 mg/l is used for white wine) and a pH of 3.65.

1. Given that the current level of FS02 is 12 mg/l and the required level is 36 mg/l, an addition equivalent to 24 mg/l (36-12) is required.

2. As approximtely 50% of the added Sulphur dioxide will bind with wine constituents the required addition of sulphur dioxide is double the FS02 requirement, or 48 mg/l. The addition of this amount would result in the TS02 of 83 mg/l (TS02 initially 35 mg/l in wine plus a 48 mg/l addition) which is below an acceptable TS02 of 100 mg/l.

3. Now, if a 48 mg/l sulphur dioxide addition is required then approximately double this amount of KMS is required (Powdered KMS is 57% sulphur dioxide by weight) which equates to 84 mg/l.

4. If the volume of wine is 2500 litres then 2500 x 84 mg of KMS is required or 210 grams.

5. Dissolve this in three times its weight of warm water and add the the tank - Simple!

6. Leave for a hour or two to allow the sulphur dioxide to diffuse through the wine and then measure in the laboratory to ensure the concentration is correct.

If you have the time, multiple additions at half the rate will provide security against over addition and allow for an adjustment (to the amount of binding) in the second addition.