I have a friend who is allergic to sulfites in wine, so he sticks to drinking beer. I use campden tablets to dechlorinate my water, at a rate of one tablet per 10 gallons. How much sulfite makes it into the final beer when treating water this way? How does it compare to sulfite levels in wine?
That would be hard to know accurately. A better way to remove the chlorine would be to use a Brita or Pur filter (an activated carbon filtering system), or expose the water in a colorless carboy or container to intense light for several hours with the top open to allow the chlorine to escape (this is a common way to allow the chlorine to escape when prepping for fish tanks). These do not add sulfur compounds.
At the rate that you are using the campden, the sulfite levels would be less than the typical ppm in wine. Before the metabisulfite reacts with the chlorine and chloramine, the concentration would be about 7 parts per million (using standard .44 g tablets). Sulfite levels of less than 10 ppm in wine are allowed for a wine to be called sulfite free ( see http://wineintro.com/glossary/s/sulfites.html). So even if none of the campden reacted, at the rate that you are adding it, the concentration would be below that accepted as "sulfite-free" in wine.
I use a brita filter and have experimentally determined that chlorine and other contaminants are reduced to sub part per million levels. BTW - the same filters also remove sulfite and sulfates. Be certain to use a relatively fresh filter.
Using campden is less expensive, faster, and more effective than using a filter. Basically, there is no sulfite left. According to BYO "The reaction converts chlorine into chloride and the sulfite is converted to sulfate." (http://www.byo.com/stories/wizard/article/section/121-mr-wizard/475-clearing-chloramine-a-historical-hopping-mr-wizard).