Measuring SO2 in our red wine

Hi everyone

We have made four different red wines, the cab sauv. has been in Hungarian oak for almost year and a half, the Grenache is in steel tank since last October, syrah is in steel tank since last October and the syrah /grenache blend is in steel tank since last October.  We bought a Hanna 84100 SO2 titrator to measure free SO2.  Our inital readings were between 2 and 5 ppm of free SO2 on all our reds. We added  potassium metabisulfite using More Wine Guide to Red Wine Making  formula.  We retested and the SO2 levels were the same.  Not sure if the Hanna instrument is malfuncting or all the SO2 is bound up?  We are reluctant to keep adding the Metabisulfite for fear of over concentrating.  Any help would be greatly appreaciated.

Thanks 
Ron Fioretti
909-797-0807