I currently operate a 7200 QTOF with an 8355 SCD. After changing the hydrogen tank the other day, the upper H2 flow and O3 generator started to have issues. The upper H2 flow is reading a negative flow value, but the lower H2 flow is normal and the O3 generator turns on, but consistently displays that the flow is too low. Eventually they both shutdown and we are unable to run the analyzer. I have tried the obvious solutions (turning the units off/on, restarting the software, etc.) and nothing has worked. Any other suggestions as to what may be the issue?