Only temp and humidity get sent to APRS/CWOP. The QC converts those to derive dew point. It's not hard to have dew point get flagged by the QC because it is a combination of data from two sensors. If both are a little off, their cumulative errors can cancel each other or magnify each other. There is also the surrounding station effect. The QC is a comparative analysis. All last summer my dew point QC was worse than usual. The culprit, I think, was a new station that came online that must have had very poor siting because their temperature was just wrong much of the time. That station has since dropped off, and now my dew point QC is something like 99% again.
The QC is only an indicator of a possible problem. But it requires some personal attention and analysis to determine if there is an actual problem or the reason for the flagging is something like normal microclimate differences, errant nearby stations, etc. If it's an actual problem, you need to determine if it's a siting issue, or a sensor issue. And you also have to determine if it's your temperature readings that are messing up the dew point, or if it's your humidity readings.
Steve
