Overlay issues

I have a list of numerical data coming from multiple machines that are being summed. When ever one of the machines is locked out for maintenance I get an overlay. I tried to bind the data quality on the summed bit to a tag with a value of 192. the overlay goes away untill any of the data changes then it goes back to 0 and the overlay reappears. Am I missing something here??

Yes, the overlay will appear if any of the tags used in the expression is bad. However, you can wrap the entire expression in a forceQuality function to guarantee the result to be of good quality. Your expression is something like:forceQuality({tag1}+{tag2}, 192)