The sensorcon s a cheap pos for this exact reason. I had the same issue as well as many buddies. We all got rid of ours. The sensor is exceptionally inaccurae, and it is due to flow rates. You can adjust the flow rate differently on the same tank and it will read differently each time. I contacted the company and they finally admitted the unit really was never made for diving and diving was an afterthought. They finally admitted that the unit has accuracy issues based on flow rates. They've become very popular on scubaboard because of the cheap price and the "advocacy". I'm glad everyone's getting serious about co testing, but spending extra for a good unit with accurate results is preferred. I and my dive group distrusted the units so much we weren't comfortable using them.