For every measurement the ODiSI makes, it is comparing the current state of the sensor to a reference state (the key). The system makes a measurement of how confident it is that the comparison was done correctly and yielded a valid data point. This measurement is called the quality factor. Many factors can impact the quality factor, but one of the most common is high strain gradients (the strain changing by a large amount from one gauge to the next). If the quality factor drops below the programmed threshold, the system will report a NaN for that gauge of that scan.
If your application is going to involve high strain gradients, there is a way to work around this. Under settings-> general there is a check box for quality factor. If you do not have this option and would like it, reach out to your Luna sales person. Activating this feature will force the system to report a measurement for every gauge. It will also export the reported quality factor for each strain measurement so the user can determine if the data is valid or not. The live plot on the ODiSI will still omit any point that is below the quality factor threshold.
How do I know if the data is valid? The best way is to understand your application. If you are expecting a sudden and large increase in strain and that is what you see then it is probably correct. If a gauge is truly giving bad data, the data will normally fluctuate rapidly and often changes sign. For example, the reported values could be: 20, 3000, -2500, 200, 205, -500. If you see a sudden change but then is fairly constant or follows a trend that you would expect, then the data is correct even if the quality factor is low.