When you need to keep tabs on a physical variable of some process, the first step is to find something that can be measured. These direct measurements will give a clear picture of the current state of the variable in question. Nice and easy… if there is something obvious to measure, that is. But many times it is not possible to obtain a direct measurement, either because it is impractical to do so, or because there is no sensor available that can do the job. In such cases, an indirect measurement may still be made by measuring proxy variables that correlate with the variable of interest.
This is a companion discussion topic for the original entry at https://www.edgeimpulse.com/blog/must-be-something-in-the-air