This paper is part of ongoing structural health monitoring in the National Physical Laboratory. The work is aimed at improving reliability of sensors and increasing the uptake of monitoring systems by improving the understanding of the fundamental interactions between the sensors and their environments. It is important to optimize and validate the selection of the sensors for each application in order to ensure that the results are meaningful. An example of unexpected aspects of a sensor performance exposed to temperature and solar radiation is demonstrated and the importance of temperature compensation is highlighted. More work is required fully to explain this phenomenon.